Residue on ignition 
281
—
Heat 50 g in an open, shallow 100-mL porcelain dish until it ignites, and allow it to burn without further application of heat in a place free from drafts. Cool, moisten the residue with 0.5 mL of sulfuric acid, and ignite to constant weight: the weight of the residue does not exceed 5 mg (0.01%).
Limit of chlorinated compounds—
Accurately weigh 5 g of Glycerin into a dry, round-bottom, 100-mL flask, add 15 mL of morpholine, and connect the flask by a ground joint to a reflux condenser. Reflux gently for 3 hours. Rinse the condenser with 10 mL of water, receiving the washing in the flask, and cautiously acidify with nitric acid. Transfer the solution to a suitable comparison tube, add 0.50 mL of
silver nitrate TS, dilute with water to 50.0 mL, and mix: the turbidity is not greater than that of a blank to which 0.20 mL of 0.020 N hydrochloric acid has been added, the refluxing being omitted (0.003% of Cl).
Fatty acids and esters—
Mix 50 g of Glycerin with 50 mL of freshly boiled water and 5 mL of 0.5 N sodium hydroxide VS, boil the mixture for 5 minutes, cool, add
phenolphthalein TS, and titrate the excess alkali with 0.5 N hydrochloric acid VS. Perform a blank determination (see
Residual Titrations under
Titrimetry 541): not more than 1 mL of 0.5 N sodium hydroxide is consumed.
Limit of diethylene glycol and related compounds—
Resolution solution—
Dissolve accurately weighed quantities of diethylene glycol and
USP Glycerin RS in water, and dilute quantitatively, and stepwise if necessary, with water to obtain a solution having a known concentration of about 0.5 mg of each per mL.
Standard solution—
Dissolve an accurately weighed quantity of diethylene glycol in water, and dilute quantitatively, and stepwise if necessary, with water to obtain a solution having a known concentration of about 0.05 mg per mL.
Test solution—
Transfer 5 g of Glycerin, accurately weighed, to a 100-mL volumetric flask, dissolve in and dilute with water to volume, and mix.
Chromatographic system
(see
Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector, a 0.53-mm × 30-m fused-silica analytical column coated with 3.0-µm G43 stationary phase, and an inlet liner having an inverted cup or spiral structure. The chromatograph is programmed as follows. Initially, the column temperature is equilibrated at 100
until the time of injection, is increased at a rate of 7.5
per minute to 220
, and is maintained at 220
for 4 minutes. The injection port temperature is maintained at 220
, and the detector temperature is maintained at 250
. The carrier gas is helium. The split flow ratio is about 10:1, and the linear flow is maintained at about 38 cm per second. Chromatograph the
Resolution solution, and record the peak responses as directed for
Procedure: the resolution,
R, between diethylene glycol and glycerin is not less than 7.0. Chromatograph the
Standard solution, and record the peak responses as directed for
Procedure: the relative standard deviation for replicate injections is not more than 15%.
Procedure—
Separately inject equal volumes (about 0.5 µL) of the
Standard solution and the
Test solution into the chromatograph, record the chromatograms, and measure the responses for all the peaks. Calculate the percentage of diethylene glycol in the portion of Glycerin taken by the formula:
100(CS / CU)(rU / rS),
in which
CS is the concentration, in mg per mL, of diethylene glycol in the
Standard solution; CU is the concentration, in mg per mL, of Glycerin in the
Test solution; and
rU and
rS are the peak responses for diethylene glycol obtained from the
Test solution and the
Standard solution, respectively: not more than 0.1% is found. Calculate the percentage of each other impurity, excluding any solvent peaks, in the portion of Glycerin taken by the formula:
100(ri / rs),
in which
ri is the peak response of each individual impurity obtained from the
Test solution; and
rs is the sum of the responses of all the peaks obtained from the
Test solution: not more than 0.1% of any individual impurity, excluding diethylene glycol, is found; and not more than 1.0% of total impurities, including diethylene glycol, is found.
Assay—
Sodium periodate solution—
Dissolve 60 g of sodium metaperiodate in sufficient water containing 120 mL of 0.1 N sulfuric acid to make 1000 mL. Do not heat to dissolve the periodate. If the solution is not clear, pass through a sintered-glass filter. Store the solution in a glass-stoppered, light-resistant container. Test the suitability of this solution as follows. Pipet 10 mL into a 250-mL volumetric flask, dilute with water to volume, and mix. To about 550 mg of Glycerin dissolved in 50 mL of water add 50 mL of the diluted periodate solution with a pipet. For a blank, pipet 50 mL of the solution into a flask containing 50 mL of water. Allow the solutions to stand for 30 minutes, then to each add 5 mL of hydrochloric acid and 10 mL of
potassium iodide TS, and rotate to mix. Allow to stand for 5 minutes, add 100 mL of water, and titrate with 0.1 N sodium thiosulfate, shaking continuously and adding 3 mL of
starch TS as the endpoint is approached. The ratio of the volume of 0.1 N sodium thiosulfate required for the glycerin–periodate mixture to that required for the blank should be between 0.750 and 0.765.
Procedure—
Transfer about 400 mg of Glycerin, accurately weighed, to a 600-mL beaker, dilute with 50 mL of water, add bromothymol blue TS, and acidify with 0.2 N sulfuric acid to a definite green or greenish yellow color. Neutralize with 0.05 N sodium hydroxide to a definite blue endpoint, free from green color. Prepare a blank containing 50 mL of water, and neutralize in the same manner. Pipet 50 mL of the
Sodium periodate solution into each beaker, mix by swirling gently, cover with a watch glass, and allow to stand for 30 minutes at room temperature (not exceeding 35
) in the dark or in subdued light. Add 10 mL of a mixture of equal volumes of ethylene glycol and water, and allow to stand for 20 minutes. Dilute each solution with water to about 300 mL, and titrate with 0.1 N sodium hydroxide VS to a pH of 8.1 ± 0.1 for the specimen under assay and 6.5 ± 0.1 for the blank, using a pH meter. Each mL of 0.1 N sodium hydroxide, after correction for the blank, is equivalent to 9.210 mg of C
3H
8O
3.
;)
