Chemistry of Silica - Ralph Iler [PDF]

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, Silica, the major component-Olthe earth's solid surface and. the constituent of ordinary sand,'becomes involved at some point in a great many phasesof .' . modern technology and science. It is an essential material in many, ·if not all, . forms of life. Its role in human disease, aging, and health is Just beginning to be explored. Here is a comprehensive account of the basic chemistry invOWed in a wide range of research and development activities, as well as a wealth of information on production and production control. Beginning with the solubility of different. . forms of silica and the factors that influence dissolution and deposition, the solution chemistry of silica Is Introduced. The author also compares and recommends analytical methOds. The digest of all currently available information provides a solid background as to the nature of soluble silicates and particularly the mechanism of polymerization of sHicic acid and formation of colloid. :.'For the first time, the mechanism by which silica sots, powders and gels -are formed and their properties controlled Is clearly described. Next, the many types and uses of commercial concentrated sols, gels, and u'trafine powders are examined, fotl~ by a discussion of the biochemical properties and many applications of the surface chemistry of silica. The finat chapter draws together all aspects of the occurrence and importance of silica in different life forms. Those engaged in research, development, and production in the many diverse fields and Industries in which silica plays a vital role-such as chemistry, biology, medicine, agriculture, metallurgy, and mining-will find THE CHEMISTRY OF SILICA an indispensable reference. 795 . -... .'"



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THE CHltMISTRY OF SILICA ;:.



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Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry



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RALPH K. ILER



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A Wiley-Interscience Publication JOHN WILEY & SONS New York • Chichester • Brisbane • Toronto



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To my wife, Mary, with gratitude for her never-ending patience during the seemiaaly interminable writing of this book



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Preface This book was at first intended to be an updated second edition of my earlier book, The Chemistry of Silica and Silicates (Cornell University Press, 1955). It necessarily covers much of the same subject matter, but with 2500 new references to consider, it had to be reorganized and expanded to such an extent that it constitutes an almost entirely new work. The purpose of the book is to present a complete and coherent account of the chemistry of amorphous silica, including soluble silica and silicate precursors of soluble silica, polymerization to polysilicic acids, colloidal sols and gels, and the surface chemistry of silica. In discussing practical applications of sols and gels, emphasis is placed on the chemistry involved. The.last chapter on silica in living organisms is especially important in view of the growing recognition that silica is present in many biological systems and can function as an essential trace element. Since publication of my earlier book in 1955, the literature on colloidal metal silicates, including minerals, and on silicic esters has grown enormously. Consequently these areas had to be omitted. The title, The Chemistry of Silica, may be misleadingly broad but is offset by a more definitive subtitle, "Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry." It is remarkable that silica, the major component of the earth's solid surface, has never become a separate branch of study or instruction. Science students graudate with little or no knowledge of its properties or chemistry. Yet sooner or later, in such diverse fields as industrial chemistry, electronics, agriculture, mining, metallurgy, petroleum, power development, and even biochemistry and medicine, problems arise involving this common element oxide. This book is written not only for those already engaged in these areas, who may find it a useful guide to the literature, but also for



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Preface



those in other fields who need specific information not otherwise easily available.



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Wilmington, De/aware November /978



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Acknowledgments I am indebted to the Cornell University Press for permission to include some of my earlier book* with the following credit: Reprinted from Ralph K. lIer: THE COLLOID CHEMISTRY OF SILICA AND SILICATES. Copyright 1955 by Cornell University. Used by permission of the publisher. Cornell University Press. I am also grateful to John Wiley and Sons for permission to include in Chapter 5 portions of my monograph on "Colloidal Silica" in Col/Did and Surface Science. VoL 6. 1973. edited by Egon Matijevic. . This work would have been impossible without the generosity of E. I. duPont de Nemours & Co. in making available to me. as a retiree. the facilities of the Lavoisier Library at the duPont Experimental Station. It is impossible to mention all those who have kindly reviewed drafts of portions of the manuscript and given invaluable advice. My friend and fellow scientist. Dr. Paul C. Yates has been very helpful with sound technical counsel. . To the late Mildred Syvertsen. who played an indispensible role in all my earlier publications. I remain grateful for help in collecting references and typing much of the present manuscript. The assistance of Patricia Cullen in final typing. of Joseph A. Pankowski. Jr. in preparing illustrations and of Jennifer J. Stiles in assembling indexes. is sincerely appreciated.



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R.K.1. • Now out of print.



, ix



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Contents Introduction



I



Previous Books and Reviews of Silica Chemistry Selection of References Terminology References



2



. 1 The Occurrence, Dissolution, and Deposition of Silica



3



The Silica-Water System Thermodynamics of the System Relating Particle Size and Composition Energy Change with Changing Particle Size and Composition Soluble Silica-Monosilicic Acid Volatility in Steam Soluble Silica in Nature Phases of Silica Anhydrous Crystalline Silicas Relation between density . .and . refractive index . Hydrated Crystalline Silicas Amorphous Silicas Microscopic sheet. ribbon, and fiberlike forms. Common amorphous forms. Hydrated amorphous silica. Biogenic sili The Solubility of Silica _ . Solubility of Quartz at Ordinary Temperature Cleaning the surface Solubility of Quartz under Hydrothermal Conditions



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3 6 7



9 10 12 13



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Solubility of Cristobalite and Tridymite Solubility of Other Crystalline Forms of Silica Adsorbed Silica on Crystalline Silica Solubility of Amorphous Silica Establishment of solubility equilibrium, Effect of heating, Solubility in water: pH 0-8. Possible solubility minimum at pH 7. Solubility in nitric acid. Solubility in NaCIO. solutions, Effect of electrolytes. Solubility under hydrothermal conditions Solubility of Hydrated Amorphous Silica Apparent High Solubility at High pH Calculation of solubility and dissociation constant Effect of Particle Size on Solubility in Water Interfacial Energy Effect of Impurities on Solubility Effect of Organic Compounds on Solubility Catechol and Related Compounds Polyhydroxy Organic Compounds N-Oxides Organic Bases Living Tissues Solubility in Alcohols . Methanol. Higher alcohols Solubility in Molten Salts" Rate of Dissolution of Silica' Mechanism Effect of pH on Rate Relation Between Rate of Dissolution and Particle Size Rate of Dissolution of Very Small Particles Rate of solution as particle dissolves Rate of Dissolution of Particles of Different Sizes Dissolution of Crushed Powders Neutral Solutions-Effect of Salts Retardants of Dissolution Rate of Dissolution in Presence of Catechol Rate of Dissolution in Aqueous HF . Comparative Rates of Dissolution Removal and Deposition of Silica from Water



32 33 34 40



46 47 49 54 56 58 59 59 59 60 60 61 62 62 62 65 65 69 72 73 74 75 75 76 76 76



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Contents



32



Removal of Silica from Water 78 Precipitation mechanisms, Nucleation of quartz, Adsorption and precipitation by hydrous oxides, Removal by ion exchange 83 Deposition of Silica from Water Rate of deposition of monomeric silica, Silicification of biogenic materials, Rate of deposition of colloidal silica



3 34 40



s, Jns 46



47 49



54 56



58 59 59 59



10 60



61 62 62 62 65 65



69 72 73



74 75 75 76



76 ~



76



xiii



2



Methods of Analysis Atomic Absorption Chemical Methods Methods Involving Silicomolybdic Acid The beta silicomolybdate method, A recom mended procedure, Interfering substances, Molybdenum blue method, For biological sample. Methods of Concentrating Silica for Analysis Depolym erizing Colloidal Silica before Analysis Standard Silica Solutions Miscellaneous Colorimetric Methods Detection of Colloidal Silica on Surfaces Rapid Titration of Total Silica as Fluosilicate Titration as the Silicomolybdic Acid References



94 94 95 95



100 101 101 101 102 102 103 104



Water-Soluble Silicates



116



Sodium and Potassium Silicates Manufacture Commercial Solutions Soluble Crystalline Sodium and Potassium Silicates Properties of Solutions Fields of Use· The Nature of Silicate Solutions Theory Physical Studies : Effects of diluting silicate solutions. Effect of alkali metal salts and other coagulants Conversion to Suicic Acids Reaction with molybdic acid. Conversion to esters of silicic acids. Conversion to trirnethylsilyl derivatives of silicic acid



117 117 119 120 120 121 123



126 130



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Silicates with Coordination Numbers Four and Six Solutions of Polysilicates Sodium Polysilicate . Potassium Polysilicate Lithium Silicates Lithium Polysilicates Uses for Lithium Silicates and Polysilicates Organic Base Silicates Mixed Organic Base-Alkali Metal Base Silicates Other Organic Base Silicates Complex Metal Ion Silicates Organic Chelates of Silicon Catechol Derivatives Humic Acids Other Organic Cornpounds : Hydrated Crystalline Alkali Metal Polysilicates Silicates Convertible to Crystalline Forms of (H 2Si 2Os),c Precipitation of Insoluble Silicates Soluble Silicate Glasses Peroxy Silicates References



142 143 144 145 145 146 149 150 153 154 154 155 156 157 157 158 160 161 163 164 165



Polymerization of Silica



172



General Theory of Polymerization Overall Effect of pH on Gelling Monosilicic Acid Preparation Dissolving silica, Hydrolysis of monomeric silicon compounds, Dissolving monomeric silicates in acid Characteristics of Silicic Acid Diffusion constant. Ionization constants. Increase in ionization constant with polymerization, Isoelectric point. Point of zero charge, Stability of monomeric silica Reactions of Monosilicic Acid Phosphoric andboric acids, Sulfuric acid, Iron and uranium, Chromium, Aluminum, Divalent cations Characterization of Silicic Acids



174 177 177 178



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145 145 146 149 ISO



153 154 154 155 156 157 157 158 160



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xv



Contents



Reaction with Molybdic Acid .. Alpha and beta silicic acids, Measurement of reaction rates, Reaction rate constants, Composition of molybdic acid reagents, Other observations Separation of Silicic-Acids Particle Size and Surface Area by Titration Correction for soluble silica Coagulation with Gelatin-Salt Mechanism of Condensation and Hydrolysis Catalytic Effect of HF Polymerization: pH 2-7 Formation ofOligomers Oligomers as Particles Nucleation Theory Particle Growth in Acidic Solution Depolyrnerization in Acidic Solution Polymerization by Aggregation-Gel Formation Molecular versus Particle Chains Mechanism of Interparticle Bonding Formation of Chains of Particles and Networks Partial Coalescence of Particles in Chains Development of Microgel and Viscosity Isolating "gel phase" or "rnicrogel", Effect of electrolytes and coagulants. Gel density and structure, Increase in viscosity Formation of Larger Particles by Coacervation Polymerization above pH 7 Spontaneous Growth of Particles Final Size of Particles versus Temperature Viscosity of Sols before Aggregation Begins Viscosity of Sols of Very Small Particles at Low pH . Decrease in Viscosity on Conversion of Microgel to Sol Thermal Effects Energy of activation. Heat of polymerization Summaries of Investigations Investigations at Low pH Iler; Alexander, Heston, and Iler: Schwarz and Knauff: Bechtold; Goto; Okkerse: Audsley and Avcston: Weitz, Franck, _...'::".1)'."0-.



195



202 203



206 209



211 213



214 215 218 220 220 222 222



223 225 227 231



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239 239 242 244



244 247 248 249 250



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Contents



xvi



and Giller; Bechtold, Vest, and Plambeck; Acker: H~ebbel . and Wieker Investigations Through the Neutral pH Range 268 Merrill and Spencer; Ashley and Innes; Baumann; Coudurier, Baudru, and Donnet; Marsh, Klein, and Vermeulen; Ginsberg and Sheidina Investigations Above pH 7 281 Greenberg and Sinclair; Greenberg; Goto; Tarutani; Iler: IIer and Sears; Richardson and Waddams; Makrides et al. Polysilicic Acids 287 Preparation of Polysilicic Acid 288 Hydrogen-Bonded Complexes with Polar Organic Compounds 288 Method of comparing hydrogen-bonding activity, Structure versus activity, Liquid hydrogen bonded complexescoacervates, Complex of silicic acid with amine salt, Interaction of silicic acid with phosphoric acid ester Combinations with Organic Polymers 297 Prevention of hydrogen bonding by negative charge on silica, Cationic organic compounds 29~ Miscellaneous Interactions with Organic Materials Interaction with proteins-e-tanning. Esterification of polysilicic acid Activated Silica Sols-Water Treatment 301 Reaction of Polysilicic Acid with Metal Cations 303 References 304



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Colloidai Silica~Concentrated Sols



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Definition of Colloidal Silica and Historical Development Growth and Stabilization of Discrete Particles Increasing Particle Size by Adding ..Active" Silica Methods of Making Particles Under 10 nm in Size Stabilization Against Particle Growth Stabilization Against Aggregation .. , Stabilization by ionic charge, Addition of salt to lower viscosity, Sterk stabilization . Porous Particles



312 313 313 317 318 323



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287 288 288



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317 318 323 '- ..



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Contents



Elongated Particles Particles with Non-Siliceous Cores Methods of Making Sols Neutralizing Soluble Silicates With Acids Electrodialysis Ion Exchange Peptizing Gels Hydrolysis of Silicon Compounds Dissolution of Elemental Silicon Dispersion of Pyrogenic Silica Purification. Concentration, Preservatives Ion Exchange Dialysis and Electrodialysis Washing Procedures Concentration Evaporation of water, Centrifugation, Ultrafiltration, Electrodecantation Preservatives Characterizing Sols Chemical Analysis Measuring pH. Electrolyte concentration Particle Characteristics Particle size. Specific surface area Ionic Charge on Particles Nature of ionic charge. Counterions and double layer Viscosity Aggregation of Particles Definitions Gelling Effect of pH. Effect of particle size and concentration. Electrolytes and organic liquids. Temperature. Theory of strength of gels Coagulation Mechanism. Coagulation by electrolytes. Monovalent cations as bridging agents. Coagulation by divalent metal ions. Coagulation by polyvalent cations-basic metal salts. Effect of silica concentration and other factors. Effect of particle' size. Partly dehydrated surface



xvii



330 330 331 33 I 332 333 334 335 335 336 337 337 338 338 338



343 344 344 345 355 360 364 364 366



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Flocculation Flocculation with cationic surfactants, Flocculation with organic polymers Coacervation Silica spheres by coacervation Aggregation into Ordered Structures-Precious Opal Opal structure, Other ordered aggregates, Formation of uniform inorganic particles, Synthesis of opal Adsorption of Silica Particles on Surfaces Sols of Silica Particles with Modified Surfaces Negatively Charged Surfaces Aluminosilicate ions, Other anions Positively Charged Particles Polyvalent metal oxide coatings, Polyvalent organic cations Organic Modified Surfaces-Organosols Organic ions, Esteri fication, Silylation Commercial Colloidal Silicas Uses of Colloidal Silicas Making Catalysts, Gels, Adsorbents Inorganic Binder, Stiffener Molded refractory bodies, Binders for fibers, Refractory coatings, Molds for casting metals Frictionizing Effects Fibers, Paper, Steel rails, Other surfaces Antisoiling Surfaces Hydrophilizing Surfaces Modifying Adhesion Increasing adhesion, Decreasing adhesion Coating Compositions Coatings on ships: tanks Reinforcing organic polymers Polishing Agent for Silicon Wafers Surfactant Effects Dispersing effects; Antifoaming effects Modifying Viscosity-e-Gelling . Miscellaneous Optical Effects. Color, Photography Use in Biological Research-. Density Gradient -



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396 398



405 407 407 410 ,



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412 415 415 420 420



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425 426 427 428 430 432 433 433 434 435 436 1, the t could be Kitchener rning the acteristics silica sols j become ghest-in Another ous silica to a sur-



where n = number of silicon atoms in a polysilicic acid molecule or particle or polymeric network x = number of OH groups per silicon atom in the polymer, not exceeding 4 m = the number of monomeric silicic acid molecules added to the polymer p = fraction of the hydroxyl groups per monomeric silicic acid molecule that are converted to water during the polymerization reaction



dehydra-



Thus when p = 1, the monomer is converted to SiO z within the polymer molecule without change in the number of OH groups in the polymer. There are, of course, restrictions such as n and In having to be integers and the values of x and p being limited by the possible structures of polymers and conditions of polymerization. However, for the case where dense amorphous silica is being deposited on extensive, massive silica surfaces from slightly supersaturated monomer solution. especially at high temperature and neutral or alkaline pH. x is very small. p is unity. and n is large. Thus the deposited silica may be essentially dense and anhydrous:



• at 25°C onnorous o or-



Even vitreous or glassy silica contains some water. probably as SiOH groups. At a given temperature and humidity there is an equilibrium "solubility" of water in vitreous silica, according to Hetherington and Jack (8). Flame-fused quartz contains 0.04 wt, % OH, whereas electrically fused material contains only 0.0003% as detected byInfrared absorption at 2.73 micron wavelength. By extrapolation to'



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The: Occurrence. Dissolution. and Deposition of Silica



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30°C. Moulson and Roberts (9) concluded the equilibrium concentration of water in ps. silica glass may be as high as 0.22% H 2 0 . probably present as internal SiOH .



Let us return to the behavior of soluble silica in water. When the solution is hfghly supersaturated and insufficient solid silica surface is available to permit rapid deposition of soluble silica. new small nuclei particles are formed by intercondensa_ tion on monomer and low polymers. Silica is also deposited on these until supersajjj, ration is relieved. It is in this manner that colloidal particles of silica are formed. These. in turn. may be aggregated to form silica gel or may be laid down as opal. both of which are highly porous with an extensive internal surface covered with SiOH groups. Thus "hydrated" silicas are formed. Very slow deposition may produce quartz. .



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Thermodynamics of the System



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The heat of formation of silica by the reaction



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was reported (10. II): ~H~ISOK ... - 217.5 == 0.5 kcal mole':' i for alpha quartz. and -215.9 ± 0.3 for amorphous silica. Greenberg and Price (12) give somewhat different estimated values:



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Alpha quartz (q) Colloidal silica (cs) Vitreous silica (vs)



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for the overall equilibrium



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Greenberg (13) calculated the following values for the thermodynamic functions:



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AH (kcal mole-I) ~F~9S0K (kcal mole ' ') .;lS~9"oK (cal deg" ' mole:")



Amorphous Silica



Quartz



+2.65 == 0.28 +3.98 == 0.04 -2.82 == 0.50



+ 7.34 ± 0.37 +5.20 ± 0.04 +4.53 ± 0.71



According to these data. the heat of formation of quartz from amorphous solid silica is ~H = -4.69 kcal mole:", which is .....areatcr than the value -1.78 found bv . W~'et /' !



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and Deposition of Silica



mcenrration of water in ir 11 SiOH groups. :n the solution is highly lable to permit rapid med by intercondensa1 these until supersatu-



irmed, These. in turn, ipal, both of which are h SiOH groups. Thus rce quartz.



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Wat.r System



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a1. (14). The latter is closer to the value 0.54 :I: 0.2 more recently calculated by Cochran and Foster (15). Other reported values for the above hydration reaction of amorphous silica were given by Morey, Fournier, and Rowe (16), who found



Kitahara (17a) measured the solubility of amorphous silica between 9 and 100°C and calculated AHa.o K .. 3.2 kcal mole:". Walther and Helgeson (17b) calculated the thermodynamic properties of aqueous silica and the solubility of quartz and its polymorphs over a wide range of temperatures and pressures. The thermodynamic constants derived from all available data were evaluated as follows: Constant Entropy, So (cal deg " mole-I) Volume, VO (em! rnole ") Gibbs free energy, AG ° (cal mole:") Enthalpy, AH (cal mole:")



Alpha Quartz



Amorphous Silica



9.88 22.69 -204.65 -217.65



14.34 29.0 -202.89 . -214.57



'or alpha quartz, and Coefficients were also given with equations for calculating the values over a wide range of temperatures and pressures. .



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Relating Particle Size and Composition . AFcs • 200°C) . AF c•• 25°C)



/narnic functions: Quartz + 7.34 +5.20 +4.53



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0.37 0.04 0.71



.rphous solid silica I f(\""d by Wise et



In most sols that consist of discrete spherical particles of amorphous silica, the interior of the particles consists of anhydrous Si02 with a density of 2.2 gem -'. The silicon atoms located at the surface bear OH groups which are not lost when the silica is dried to remove free water." The relation of particle composition to particle size can be calculated purely from geometry and densities of the components. Let . . . n, = total number of silicon atoms in a particle n, = number of silicon atoms at the particle surface d . = diameter of particle on anhydrous basis (nm) d" ~ diameter of hydroxylated particle (nm) x = ratio of SiOH groups to total Si atoms = ns/n t assuming one OH per surface silicon



w ... weight of one anhydrous Si0 2 particle (grams) WIt



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weight of one surface hydroxylatcd particle = average number of silicon atoms across the diameter of a particle



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The Occurrence. Dissolution. and Deposition of Silica



surface had decreased and the particles have grown to a certain size. Further spon-



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taneous changes are unlikely to occur. One point that has not been considered in the foregoing discussion is that the energy values have been generally determined on types of silicas that have already reached a relatively stabilized state of particle growth. On the other hand. for much finer silica, for example, with a specific surface of more than 600 m Z g-l, the radius of curvature of the surface is then less than 25 A, and the silanol groups must be spread apart so that less hydrogen bonding can occur between neighboring hydroxyl groups. In turn. it might be expected that this would increase the heat of "wetting, decrease the heat of dehydration, and decrease particle density and surface energy. Under these conditions, it is certain that particle growth occurs with decrease in the radius of curvature, but energy data on such materials have not been obtained, particularly in regard to surface energy of the silanol-water interface.



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SOLUBLE SILICA-MONOSILICIC ACID



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The soluble form of silica is monomeric, containing only one silicon atom and generally formulated as Si(OH)•. This is often called monosilicic acid or orthosilicic acid. The state of hydration is not known. although at high pressure there is some indication that one water molecule is linked to each OH group, probably by hydrogen bonding, so the hydrated molecule is represented by Willey (20) as Si(OH: OH z).' The structure of monosilicic acid is assumed to involve silicon coordinated with four oxygen atoms as in amorphous vitreous silica and in crystalline quartz. Although there are rare minerals such as the stishovite form of SiO z (21) or thaumasite (22), in which silicon is coordinated with six oxygen atoms, silicon in most oxides and silicates is surrounded by only four oxygen atoms. If the monomer had the structure HzSi(OH)., one would expect it to be a strong acid like the analogous HzSiF., but in fact it is a very weak acid. It is essentially non ionic in neutral and weakly acidic solution and is not transported by electric current unless ionized in alkaline solution . .lt is not salted out of water nor can it be extracted by neutral organic solvents. It remains in the monomeric state for long periods in water at 25°C, as long as the concentration is less than about 2 x 10 -3 M. but polymerizes. usually rapidly. at higher concentrations. initially forming polysilicic acids of low molecular weight and then larger polymeric species recognizable as colloidal particles. The question often arises as to whether the term "soluble silica" should include the low polymers such as tetrarner or decarner, which are classed as "oligorners." It becomes a matter of definition. "Soluble" materials have been recognized as those that pass through a dialysis membrane. whereas colloids do not: but even though membranes can now be made with pores sufficiently small to separate dextrose from sucrose. we think of sucrose as being "soluble," On the other hand. sucrose is certainly not colloidal. For the purpose of this book. the following terminology is used:



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Deposition of Silica



Soluble Silica-Monosilicic Acid



size. Further spon-



Soluble silica (or monosilicic acid). Si(OH)4' Polysilicic acid (oligomers). Polymers with molecular weights (as Si0 2) up to about 100,000, whether consisting of highly hydrated "active" silica or dense spherical particles less than about 50 A in diameter. Colloidal silica. More highly polymerized species or particles larger than about 50 A, although sometimes down to 10-20 A. Silica sol. May refer broadly either to polysilicic acid or colloidal silica.



cus.. • is that the that have already ier hand, for much m2 g the radius 01 groups must be ghboring hydroxyl ie heat of wetting, nd surface energy. lith decrease in the lot been obtained, Ice. r



',



11



The arbitrary borderline of 50 A or mol. wt. 100,000 is based on the general observation that below this point the polymer species are generally unstable, having only a transient existence owing to gelling or particle growth. Also, as has already been shown, it is below this size range that less than half of all the silicon atoms are present as Si02 that is, as "silica," whereas more than half are each associated with at least one hydroxyl group. The term "silicic" acid is thus justifiable. The preparation and reactions. for example. polymerization. of dilute solutions of monosilicic acid are further described in .Chapter 3. Meanwhile, some of its characteristics are noted, as follows, prior to discussing solubility:



silicon atom and acid or orthosilicic .sure there is some 'oup, probably by Jy Willey (20) as



1. It is characterized by its rapid rate of reaction with molybdic acid to form the yellow silicomolybdic acid. 2. It is generally inert in neutral solution if the concentration is below the saturation level with respect to amorphous silica. Thus it is almost universally present at a concentration of a few parts per million in most natural waters and in living organisms. 3. It combines with metal ions to an increasing degree with increasing pH, thus reducing the concentration of free monosilicic acid. (Ferric and uranyl ions react at a pH as low as 2, whereas most other metal ions combine only at higher pH.) 4. Above pH 9 it is ionized first to (HO)3SiO - or at still higher pH to (HO)2Si022-. The first equilibrium constant (13.23) is approximately (25°C)



c. dinated with crystalline quartz. Si02 (21) or thauns, silicon in most the monomer had like the analogous



1



lution and is not .lt is not salted out



[(HO)3SiO-] [OH-] [Si(OH)4]



t 25°C. as long as usually rapidly. at ilecular weight and



1.5



X



104



. or [(HO)3SiO-] [H -]



ca" should include as "oligorners." It ecognized as those t: but even though .rate dextrose from LOd. sucrose is cer-



[Si(OH)4] Even though the silica solution is neutral. if it is passed through a bed of strongbase cation-exchange resin in the free-base form. the soluble silica in contact with the resin is ionized and is then held as silicate ions. In a mixture of Si(OH)4 in equilibrium with colloidal silica particles at pH 7-8, the particles bear a negative charge. According to Goto, Okura, and Kayarnu (24) electrophoresis and trans-



.,



.l.



....



• port studies show that the colloid. not the "molecularly soluble" Si(OH)., is the charge carrier. When the mixture is passed through a mixture of strong-bas' anion- and cation-exchange resin, monosilicic acid is removed. but not the colloidal particles. [After a time the particles dissolve surticiently to reestablish the equilibrium concentration of Si(OH)•.] 5. It is converted to H 2SiF. by reaction with HF in aqueous solution:



;I' . til: ,



'



l:



~



-



--The Occurrence. Dissolution. and Deposition of Silica



12



,



~ ...



:



. :.



!;.,. ~"



n ~ . t!r..



"1( :'



Si(OH).



+ 6 HF



== 2 H+



+ SiFi- + 4 H20



6. It is converted to a complex anion by reaction with o-dihydroxy aromatic compounds, such as catechol, in neutral solution: Si(OH).



+ 3 o-C 2H.(OH)2 + 2 NH.+ + 20H-



(o-C 2H.02)3Sj2- + 2 NH. + + 8 H20



Volatility in Steam Although Si(OH). is nonvolatile at ordinary temperature and polymerizes quickly when heated, nevertheless at elevated temperature and pressure in'water its solubility is greatly increased and it can exist in equilibrium as the vapor phase in the steam, as shown by Kennedy (25). This is of importance in very high pressure boilers in power plants where deposits build up on turbine blades unless all silica is remover from the feed water. Brady (26) supposes the volatile species is Si(OH). 0,. (HO)3SiOSi(OHh. Astrand (27) found that volatility increased with decreasing alkalinity in experiments conducted up to 350°C and 300 atm. This. of course. suggests that Si(OH). is more volatile than the silicate ion. Wendlandt and Glemser (28) reviewed evidence from earlier workers and calculated the equilibrium constants involved whence the species in the vapor were related to the density of the water vapor:



I



.



. ,



:



:



I



I



.. I. I



Quartz



Gas Density Range (g em -3)



Si0 2 + 2 H20 == Si(OH). 2 Si0 2 + 3 H20 == (HO)3SiOSi(OH)3 Si0 2 + H20 == OSi(OH)2



Up to 0.05 Up to 0.45 Above 0.65



Similarly. Martynova, Fursenko, and Popov (29) found that in a solution saturated with soluble silica at 263-364°C about a third of the silica in the vapor was present as disilicic acid, whereas in the range lSI-223°C it was all monomeric. Heitmann (30) concluded that deposition in turbines was minimal if the silica concentration was less than 0.01 ppm. iron concentration less than 0.005 ppm. and the conductivity less than 0.1 micromho cm -I. According to Heitmann's measurements (3 I). the silica concentration in the vapor phase ranges from O. I rng kg -I r 400°C to 5 mg kg:" at 600°C at a pressure of 0.3 kg em -2, but increases to mor.. _ than 100 mg kg -I under an applied pressure of 300 kg ern -2.



isition of Silica



,i(OH)., is the



.r ~··"'ng-base



i



I



b, lot the to reestablish



romatic com-



erizes quickly r its solubility in the steam, ure boilers in :a ie removed H). or th decreasing ,f course, sug. Glemser (28) urn constants of the water



~-



.



' -., r,



: (g cm -3) " :'~.



~ -.



solution satu.he vapor was meric. I if the silica ~05 ppm, and nn's measure.1 rn~. kg-I at ea. 0 more



_ _ _ _ _ _t;;...'



"'';--'..



Soluble Silica-Monosilicic Acid



13



Soluble Silica in Nature



Silica is constantly dissolving and precipitating over a large part of the earth's surface. The sedimentary cycles have been described in complex detail by Siever (32). Soluble silica is mainly derived from the weathering of minerals which, in some cases, results in amorphous silica residues that then dissolve. Very little can come from the "sands of seashore," or quartz, which is soluble to only a few parts per million; furthermore, the rate of dissolution is extremely slow. River waters range from S to 3S ppm Si0 2, a few up to 7S ppm, and by the time they reach the sea may range from 5 to 15 ppm. Seawater varies widely but the silica content may range from 2 to 14 ppm (33). However, Lisitsyn and Bogdanov (34) report that surface waters in the Pacific Ocean contain only 0.0001-0.3 ppm Si02 • Plankton convert 6 X 10' tons Si0 2 from soluble to suspended form each year, but this is only 0.16% of the available silica. In addition to the silica carried into the sea by fresh water, additional soluble silica comes from the suspended colloidal clays and related minerals. Tests show that common colloidal silicates like clay will dissolve in seawater sufficiently to give a silica concentration of 10 ppm (35). Concentrations of silica of around 2 ppm were reached in dilute salt solution with mica and kaolin and up to 15 ppm with montmorillonite (36). When seawater was enriched with soluble silica to 25 ppm Si02 , it remained at that level for a year in the absence of these minerals, but when the latter we~e then added, the silica was removed from solution down to the 2-15 ppm level that was reached when the minerals alone were added. Since many ocean waters contain 2-10 ppm Si0 2 , it is possible that this value is reached as the equilibrium solubility of colloidal aluminosilicate in suspension. The above experiment is consistent with the fact that in pure water, pure amorphous silica dissolves to give a concentration of monosilicic acid of 100-110 ppm, but in' the presence of polyvalent metal cations such as iron, aluminum, and other metals, colloidal silicates are formed with a much lower solubility with respect to monosilicic acid. lIer (37) has shown that soluble aluminum reduces the solubility of amorphous silica from about 110 to less than 10 ppm. Willey (38, 39) has studied the natural interaction of soluble alumina and soluble silica in 0.6 N sodium chloride solution. Addition of aluminum ion to 200 ppm Si(OH). retards polymerization. Probably there is formed a colloidal complex which reacts as monomer when put into the strongly acidic molybdate reagent. A very low concentration of soluble silica also causes the precipitation of alumina. Soluble silica as determined by the molybdate: method is not necessarily present as Si(OH)•. Bogdanova (40) reported that in natural waters that contained only about 5 ppm total silica, 4-9% of the silica was polymeric but was converted to monomer by acid. It is most likely that the "polymeric" silica was actually very small colloidal particles of aluminum silicate that liberated monomer when acidified.. Silica is continuously removed from seawater by biochemical processes. Diatoms and sponges as well as plants remove silica which is stored within the organism. Although Calvert (41) believes that the concentration of silica in the sea is mainly controlled by biological activity, Harder (42) reports that amorphous hydroxides of AI, Fe, Mn, or Mg can react with and precipitate soluble silica, thus reducing the concentration to as low as 3 ppm. Both processes are no doubt operative.



_



-'.;..... -



-The Occurrence. Dissolution. and Deposition of Silica



14



Amorphous silica is probably more soluble in seawater at great depths owing to the higher pressure. Willey (20) and Jones and Pytkowicz (43) found that at about O°C, the solubility increased with pressure as follows: Willey (O°C)



-'



Ib in. -2



ppm Si02



15 4,000 8,000 12,000 18,000



64 74 80 85 94



"



Jones (2°C) Ib in.- 2



ppm Si02



15



56



7,500



62



15,000



70



The salt at this concentration no doubt promoted more rapid establishment of equilibrium but would have little effect on solubility. Hot springs in some areas produce a supersaturated solution of silica. Knowing the solubility of quartz, Fournier and Rowe (44) have shown that the total silica content of the water permits an estimate of the subterranean temperature at which the water has become saturated with respect to quartz, which is the major phase that usually determines the solubility. Typical results are as follows:



..



Dissolved Silica (ppm) 660 . 425 .' . 245



.'



I.



I



,



. :>



I



,-



l



j .



,



~



~



.



Estimated Temperature (OC)



Maximum Measured Temperature in Drill Hole (0C)



246-252 215-220 178-180



250 220 170



~-



The method is, of course, not dependable if the water encounters previously deposited amorphous silica at any point. Also, the loss of water as steam must either be prevented or taken into account. . Major studies of silica in geothermal waters have been made in 1956 by White. Brannock: and Murata (45) and in 1970 by Fournier (46). all of the united States Geological Survey. Detailed analyses of waters from Yellowstone National Park, Wyoming, have been published by Rowe, Fournier, and Morey (47). Soluble silica is found in essentially all plants and animals. For example. human blood contains I ppm. Ingested monosilicic acid as an undersaturated solution rapidly penetrates all tissues and body fluids and is excreted apparently without any effect (48). Plants, especially grasses. including the grains and rice. take up silica and deposit it in the tissues as characteristic microscopic amorphous opaline particles. which are later found in the soil and in the intestinal tracts of grazing animals (49). The widespread occurrence and possible role of silica in living systems are more fully discussed in Chapter 7. In regard to the weathering of soils. it is noted that aluminosilicates (clays) . undergo weathering in the tropical regions with dissolution of silica, leaving a .-' residue high in alumina (bauxite) whereas in colder regions alumina seems to be removed preferentially, leaving more highly siliceous residues (50).



....



15



Phases of Silica



sition of Silica



A possible explanation is that in the tropics the decomposing vegetation produces tannins and other catechol-like materials that are known to dissolve silica in neutral solution. In colder regions, .less organic matter is likely to be present and the pH may be lower because more dissolved carbon dioxide is present, so that alumina is preferentially dissolved.



ths owing to hat '1bout



PHASES OF SILICA Since different phases of silica exhibit different solubility behavior, they are briefly described. By far the commonest crystalline form is quartz, the main constituent of common sand. However, under certain conditions in nature and in the laboratory, other forms are produced. These forms in turn may be divided into the following classes: nent of equi-



1. Anhydrous crystalline Si02 • 2. Hydrated crystalline Si0 2·xH 20. 3. Anhydrous amorphous silica of microporous anisotropic form such as fibers or sheets. 4. Anhydrous and hydrous amorphous silica of colloidally subdivided or microporous isotropic form such as sols, gels, and fine powders.



:a. Knowing e total silica ure at which or phase that



S. Massive dense amorphous silica glass. Temperature



Of these, 2, 3, and 4 exhibit extensive external or internal surfaces and are thus pertinent to the present study.



(0C)



Anhydrous Crystalline Silicas Sosman (51) classified the more common phases as follows:



s previously 1 must either _



Thermodynamically Stable at Atmospheric Pressure



;6 by White, mited States tional Park,



Quartz low Quartz high Tridymite S-1 Tridyrnite S-II Tridyrnite S-lIl Tridymite S-IV Tridymitc S-V Tridyrnite S- VI Tridyrnite 1\1:1 Tridymitc M-I r Tridymitc M-1I1 Cristobalite low Cristobalitc high



nple, human ted solution without any up silica and ne particles. mirnals (49). -e more fully



cates (clays) a, l-uving a se, to be



_ _ _ _ _ _ _---..J:.



"



_



Thermodynamic Stability Range (OC) To 573 573-867 - tridyrnite



To'64 64-117 117-163 163-210 210-475 475-I·HO - cristobalite To 117 117-163 Above 163



To



~72



272-1723



The Occurrence. Dissolution. and Deposition of Silica



16



The different forms of quartz. tridyrnite, and cristobalite are transformed spontaneously with temperature so that from the standpoint of solubility there are only the three phases to be considered. The next group of three phases are those formed only under conditions of high temperature and pressure. ' Thermodynamically Stable Range Therrn odynam ically Stable at High Pressure :! .



't· . rI ' (



Keatite Coesite



Temperature (OC) 400-500 From 300 To 1700 1200-1400



Stishovite



Pressure (kilobars) 0.8-1.3 15 40 160



,



r L



E i I



L r I'



Surveys of these phases and their properties have been published by Fronde! (52). Sosman (53). and Fldrke (54). Wells relates the structure of the different forms of silica to various crystalline silicates (55). Quartz, the commonest phase found in nature. ranges from huge crystals. to amorphous-looking powders a few microns in size. to shapeless masses of chalcedony agate or flint consisting of densely packed. interlocked microscopic crystals. The transformations between the three common forms and vitreous silica is as follows: quartz



tridymite



147QOC ~



~'.



cristobalite



vitreous



t',



.!~ .j'



.•' j



1



I



J



1 i



f I



,,



,



i



~ . .



f j .;



i ' ~



•e "



;;



.";



L. i .



.,



.t



~



The transformation to tridymite apparently requires traces of certain impurities or mineralizers. The three phases metastable at ordinary pressure were recognized only recently. Keatite was discovered by Paul Keat (56) in 1954. and its formation via cristobalite and transformation to quartz were studied by Carr and Fyfe (57). Hoover (58), in a patent filed in 1954. described the preparation of a very similar if not identical material from "silicic acid". that is. hydrated amorphous silica powder. by heating it in water at about 3000 atm pressure and 500-625°C in the presence of about 1% alkali based on silica. Coesite was discovered by Coes, in 1953 (59). It is made from amorphous silica in the same temperature range as for keatite, but at 10 times the pressure and with weakly acidic catalysts such as boric acid or ammonium chloride (59). It was found in nature in 1960 at Meteor Crater, Arizona. apparently formed under the high temperature and pressure conditions of the im pact. Similarly. stishovite was first made in the laboratory in 1961 by Stishov and Popova (60) and discovered in Meteor Crater by Chao. Shoemaker. and Madsen. in 1962 (61). A most interesting story of the isolation of substantial amounts of coesite and stishovite from the crater is told by Bohn and Stober (62.63). There arc also some unusual anhydrous crystalline forms. as follows (64). \.



-



~-_.



:ion of Silica



°med spononly re ft - - :



ons



of high



mge kilobars) 1.3 5



o o



'ondel (52), nt forms of Is. to arnoredony agate /stals. The 'ollows:



17



Phases of Silica ~



Silica W is a fibrous crystalline silica with a density of 1.97 g em -, described by Weiss and Weiss (65), formed in the gas phase by oxidizing silicon monoxide vapor at 1200-1400°C and deposited as paperlike films. It is unstable above about 1400°C. It is fairly stable in dry air, but is converted by moisture to amorphous hydrated silica, still retaining a swollen fibrous form. In this transformation. only about 0.08 mole of H 20 is taken up per mole of Si0 2, forming SiOH groups. Silica W can have no true equilibrium solubility in water. Instead, it must decompose rapidly in water to give monosilicic acid. When the powder is suspended in water and within 2 min centrifuged to obtain a clear solution. then titrated with NaOH solution at pH 10.2-10.5 (thymolphthalein). 2 equivalents of base are required per mole of Si0 2 in solution (66). After the solution has been aged for 1 hr, only 0.1 equivalent is required .. Initially. therefore, the solution must have been supersaturated with Si(OH)4, which when titrated with base, requires 2 equivalents of alkali. but after the monomer has polymerized. much less alkali is required to neutralize the surface acidity of the colloid. If the solution is mixed with silver salt an orange. light-sensitive AgSiO, is precipitated. In absolute methanol the fibers swell and form a polymeric methyl ester containing one methoxy group per silicon atom which, when heated in vacuum at 300-500°C, yields cyclic methyl esters [(CH,OhSiO]II.'.u, _When hydration of the fiber by water is followed by suitable technique under the microscope it can be seen that the reaction starts at the end of the fiber and proceeds rapidly along its length as the crystal swells and is converted to hydrated amorphous silica:



vitreous -



npurities or Iy recently. cristobalite er (58). in a ot identical 'y heating it f about 1% ous silica in ... re and with t was found er the high Stishov and Madsen. in lS of cocsitc . 4).



Anhydrous fibrous silicas formed in connection with high temperature metallurgical operations were noted as long ago as 1852 by Schnabel and 1859 by Rose. Soft. silky fibers of more than 98% Si0 2 were classed as aphanitic (invisible) silica. .' and also known as lussatite. Around 1910. in the mouths of electric furnaces making silicon carbide. a sort spongy gray deposit called "elephant's ear" was identified as microfibrous amorphous silica (67). It is likely that all of these were silica W. . Melanophlogite, a long known but strange and little understood mineral. is found in volcanic sulfur deposits in Sicily. Skinner and Appleman reviewed its history and showed that it was a new cubic ·polymorph of silica (68). It has a cubic. very open structure. containing 92.4% SiO; and about 5.7t:(. SO, (2.2SC;c as sulfur). 1.2% carbon. and 0.81 % hydrogen. The density is 2.052 ± 0.013 g COl -'. The initial refractive index is 1.467. but when these volatile materials are driven off by heating. the crystalline silica residue has a refractive index of 1.425 ± 0.002 and a density of 1.99 g COl -'. which arc substantially lower than those of amorphous or glassy silica. The silica crystal is stable up to about 900°C. above which it changes into cristo. balite. However. ,vhen crystals arc subjected to grinding in a mortar at ordinary temperalure.· the open structure collapses to fine-grained quartz. Its solubility h~IS not been measured. and it is doubtful if il can ha v c a true equilibrium solubility in



,



-~



18



I, I'



.



I. I,·



. '; .



I



, J.



r



,



-



- The Occurrence. Dissolution. and Deposition of Silica



water. The heat-purified material will probably react with water rapidly to give a highly supersaturated solution of Si(OH)4' similar to the behavior of silica W. The nature of the hydrocarbon and sulfur content is still not clear. However. calculations based on density data would seem to support earlier suggestions that the sulfur must be present as S03 or H 2S0 4 within the silica lattice. The optical characteristics of the mineral show that the organic matter occurs in films between the faces of the crystals. On the other hand, calculations based on the difference in densities of the original mineral and the pyrolyzed silica crystals show that the sulfur compounds at least must be within the crystal lattice. Kamb offers evidence (69) that the silica structure is a clathrate with S02. H 20. and CH 4 in the lattice analogous to the known 12 A gas hydrates of water, 6X .46H 20, where X is CH 4, H 2S, CO 2, S02. C12• etc., and in fact the structure is the complete analogue of 6C1 2 • H 20. Silica 0 crystallizes from lithium silicate glasses during devitrification at low temperature. It has a crystal lattice similar to quartz and may simply be "high quartz" stabilized below 573°C. the normal transition temperature, to low quartz (53. 70) by inclusions of metal ion impurities. The only way pure material can be obtained is by neutron bombardment of quartz. Silica X is a microcrystalline form obtained as spherical aggregates of radial fibers up to 12 microns in diameter by heating pure amorphous hydrated silica ("silicic acid") with 2% KOH solution in sealed tubes at 150°C for a few weeks. The refractive index of 1.484 ± 0.004 is close to that of cristobalite. It is anhydrous. maintaining its structure up to about 600°C. above which it is converted to cristobalite (71a. 71b). Silica lite. a very unusual new form of anhydrous crystalline silica hornogeneousl.. permeated by uniform pores 6 A in diameter and having a density of only 1.76 g ern -3, has been described by FJanigen et al. (71 c) and patented by Grose and Flanigen (71d). The pores constitute 33% of the volume of the crystal. A most remarkable feature is that this silica is hydrophobic; the pores are lined with oxygen atoms that are highly hydrophobic and organophilic or oleophilic. Thus the crystals preferentially absorb hexane in the presence of water. which does not enter the pores even at saturation pressure. This type of silica is made first as a crystalline quaternary ammonium silicate. for example, tetrapropylammonium silicate: (TPA).O .48Si0 2· H 20. It is then heated to red heat to remove the organic matter and water. leaving uniform cylindrical channels throughout the three-dimensional crystalline framework of silica. A similar but even more hydrophobic. anhydrous. microporous crystalline silica was obtained by Flanigen and Patton (7le) by conducting the hydrothermal synthesis in the presence of some ammonium fluoride. which facilitated the formation of crystals 2-15 microns in size at only 100°C rather than at the higher temperature and pressure required for making silicalitc. After crystallization from solution a typical composition was about 88% by weight of silica. 11.0% tetrapropylarnmonium oxide. and 0.9% fluorine, but after calcination at 600°C the porous crystalline product was essentially pure Si02 (containing less than 0.1 % fluorine) with a mean' refractive index of 1.39 ± 0.01 and a specific gravity of" 1.70 ± 0.05. These value" arc the same as those of silicalite and fall on the same curve with other forms 0 silica in Figure 1.1. "'~



-..,. on of Silica



to give a W veve. callS



that the



.al characetween the



Terence in : the sulfur e (69) that alogous to CO2, S0 2. on at low , be "high .ow quartz -ial can be ; of radial ated silica veeks. The anhydrous, I to cristoog, usly mly 1.76 g



Grose and J. A most ·ith oxygen he crystals r the pores



Relation Between Density and Refractive Index



The anhydrous crystalline phases were arranged in order of increasing density and refractive index, and found to have a linear relationship, by Skinner and Appleman (68). Stishovite falls on the s~me line (72) (Figure 1.1). It will be noted that the line for silica polymorphs has been extrapolated to meet that of two forms of water. It seems odd that neither of these lines extrapolates to a refractive index of 1.0 (for a vacuum) at zero density. The analogous structures of Si0 2 and H 20 have been compared by Kamb (69), who points out that the ratios of the densities of the various phases or polymorphs of Si0 2 and water to those of the corresponding forms of Ice I and cristobalite are very similar, and for each type of silica there is an ice counterpart with the same type of crystal structure. Hydrated Crystalline Silicas



Until the advent of X-ray diffraction, it was not clear whether solids containing only silica and water were definite compounds. that is, had a definite stoichiometry or structure. In 1905, Tschermak (73) believed that he had obtained definite hydrates based on ratios of silica to water corresponding to Si0 2 : 2 H:O, 2 Si0 2 : 2 H20, 3 Si02 : 2 H20 , etc.• by carefully leaching the metals out of certain crystalline silicates and drying in air. Then Van Bemmelen (74) and Theile (75) gave evidence that no definite silicic acids were thus produced. Nevertheless, since then· numerous instances have been found where definite crystalline materials. having characteristic X-ray diffraction patterns and crystal structures. have been made by extracting the cations from certain crystalline silicates . with a c i d . ' 2.0



x o



silicate, for



w



heated to rical chan-



w



1



19



Phases of Silica



~



>



i=



illine silica nal syntherrnation of emperature solution .1 irnmoniurn crystalline ith a mean 11:S" values :r . s of



COESITE



~ 1.5



QUARTZ



0::



l.l.



KEATITE



W 0::



' - - - - - - CRIST06:'UTE



Z



o



::t



00:



o



~



oCt



~



500



90°C---73°C------



>-



I-



---~~



-J



iii



-~



::> ...J o



(/l



a 5



6



7



B pH



9



10



II



Figure I. 7. Solubility of amorphous silica versus pH at di Iferent tern peratu res [from Goto (167a)].



ion of Silica



Effect of Particle Size on Solubility in Watee., -



reasonably of ~;Iicate



undoubtedly because the colloidal particles were smaller than 50 A. since they were prepared at room temperature from sodium silicate by ion exchange. However, at 155 and 200°C in water, the particle size of the silica undoubtedly increased and the solubility was the same as reported by others (see Figure 1.4).



1



ie I.



.5



X



ause of the



49



Calculation 01 Solubility and Dissociation Constant at equilib- _ From these



Where the ionization constant and solubility are not known, as for example at some unusual pH. Van Lier (114) has shown how the ion concentration and solubility can be calculated easily from data on pH and total soluble silica. which includes both monomer and ionic silica as determined by the molybdic acid reagent. In general terms their method is as follows. Let S, = concentration of total soluble silica at pHI S2 = concentration of total soluble silica at pH 2 So = concentration of Si(OH)•• which is the solubility of silica in neutral water Then concentration of ionic silica at pH 1 = S1 - So concentration of ionic silica at pH 2 = S2 - So Since the decrease in H· ion concentration is accompanied by a corresponding increase in concentration of H 3SiO. -. then



20r - .... was lub•.. j was 1.7. This is By rearrangement.



The concentrations of silicate ion can be obtained by difference and the ionization constant calculated. using the appropriate value for the ionization constant of water: °C



0



25 50 90 100



K 14.944 13.996 [3.26 12.42 12.26



EFFECf OF PARTICLE SIZE ON SOLUBILITY IN \VATER amorphous em 'lures



The behavior of polysilicic acids. colloidal silica.•ind silica gels cannot be understood without taking into account the fact that the solubility or silica is higher when



The-occurrence. Dissolution. and Deposition of Silica



the silica surface is convex. and lower when it is concave. It is a matter of'the radius of curvature of the surface; the smaller the radius the greater the effect on solubility (l67b). As shown in Figure 1.8 smaller particles with a smaller positive radius of curvature have a higher equilibrium solubility. On the other hand. in a crevice. such as where two particles are in contact, the radius of curvature is negative and the equilibrium solubility is low. There are two important practical consequences:



I



-j



•



1



".



I



!



2. If there has been an aggregation or flocculation of colloidal silica particles. so that two or more particles are brought together, then at the point of contact the



.:



.



\,



Si02



INCREASING POSITIVE CURVATURE



SOLUBILITY, PPM



200



L



100



o



_



S=77



5



10



OF CURVATURE-NANOMETERS



Figure 1.8. Variation in solubility of silica with radius of curvature of surface. The positive radii of curvature are shown in cross-section as particles and projections from a silica surface. Negative radii are shown as depressions or holes in the silica surface. and in the crevice between two particles.



I



I



1. When very small individual silica particles are brought into the same solution as larger ones. especially at pH 9-10 where hydroxyl ions catalyze dissolution and deposition of silica. the smaller ones dissolve and the larger ones grow.



INCREASING NEGATIVE CURVATURE



~"J,



~.



'".po"";



''''i'



SI



Effect of Particle Size on Solubility in WatCI- -



on of Silica



radius of curvature is negative and extremely small, the solubility of silica in this region is very low, and silica dissolves from the particle surfaces and is deposited around the point of contact to minimize the negative radius of curvature, thus forming a coalescence or neck between particles.



the radius \ s('I-'l,ility ; of curva.e, such as j the equi-



,



Even at low pH the same phenomenon occurs if the particles of silica are less than about 5 nm in diameter, so that the solubility changes rapidly with radius of curvature. The coalescence between adjacent silica particles in an aggregated gel structure by this spontaneous process or by adding soluble silica to be deposited at the points of contact between particles is described by Alexander, Broge, and lIer (168) and is further discussed in Chapter 5. Kitahara (169) shows that the effect of pH, salt, and temperature have the same influence on the rates of polymerization of monosilicic acid as they have on the gelling of sol, showing that both phenomena, that is, the growth of particles and the cementing together of particles once they are in contact, are influenced by the same factors; that is, those jhat affect the rate of dissolution and deposition of monomeric silica. . Greenberg (13) concluded that although theoretically solubility is a function of particle size, there was no published data to bear this out. Alexander (152) was the first to obtain data that showed that for a given type of silica, solubility increased with decreasing size. Silica containing different impurities or having different degrees of hydration within the particles cannot be used for comparison. The. Ostwald-Freundlich equation, applied to solubility (known as the Thompson-G,ibbs effect), is as follows:



.olution as lution and irticles, so .ontact the



. 5, = exp(2 EVR -IT-1r 1) . 5,



;".



r F.. r'



where S, = solubility of particle or radius r 5, = solubility of a nat surface or particle of infinite radius (nm) E = interfacial surface energy (ergs em -2) V = molar volume = 27~2 ern" for amorphous silica R= gas constant (8.3 X 107 ergs mole ? deg ") T ~ temperature (degrees absolute) ." r = radius of curvature (cm) . . d= particle diameter (nm) E .: I04.6(J0 7 )(r )log(5 , / 5 ,) at 25°C



-.



.



The positive lica surface. l~ reVlce



......



Thus



. s:



Jog l o c,



o.



[~:]



= 2.85 x 10- 7



£



Tr



5.7



£ Td



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Theoccurrence. Dissolution. and Deposition of Silica



There have been past indications that very small silica particles are ibnormally soluble, but no measurements were made. In studying the solubility of finely ground quartz, Stober and Arnold (122) concluded that the silica which dissolves very rapidly at first when immersed in water is much more than an adsorbed layer of Si(OH)., but instead is a minute fraction of the powder which is finer than 0.1 micron and is thus, owing to the Thompson-Gibbs effect, much more soluble. . The radius of curvature of the silica-water interface is of critical importance even with a porous silica solid. Charles (170) found that the rate of dissolution of porous high silica glass could be explained on the basis of a high local solubility of the silica surface owing to its small radius of curvature. Over millions of years the conversion of the amorphous siliceous remains of large sponges to solid rounded flint boulders in chalk beds is an extreme example of the conversion of a high surface area form of silica, through dissolution and deposition, to a dense low area form. Examination of flint boulders with inclusions of oyster shells and belemnites indicates that a once highly extended skeleton of a sponge had been drawn together into a round black boulder (171). Between the flint and the CaC03 there is a film 10 nm thick of intermediate hydrated calcium silicate, along which the soluble silica must have been transported. Another mysterious phenomenon is "ambient pyrite," Grains a f~w microns in diameter have moved through solid chert (an extremely tine-grained quartz). leaving a trail of coarse-grained quartz. This was described by Tyler, Knoll. and Barghoorn (l72)(Figure 1.9). One explanation is that there is a slow transport of silica from the more soluble tine grains ahead of the crystal of pyrite to the growing quartz crystals behind the pyrite. The pyrite is hydrophobic in nature and not chemically bonded to the surrounding silica. Thus it is possible that the pyrite grain is pushed ahead by, the growing quartz crystals. The resulting pressure ahead of the grain on the finer crystals of quartz must also increase their already higher solubility. However, organic matter is known to be present, and it is postulated that gas evolution and pressure buildup are also involved.. The particle size effect is probably also involved in a phenomenon described by Baumann (173). Amorphous silica powder condensed from a name consists of fine spherical particles generally less than ISO A diameter. When such a pov..' der is placed in water, a supersaturated solution of silica is obtained, no doubt owing to a very small percentage of more soluble particles under 50 A diameter. The dissolved monomeric silica then rapidly polymerizes to polysilicicacids. but these later disappear as supersaturation is relieved by deposition of silica on the larger amorphous particles in suspension. Agitation of granular silica in water may cause abrasion. liberating very fine particles which then give erroneously high solubility data. Morey. Fournier. and Rowe (16) measured the solubility of amorphous silica in the form of commercial pure silica gel, crushed vitreous silica (fused quartz), colloidal silica formed by cooling a supersaturated solution (720 ppm) dissolved from quartz, and colloidal silica from supersaturated hot spring waters. The silica gel and colloid from supersaturated solutions showed a reproducible solubility of 115 ppm at 25°C. However. silica



\ .



.,-.--



--~ ..



-



:ion of Silica



abnormally -,



I (I.., , conl in water is fraction of ng to the



rtance even n of porous of the silica tins of large mple of the deposition, 1S of oyster sponge had .int and the icate, along microns in rtz), leaving I Barghoorn .luble behind the to the surly the growr crystals of lie matter is buildup are



10,



lescribed by isists of fine Ier is placed 19 to a very ie dissolved later disapamorphous 19 very Ii ne



rurnicr. and commercia! ned by coollloidal silica



.



.



-- .r:ing



s t.



Jt



, ).



is a rich he silica ved duriediately away, it is amorice plant t not by occurs ninerals ion: residues, ded, sec-



wi,



.d



rhus, the of strucfrom the



ire in a



; carbon ion of a ould be .h faster of these lignin is he final iversion riations ie final when it esulting



Removal and Deposition of Silica from Watsr,



91



The silicification of various types of biogenic materials was reviewed' by Siever and Scott (278). Possible mechanisms are discussed, but they believed that exact duplication of the silicification process in the laboratory is impossible, because of the time required. The youngest silicified wood is of the early Pleistocene age. It is not only a matter of how rapidly silica is deposited, but also how rapidly the organic material inside a partially silicified specimen decomposes in the absence of microorganisms within the structure. Chemical degradation must occur before the resulting space can be filled by further silica deposition. This fact, furthermore, precludes deposition of colloidal silica except initially in the open pores, since' colloidal particles cannot pass through the cell walls. For this reason, silicification for the most part involves diffusion of a supersaturated solution of soluble Si(OH)4 through the structure. If the process occurs at ordinary temperature, the silica concentration is not likely to be more than 200 ppm and the rate of deposition not more than 1 mm in 1600 years. Since rate of diffusion, even through micropores, is relatively rapid, it means that silica deposition can continue to occur even in cavities to which there is access only through a few pores. Thus dense, almost impervious masses of silicified structures can be formed. It is difficult to v isualize how this could be accelerated, say, a thousandfold, in the laboratory. The most successful approach to duplicating fossilized wood has been made by Leo and Barghoorn (279), who also presented a survey of the subject since the attempts of Basil Valentine, the alchemist, in 1520. The wood specimen is boiled in water until free of gas and then alternately immersed in separate sealed jars containing water and ethyl orthosilicate, stored at 70°C. Exposure time in each fluid may vary from a few days to a month and the alternate immersions are continued for us to 1 year or more. Several times during the ethyl silicate immersion vacuum is applied to remove water vapor from the wood to facilitate further penetration by the ester. The ethyl silicate is replaced with fresh liquid whenever it becomes viscous or cloudy. During the final cycle in the ester the specimen is soaked in 0.004% HN0 3 to hydrolyze the residual ester in situ. The silica-filled specimen still contains all the original organic matter and closely resembles natural petrefaction of geologically young age. After organic matter has been removed, as shown by a fragment not turning dark when it is place in concentrated H 2S0 4, the silica Iithomorph is found to faithfully reproduce the original organic structure. Unlike previous methods, this technique gives a strong coherent lithomorph. Since the organic matter can be removed by oxidizing agents it is obvious that the lithomorph is highly porous. Presumably, this would be an ideal substrate for further deposition of silica, perhaps colored with a little iron, to more closely approach the appearance of a natural, highly petrified specimen. Duplication of chert containing microorganisms is feasible in the laboratory because diffusion distances are so small. Oehler and Schopf (280) made specimens by embedding filamentous algae in silica gel and then autoclaving at 2-4 kilobars for 2-4 weeks at 150°C. Under these conditons, the gel undergoes syneresis until completely solid and is converted to the microcrystalline state, as further described by Oehler (l64b).



. The Oeeurrence, Dissolution. and Deposition of Silica



92



Rare of Deposition of Colloidal Silica There are some situations, as in equipment handling hot geothermal brines, where the cooled liquid deposits silica orders of magnitude faster than is possible by the deposition of molecular silica. Such deposits invariably are hydrated. and although often very hard, have a microporous structure. A high buildup rate is to be expected, since particles corresponding to molecular weights of thousands (several nanometers in diameter) are being deposited instead of single SiO, units. Also porosity, corresponding to the spaces between the colloidal particles, is to be expected. . . There is one major difference in mechanism: deposition of colloidal particles requires the presence of a potential coagulating agent, usually small concentrations of polyvalent metal ions, although monovalent ions such as sodium have a similar effect above about 0.3 N concentration. In the absence of flocculating ions, in neutral or alkaline solution, a colloidal particle of pure silica bears a negative charge and so does the silica surface of the substrate. There is mutual repulsion so that the collision rate is low. However, in this pH range metal ions are adsorbed on the silica surfaces in some degree and upon collision, adhesion occurs. For deposition on a surface to occur, the metal ion should not be present at such concentration as to coagulate the colloidal particles in suspension (Figure 1.17).



~.



.,. ~,



PARTICLES OF Si02



REARRANGEMENT TO SMOOTHER SURFt.CE



MilI :. • I



I



I



ORIGINAL SURFACE



Figure 1.17. Flocculating action of calcium ion followed by spontaneous cementing effect through dissolution and redeposition of soluble silica.



.



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.



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Removal and Deposition of Silica from WateL_



n of Silica



es, .re ile by the although expected, .nometers sity, corparticles



mtrations a similar



s, In neuIe charge J' that the the silica tion on a 1S to coa- .



f I



ing effect



I



II



I



93



Deposition is likely to be unusually rapid when the colloidal particles are less than 5 nm in diameter. Such small particles are in equilibrium with a concentration of monomer that is supersaturated with respect to a flat surface. Hence, as shown in Figure 1.17, the surface of deposited silica rapidly fills in so that the radius of curvature is much greater than that of the colloidal particles. In effect, the surface tends to be filled in and flattened out. This is an important factor in rapid desposition because the small colloidal particles are then much more likely to adhere to the surface than to each other in suspension. Also, the presence of small colloidal particles, especially in a hot sol that' is being cooled, furnishes a high degree of supersaturation that causes rapid polymerization of monomer at the moment of contact between particle and surface (Figure 1.17), .thus increasing the rate of deposition beyond that initiated by the divalent cations. As pointed out by lIer (281) in the slightly alkaline pH range, a concentration of 100 mM calcium ion is required to flocculate 4 nm particles (700 m 2j g- l ) whereas for very large particles only abou t 10 mM is required. A flat surface corresponds to the radius of curvature of a very large particle. Hence when 4 nm particles are in the neighborhood of a relatively smooth surface, there is an intermediate calcium concentration that will promote adhesion of particles to the surface, yet not cause flocculation in solution. It is probable that deposition of more than a monolayer of colloidal particles larger than about 10 nm by the described local coagulation mechanism is unlikely to occur. Much more monomeric silica would have to be deposited to smooth out the surface. It is conceivable that by careful addition of monomeric silica to the sol to maintain an optimum degree of supersaturation along with careful control of the concentration of coagulent, continuous deposition could be maintained without coagulating the sol. The importance of flocculating ions is borne out by Midkiff (282, 283) who found that cooling water containing more than 300 ppm silica did not deposit a scale if cal. cium ions were first converted to soluble chelates: otherwise, a deposit of colloidal silica. associated with calcium carbonate, was formed. At the concentration involved, over half of the silica in solution must have been colloidal and the particles quite small. The rate of deposition was over 100 times faster than would be possible if monomeric silica alone were involved. Wohlberg and Bucholz (246) reported that if the silica concentration exceeded about 240 ppm (at which point colloidal silica would be expected to be nucleated). scale formation occurred when calcium was present. The deposits from hot deep-well brine near the Salton Sea in Cali fornia build up very rapidly as the brine cools while going through pipes. The silica content is 400 ppm in a solution containing up to 15% NaCl as well as,a few percent of CaCl 2 and KCI. The brine is slightly acidic. so there is no interaction of silica with calcium ion. but iron. which is present at only 0.2%. is adsorbed on silica at this pH and is a major component of the scale. More striking is that up to 20% copper and 6% silver are found in the scale as sulfides. The deposit is amorphous to X-rays and consists of a hydrated silica. classed as opal, but is actually a microporous silica gel. under the coagulating influence of the metal ions. Since the brine contains 1-2 ppm H 2S, the



- _.-. 94



The Occurrence. Dissolution. and Deposition of Silica



adsorbed metal ions eventually migrate and nucleate as fine sulfide crystals embedded in the silica. The difference between depositing material molecularly and as particles has been noted in other systems. Thus Howard and Parfitt noted that in depositing silica on titania pigment either a layer, could be deposited as polysilicate ions by an isothermal, pH-dependent process or colloidal silica particles could be deposited by a coagulation mechanism (284a). Silica is rapidly deposited on receptive surfaces from a hot lithium silicate solution of 2 Si02 : I Li~O ratio. Sams (284b) describes the character of the coatings obtained on glass from solutions of different ratios of silicate, from a solution containing 12.5% SrO, heated at 95°C. The opaque white color of coating deposited during 1~ hr indicates that it is porous and that the silica was deposited as colloidal particles. Even after only 60 sec the appearance of a hazy blue coating (after being washed and dried) indicated that colloidal rather than monomeric silica was being deposited. It was essential that the substrate be immersed in the solution as it is heated. Merely drying the solution on a surface gave no adherent film. This behavior of 2: I lithium silicate is associated with its unusual property of becoming insoluble and forming a precipitate, apparently amorphous, when the solution is heated above about 48°C, yet redissolving slowly after the solution is cooled to ordinary temperature. However. at 100°C, the precipitate apparently disproportionates to insoluble silica, probably colloidal in size. Further discussion of forming films from colloidal silica is reserved for Chapter 4.



METHODS OF ANALYSIS Various methods of analysis are involved in every aspect of silica chemistry, For convenience, sources of information are assembled here, along with a few methods particularly useful for research purposes. 1 ,.



.



Atomic Absorption



,. '::



if,



Analysis for silicon in the larger laboratories has been revolutionized by the atomic adsorption method. Although equipment represents a large investment, the method can be used for a wide variety of elements and, once samples have been prepared in solution, permits dozens of samples to be run in a few hours. Usually the instrument manufacturers can recommend suitable methods for preparing solutions for analysis. Bowman and Wills (285) have recommended specific procedures for silicon. Dissolution of solid samples and preparation of suitable solutions have been described by Terashima (286). Dissolution of mineral samples in H3PO~ is a convenient method, according to Horton and Baines (287), especially since it eliminates the background correction when the silicon is dissolved after alkali fusion. Spectral interference of vanadium can be a problem (288). The method is ideal for determining total silicon.



of Silica



Methods of Analysis



embed-



Chemicall\lethods



]a~



:- .. -



95



A wide range of chemical methods and procedures is found in the treatise of Kolthoff and Elving (289). They also give a summary of the chemistry and solubility of silica. Preparation of solutions for analysis, the silicomolybdic acid yellow and blue colorimetric methods including interferences, gravimetric procedures, and special procedures for biological materials' are discussed in concise detail. Procedures especially suited to silicate rocks, minerals, and refractory silicates and aluminosilicates have been described by Bennett and Reed (290). A history of analytical methods was published by Andersson (291), and three new spectrophotometric procedures were developed. Available chemical methods listed by Meites (292), in addition to the conventional gravimetric and colorimetric methods, also include precipitation of the silicate ion as the cobalt salt, which is then determined by chelometric titration, and as a nitrogen base salt which is titrated with perchloric acid. It has been my experience that for research purposes, in addition to the atomic absorption method for total silica, the alkali titration of silica as SiF.2- is most useful for concentrations greater than 0.1 %, and the yellow and blue silicomolybdate methods for concentrations down to I and 0.1 ppm, respectively.



.1



silica on , by an .sited by solution rbtained ntainin,g uring I ~ .articles,



.hed and sited. It Merely lithium rming a It 48°C,



.erature. Ie silica, silica is



Methods Involving Silicomolybdic Acid The reaction of molybdic acid with monomeric Si(OH). to give the yellow silicomolybdic acid is indispensable in investigating the behavior of soluble and colloidal silica. The literature is too voluminous to cover here. Morozyuk (293) made a chronological list of the literature to 1971. This section deals with the chemistry of the reaction and gives some recommended analytical procedures. Application of the reaction to determining the nature of polysilicic acids and colloidal particles is reserved for Chapter 3. It is obvious that only Si(OH)., but not polymers thereof, can react directly with acidified ammonium heptarnolybdate to form the yellow silicomolybdic acid, since the latter molecule contains only one silicon atom:



try. For nethods



; atomic method Jared in trument .nalysis. silicon. le been ; a conrninates Spectral :termin-



or



Since the molecular weight of ammonium molybdate is 1235.9. I g SrO, consumes 35.3 g of the ammonium molybdate. Polysilicic acid de polymerizes slowly enough that it is possible to determine the monomer in the presence of polymers by noting the rate of color development



! I,



(294-297).



., .. E~' , It.· ;j .



.,



_._~



The-Occurrence. Dissolution, and Deposition of Silica



Although this reaction was discovered in 1898 by Jolles and Neurath (298) and used for many years. it was only in 1952 that Strickland (299) showed that variations in the extinction coefficient under some conditions were due to the existence of two forms of molybdic acid. alpha and beta. that react with silica to give yellow silicomolybdic acids having, different extinction coefficients and absorption peaks. The beta form is obtained at lower pH and is used in the "yellow" method. but unless conditions are optim urn, it changes with time to the alpha form, which is less colored but more stable. The beta form is the only one that is used for determining monosilicic acid in the presence of polymers because it is formed at once. and its color is more intense. To eliminate the problem of beta changing slowly to alpha, Garrett and Walker. (300) proposed measuring the color at 335 nm, where the alpha and beta forms have the same absorption coefficient. They also studied the kinetics of formation of silicomolybdic acids in very dilute molybdic acid solutions (0.0025 M) and concluded the rate was proportional both to silica concentration and molybdate ion concentration; the alpha .forrn had a constant optical density between pH 2 and 4.5. Alcohol catalyzes the color formation, especially when present at about 30 vol, %. even at pH 5. In contrast to the above. Andersson (30 I) reported that at a wavelength of 325 nrn, at pH 1.5. alpha silicomolybdate had a very low extinction coefficient as compared with the molybdic acid blanks, and for this reason he used 400 om. However, his method involves a heating step to convert the beta to alpha form. and so is timeconsuming and inconvenient for studying silica polymerization. It is useful for accurate determination of total silica with a standard deviation of less than 0.5%. The Beta Silicomolybdate M ethod This method was used long before Strickland's (299) discovery of the existence of the alpha form. Thompson and Houlton (302) had used conditions that resulted in the beta form and later Alexander (303) used it in his study of monosilicic acid. . . Govett (304) determined the critical factors in obtaining the beta form:



.



! j



I!



I. Adjust the stock ammonium molybdate solution to a pH of 7.5 with NaOH to



ensure that only MOO.2- ions are present. 2. Use an amount of molybdate in the reaction solution to give a 0.06 M concentration of MOO.2-. 3. After the reaction with silica there should still remain a 0.05 M concentration of M00 4 2- ; that is. a fivefold excess should be used. 4. About 2.7-5.0 equivalents of H 2S0 4 should be added per mole of MoO/-. Guignard and Hazebrouck (305) further examined this method and found that over 3-4 hr the absorbance increased and then decreased as alpha was formed. In 20-50% alcohol. the beta form was more stable.



.i



-97



Methods of Analysis



of Silica



A Recommended Procedure



298) and at v-riaste. Jf



For most purposes I have found, it is possible to make up a dilute molybdic acid reagent solution that is stable for a week. Thus to determine silica it is necessary only to add the sample solutio~. Standards should be run each day.



e yellow n peaks. hod, but ch is less errnining . and its



REAGENTS.



(A)



Add 41.0 011 95.5% H 2SO. to 800 ml water and dilute to 1 liter (1.5 N) .



(B)



Walker 'ms have at ion of



(C)



mcluded



ncentraAlcohol even at



Dissolve 100 grams (NH.)6M0102•. 4H 20 (mol. wt, 1235.9) in 900 ml H 20, add 47 ml concentrated NH.OH solution (28% NH 3 ) , and dilute to 1 liter (0.566 M MOO.2- and 1.18 N NH.';' ion). To 500 011 H 20 add 200 ml solution A and 100 011 solution B (800011 total). This is 0.0707 JI;[ with respect to MoO/-, 0.148 N in NH.';' ion, and 0.375 N in SO/- and has a pH of about 1.2.



To 40 ml reagent mixture C add up to 10 ml of sample solution containing not more than 2000 p.g Si0 2, and adjust the volume to 50 011. Measure absorption at 410 nm wavelength. PROCEDURE.



h of 325



as comIowever, is time.eful for .5



The number of H.;. ions added per MOO.2- ion is 5.3. which is slightly more than recommended by Govett. If 2000 p.g of Si0 2 is present. it will consume 0.2 millimols MoO/-. The mixture contains 40 x 0.0707, or 2.83 millimoles MOO.2-, so there is a sevenfold excess. This composition is compared with that of Govett, Alexander, and Kautsky et al. (306) as follows:



tence of suited in cic acid.



Composition of Color-Forming Mixture



Authors aOH to



Her Alexander Govett Kautsky et al.



ncentra-



Maximum mg Si0 2/ 50 01 I



Molar MoO/0.0566 0.0227 0.060 0.0566



5.3 4.4 3.3 6.5



2.0 1.0 2.0 1.5



ation of Her found that with a H": M 00. 2- ratio of 5.3. the p H was 1.2 and monomer reacted fully in 2 min. and the color was stable for 2 hr. At 3.7 ratio. the pH was 1.5 and the monomer reacted fully in I min. but the color began to fade in less than 100 min. At a ratio of 10.6. at a pH of 0.8. the monomer did not completely react in 100 min. The reaction rate appears to be proportional to the hydroxyl ion concentration. even at this low pH.



md that



med. In



I



98



.::...'--



The-Occurrence, Dissolution. and Deposition of Silica



Interfering Substances Iwasaki and Tarutani (307) found that in concentrated salt solutions, the silicomolybdic colorimetric method gave low values. (Presumably the method can be standardized using salt solution.) Fluoride ion interferes if it is present before the molybdic acid is added (308). It can be masked by Al H • Tannins, especially in natural waters, interfere with the yelIow silicomolybdate method because of their yellow color. Thus 6.8 ppm of pyrocatechol tannins or 2.7 ppm of pyrogallol tannins give a color equal to 1,0 ppm of silica. Reduction of the yellow to molybdenum blue by sulfite at low acidity (pH 2.5) and measurement of light absorption at 620 nm avoid the problem, and at the same time elirnihate interference by phosphate ion (309). Traces of ferric iron may contribute to the yellow color and for this reason Kenyon and Berwick (310) prefer to have tartaric acid present to form a less colored complex with iron. In analyzing caustic liquors they added varying amounts of sodium or potassium chloride, even though color is suppressed, because this permitted them to maintain a constant salt level when the alkali samples were neutralized with hydrochloric acid. Interference by phosphate ion is an especially common problem, Since the phosphate ion reacts like silica to form a yellow phosphomolybdic acid, its' interference must be eliminated. Numerous techniques have been proposed. either for separating the silica and phosphorus before analysis or preferentially reducing silicomolybdic acid to molybdenum blue in the presence of the phosphomolybdic acid (311-313). Snell and Snell (314) summarized the possible procedures: (a) precipitating and removing phosphate as the calcium salt. (b) adjusting pH so only silica will form the yellow color, (c) destroying the yellow phosphate complex with citric. oxalic, or tartaric acids, and (d) preferentially reducing the silicomolybdic acid to molybdenum blue.



Molybdenum Blue Method



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When the silica concentration is only a few parts per million, the absorbance of the yellow complex is insufficient for accurate measurement. If the yellow complex is reduced to molybdenum blue, and a reagent is used that will not reduce the residual excess of molybdic acid, the much greater absorbance increases sensitivity perhaps tenfold. The best reducing agent, according to a study by Mullen and Riley (315). is a mixture of rrietol, sulfite. and oxalic acid which gives complete reduction in 90 min at 20°C and stable color for 48 hr. Their method measured silica at a concentration of 0.4 J.Lg of SrO, in 20 ml of sample (0.02 ppm) with a standard deviation of I % and of about 0.3% at concentrations up to 6 J.Lg (0.3 ppm). Possible interferences by all common metals and anions were checked. eerie ion, fluoride, germanium, and vanadium gave serious interference; interference by phosphate was eliminated by the oxalic acid. Their method was further developed by Yolk and Weintraub (3 J 6), expc-



of Silica



99



Methods of Analysis



cially for analyzing plant tissues. which were first ashed and fused with l'::lzC0 3 • The following reagent solutions are prepared in plastic bottles. rn,



.re



d can be



REAGEl'TS.



(A)



(308). It



(B)



olybdate 1S or 2.7 >n of the -rnent of .lirnihate



(C)



reason colored ounts of ; permitrtralized i



i



he phos.rference parating nolvbdic I:



1}.



:ing and form the :. or tar'bdenum



(D) (E)



PROCEDURE. Dilute a 1-20 ml sample containing 10-50 J.Lg Si0 2 to 20 mJ. Add 3 ml solution A and let stand 10 ± 0.5 min at 25°C. Add 15 ml reducing solution E and dilute to 50 mJ. Wait 3 hr and measure absorbance at 810 nm wavelength.



A similar method is described by Jarabin, Vajda. and Szarvas, except that the color is measured at 660 nm (317). Another modification of the molybdenum blue method is to develop the color in perchloric acid medium, by reduction with stannous ion and ascorbic acid (318), which eliminates interferences and gives a stable color. Kahler, Betz, and Betz (309) and Milton (319) favor reduction with sulfite ion for water analysis. For Biological Samples



For analyzing biological materials the molybdenum blue method is invariably used because of the very low concentrations of silica. For this purpose a method of analyzing for traces of silica in the presence of iron. phosphorus. arsenic, and reducing substances was developed by Baumann (320). REAGEl'TS.



:e of the



nplex is residual perhaps s a mi: