Glycerine Determination [PDF]

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Glycerine Determination (USP) ============================================== ================= Scope: This method is applicable to the assaying of USP grade glycerine. Summary: The sodium metaperiodate oxidizes all glycerine present in the sample to form 2 moles of formaldehyde and 1 mole of formic acid per mole of glycerine. The formic acid is neutralized to a pH of 8.1+0.1 using 2.0N sodium hydroxide. The glycerine content is calculated after determining the volume of titrant needed to react with all formic acid present. Apparatus: 1. Analytical balance, capable of determining weights to four decimal places. 2. pH meter capable of +0.05 pH unit readings. 3. Electrode, combination (Remark 1). 4. Burette, 50 mL class A with 0.2 mL divisions. 5. Beakers, 600 mL. 6. Volumetric flasks, 100 and 1000 mL. 7. Pipettes, 1, 10, and 50 mL, class A volumetric. 8. Reagent bottle with ground glass stopper, 1 liter amber. 9. Syringes, disposable 3cc plastic. 10. Watch glasses. 11. Timer. 12. Stir plate. 13. Stir bars. Reagents: 1. Sodium metaperiodate, reagent grade. 2. Sulfuric acid (0.1N), concentrated (80%). Sodium hydroxide (0.05N) 4. Ethylene glycol, reagent grade. 5. Glycerine, 99.5%+ purity. Procedure: Preparation of Reagent Solutions: Sodium Periodate Solution Dissolve 60 grams of sodium metaperiodate in sufficient water containing 120 mL of 0.1 N sulfuric acid to make 1000 mL of solution. 0.1N Sulfuric Acid Solution Weigh about 5.15 grams of 80% sulfuric acid into a 1000 mL volumetric flask. Dilute to volume with deionized water and mix well. Label accordingly. Ethylene Glycol Solution Pipet 50 mL of ethylene glycol into a 100 mL volumetric flask. Dilute to volume with deionized water. Bromothymol Blue Indicator Solution Weigh 0.1 grams of bromothymol blue into a 100 mL volumetric flask. Pipet 1 mL of ethanol into the flask and dilute to volume with deionized water. Mix well and label



accordingly. Sample Preparation and Analysis: 1. Weigh about 0.4 grams of sample into a 600 mL beaker. Record the weight and label accordingly. 2. Add 50 mL of deionized water and 1 mL bromothymol blue indicator solution to the beaker. Add a stir bar to the beaker and mix well. 3. Acidify with 0.1N H2SO4 to a green or greenish yellow color. 4. Neutralize with 0.1N NaOH to a definite blue endpoint (no green). 5. Prepare a blank by adding 50 mL deionized water and 1 mL of bromothymol blue to a 600 mL beaker. After adding a stir bar and mixing well, acidify and neutralize (Steps 3 & 4) the blank solution. Label the flask as "Blank". 6. Pipet 50 mL of the Sodium Periodate solution into each of the beakers. Swirl gently. Cover each beaker with a watch glass and allow to stand in the dark for 30 minutes. 7. Pipet 10 mL of the ethylene glycol solution to each of the beakers. Swirl gently. Allow to stand for 20 minutes. 8. Add 150 mL of deionized water to each beaker and mix well. Titrate each blank and sample solution with 2.0N sodium hydroxide. The "Blank" endpoint is reached when the pH reaches 6.5 + 0.1. The "Sample" endpoint is reached when the pH reaches 8.1 + 0.1. 9. Using Equation 1 in the Calculations section of this method, determine the % glycerine content to one decimal place accuracy. Calculations: Equation 1 (mL Sample - mL Blank)(N)(92.10)(100) % Glycerine = -------------------------------------------------------(Sample wt.)(1000) Precision: Initial studies indicate that the relative standard deviation of the glycerine determination is +0.6%. Safety: Sodium metaperiodate is an oxidizer. Use caution when handling and storing. Do not get in eyes, on skin, or on clothing. Ethylene glycol is an eye irritant. Do not get in eyes, on skin, or on clothing. Use in well ventilated area. Sodium hydroxide is corrosive. Do not get dilute solutions in eyes, on skin, or on clothing. Sulfuric acid is extremely corrosive. Do not get dilute solutions in eyes, on skin, or on clothing.



Remarks: 1. Follow the care and maintenance procedures outlined by the manufacturer of the electrode. References: 1. U.S. Pharmacopeia and National Formulary, 1995, USP 23 & NF 18, Glycerin Official Monograph, pages 713-714, 2056, and 2057.