AOCS Ea 8-58-2009

AOCS Official Method Ea 8-58Reapproved 2009Moisture, Karl Fischer Volumetric Method

DEFINITION This method determines the actual water content of glycerol by reaction of the water with a pyridinesulfur dioxide solution and titration with an iodine-methanol solution. SCOPE Applicable to distilled glycerin. May be used with any glycerol-water solution which contain no interfering substances.

SAMPLING AND ANALYSIS OF CRUDE GLYCERIN

APPARATUS 1. See Notes, Recommendations, and Numbered Notes, 2 2. Bottle-burette assemblies—each consisting of a storage bottle and 50 mL burette with side filling tube, properly vented and protected against moisture absorption with drying tubes filled with Drierite, indicating type (see Numbered Notes, 3). The delivery tip of each burette is equipped with a stopper so that the solution is protected from the moisture in the air during the measurement of Solution B (Reagents, 7) and titration with Solution A (Reagents, 6). The rubber stopper on the burette tip is fitted with a capillary tube which allows air to escape from the flask when titrating, thus assuring a continuous and even flow from the burette without loosening the flask from the stopper to relieve pressure which would otherwise develop. 3. Erlenmeyer flasks—narrow neck, 3,000 mL, with rubber stoppers. Insert a drying tube filled with Drierite in the stopper fitting the 3,000 mL flask. 4. Flask—150 mL, Soxhlet extraction type with rubber stoppers to fit opening. 5. Drying tubes—straight form. Insert glass fiber in the bulb of the tube, fill with Drierite almost to the top and cover with glass fiber. 6. Neoprene tubing—6.35 mm i.d., length about 1 m. 7. Glass tubing—76 mm o.d., wall thickness 1 mm., length about 30 cm. 8. Bottle—glass-stoppered, 2,000 mL. 9. Weighing bottle—glass-stoppered dropping bottle or Lunge weighing bottle.

REAGENTS 1. Drierite—indicating type, 8 mesh. 2. Pyridine—anhydrous, reagent grade. 3. Sulfur dioxide—anhydrous, liquefied gas in cylinder. 4. Methanol—anhydrous, reagent grade. 5. Iodine—reagent grade. 6. Solution A, Iodine-Methanol Solution—dissolve 60 g of iodine, reagent grade, in 1,000 mL of methanol, protecting so that solution does not absorb moisture from any outside source. 7. Solution B, Pyridine-Methanol-Sulfur Dioxide Solution—Karl Fischer reagent (see Numbered Notes, 1), stabilized single solution, available commercially or prepared as follows. Add 900 mL of anhydrous pyridine and 900 mL of anhydrous methanol to a 3 L Erlenmeyer flask. Stopper and cool to 4°C. Tare on a scale or balance (sensitive to 0.5 g), add 180 g sulfur dioxide at a fairly rapid rate by discharging from a cylinder through a tube into and beneath the surface of the solution. If the solution becomes hot, interrupt the addition of sulfur dioxide, close flask with a stopper fitted with a drying tube, and cool the solution. When the addition of sulfur dioxide is complete, close flask with a rubber stopper fitted with a drying tube. Caution—Do not cool at this or any subsequent stop while tightly stoppered. Note—In case the SO2-pyridine solution is to be used immediately after preparation, cool if necessary, under tap water. Do not cool below room temperature.

CALIBRATION OF SOLUTION A 1. Run 25.0 mL of Solution B into a clean dry flask. 2. Attach flask to stopper on delivery tube of burette filled with Solution A. Shake with a rotary motion and titrate with Solution A to first appearance of a red color that persists after 10 sec shaking. Add Solution A in increments of 0.2 mL when approaching the end point. Remove flask and stopper. 3. Add 1 drop of water from a weighed weighing bottle to the solution. Adjust the meniscus in the buret to the “0” mark and titrate with Solution A as directed in Calibration, 2.SAMPLING AND ANALYSIS OF CRUDE GLYCERINEa 8-58 • Moisture, Karl Fischer Volumetric Method

4. Weigh the weighing bottle and determine weight of the water added. Repeat Calibration, 3 and 4 using titrated solution from Calibration, 3.

5. Water factor for Solution A (g water titrated per mL) =

mass,gofwaterfromCalibration,4

volume,,mLsolutionfromCalibration,3Use the average of two single determinations as the factor if they agree within 0.0001. If the factors do not agree within 0.0001, repeat calibration.

PROCEDURECaution—Glycerin absorbs water very rapidly so that the sample must be protected from the atmosphere at all times. 1. Measure from burette 25 mL of Solution B (Reagents, 7) into a clean, dry flask. 2. Attach flask to stopper on delivery tube of burette filled with Solution A. Shake with a rotary motion and titrate with Solution A to first appearance of a red color that persists after 10 sec shaking. Titrate 0.2 mL at a time when nearing the end point (yellow to red). 3. Remove flask and stopper. 4. Weigh into the flask a test portion of such size that from 10–40 mL of Solution A will be required. This flask must not be left unstoppered except when introducing the test portion. Table 1 is a convenient guide of the size of test portion to weigh and the accuracy with which the test portions are weighed. 5. Immediately stopper the flask after introducing the test portion and keep closed until ready for the next step. 6. Fill the burette with Solution A and adjust meniscus to “0”, attach flask to stopper on delivery tube and titrate. Add reagent slowly, but steadily, rotating flask just enough to keep the color dispersed. 7. Titrate 0.2 mL at a time when nearing the end point (yellow to red). 8. Titrate to the first appearance of a red color, which persists for 5 min when the flask is stoppered immediately after reading the end point.