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AOCS Ea 8-58-2009

AOCS Ea 8-58-2009
AOCS Ea 8-58-2009

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.

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Page 2 of 3SAMPLING AND ANALYSIS OF CRUDE GLYCERIN Ea 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,g of water from Calibration,4

volume,,mL solution from Calibration,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.

PROCEDURE Caution —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.

CALCULATIONS Moisture,%volume of titrant,mL of test =portion F mass,g of test portion

×100×Where— F = g of water titrated by 1 mL of Solution A (water factor as determined in Calibration of Solution A, 4).

PRECISION Collaborative studies have shown that the following 95% confidence limits may be expected (see Table 2):

NOTES Recommendations Responses to the industry survey carried out by the Associate Methods Editor of Section E in 1989, showed that the Karl Fischer method for determining moisture in glycerin is outdated and no longer used routinely. The survey indicated that the preferred method is the use of commercial automatic coulometric titrators, e.g., the Aqua Star C-1000 or equivalent. Methods based on coulometric instrumentation have not been studied collaboratively within the AOCS.

NUMBERED NOTES 1. Because of its toxicity, pyridine is an undesirable constituent of the Karl Fischer reagent. Both one- and two-component Karl Fischer reagents, based on aliphatic amines and heterocyclic compounds and containing no pyridine, are available commercially. 2. The procedure employing a visual end point is not intended to preclude the use of any satisfactory potentiometric apparatus, of which several are available. The use of a closed system with mechanical stirring is to be encouraged, although it is not essential for accurate analysis. The method prescribed will give satisfactory results on any distilled glycerin not too dark in color to obscure the visual end point. 3. In humid climates or during humid seasons, the drying tubes used to protect the reagent against moisture in the air must be watched closely. The Drierite must be changed at the first evidence of color change.

SAMPLING AND ANALYSIS OF CRUDE GLYCERIN

Ea 8-58 ? Moisture, Karl Fischer Volumetric Method

Table 1

Expected % moisture mass of test portion, g

0–1.5 10.000 ± 0.01

1.5–3 5.000 ± 0.001

3–8 2.000 ± 0.0001

Table 2

Moisture present at level of

5.0% 0.5%

Duplicate determination on the

same day by a single analyst should not

differ more than approximately 0.1 0.02

Single determinations made in two

different laboratories should not

differ more than approximately 0.25 0.07

Averages of duplicate determinations

made in two different laboratories

should not differ more than approximately 0.23 0.07

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