Isopropyl alcohol-free dihydroxyaluminum sodium carbonate— Use a portion of Dihydroxyaluminum Sodium Carbonate that has been previously tested as directed in this section and found to be free of isopropyl alcohol.

Sodium chloride solution— Prepare a solution (1 in 5) of sodium chloride in water.

Standard stock solution— Transfer about 1 g of isopropyl alcohol, accurately weighed, to a 50-mL volumetric flask containing about 10 mL of Sodium chloride solution, dilute with Sodium chloride solution to volume, and mix. This Standard stock solution contains about 20 mg of isopropyl alcohol per mL.

Standard solutions— Transfer 2.0, 4.0, 5.0, and 6.0 mL of the Standard stock solution to four separate 100-mL volumetric flasks, dilute with Sodium chloride solution to volume, and mix. These solutions contain about 0.4, 0.8, 1.0, and 1.2 mg of isopropyl alcohol per mL, respectively.

Headspace containers— Use suitable 20-mL containers capable of being tightly closed with an inert septum and a metallic crimp cap.

Standard preparations— To four separate 20-mL Headspace containers add 1.0 g of Isopropyl alcohol-free dihydroxyaluminum sodium carbonate. To the containers add, respectively, 10.0 mL of the appropriate Standard solution. These containers contain about 4, 8, 10, and 12 mg of isopropyl alcohol, respectively. [note—Keep the containers cool until sealed.] Seal the containers, place in a water bath maintained at 70, and allow to stand for 1 hour.

Test preparation— Transfer 1.0 g of the Dihydroxyaluminum Sodium Carbonate under test to a Headspace container, and add 10.0 mL of Sodium chloride solution. [note—Keep the container cool until sealed.] Seal the container, place in a water bath maintained at 70, and allow to stand for 1 hour.

Chromatographic system (see Chromatography 621)—The gas chromatograph is equipped with a flame-ionization detector and a 0.9-m × 3-mm column packed with support S3. The injection port temperature is maintained at 200, the detector temperature is maintained at 250, and the column temperature is maintained at 180. Inject 1 mL of the gaseous phase of the Standard preparation containing 10 mg per container as directed for Procedure: the relative standard deviation for replicate injections is not more than 4%.

Procedure— [note—Use peak areas where peak responses are indicated.] Using a gas-tight syringe, separately inject equal volumes (about 1 mL) of the gaseous headspace of the Standard preparations and the Test preparation into the gas chromatograph. Record the chromatograms, and measure the peak responses. Determine, based on a retention time comparison, if isopropyl alcohol is detected in the Test preparation. Plot the responses of the Standard preparations versus the content, in mg, of isopropyl alcohol in each container, draw the straight line best fitting the plotted points, and calculate the correlation coefficient for the line. A suitable system is one that yields a line having a correlation coefficient of not less than 0.99. From the graph, determine the total amount, TU, in mg, of isopropyl alcohol in the Test preparation. Calculate the percentage of isopropyl alcohol in the Dihydroxyaluminum Sodium Carbonate taken by the formula: in which WU is the weight, in g, of the Dihydroxyaluminum Sodium Carbonate taken. The limit is 1.0%.

Potassium chloride solution— Prepare a solution of potassium chloride in water containing 38 mg per mL.

Sodium chloride stock solution— Dissolve a suitable quantity of sodium chloride, previously dried at 105 for 2 hours and accurately weighed, in water, and dilute quantitatively and stepwise with water to obtain a solution containing 25.42 µg per mL (10.0 µg of sodium per mL).

Standard solutions— On the day of use, transfer 4.0 mL of 1 N hydrochloric acid and 10.0 mL of Potassium chloride solution to each of two 100-mL volumetric flasks. To the respective flasks add 5.0 and 10.0 mL of Sodium chloride stock solution. Dilute with water to volume, and mix. These solutions contain about 0.5 and 1.0 µg of sodium per mL, respectively.

Test solution— Transfer about 250 mg of Dihydroxyaluminum Sodium Carbonate, previously dried and accurately weighed, to a 200-mL volumetric flask, add 40 mL of 1 N hydrochloric acid, and boil for 1 minute. Cool, dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a 100-mL volumetric flask, dilute with water to volume, and mix. Transfer 5.0 mL of this solution to a 100-mL volumetric flask containing 4.0 mL of 1 N hydrochloric acid and 10.0 mL of Potassium chloride solution, dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Standard solutions and the Test solution at the sodium emission line at 589.0 nm with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering 851) equipped with a sodium hollow-cathode lamp and an air–acetylene flame, using as a blank a solution prepared by pipeting 4 mL of 1 N hydrochloric acid and 10.0 mL of Potassium chloride solution into a 100-mL volumetric flask, diluting with water to volume, and mixing. Plot the absorbances of the Standard solutions versus concentrations, in µg per mL of sodium, and draw a straight line between the plotted points. From the graph so obtained, determine the concentration, C, in µg per mL of sodium, in the Test solution. Calculate the percentage of sodium in the portion of Dihydroxyaluminum Sodium Carbonate taken by the formula: in which W is the quantity, in mg, of Dihydroxyaluminum Sodium Carbonate taken: between 15.2% and 16.8% is found.

Edetate disodium titrant— Dissolve 18.6 g of edetate disodium in water to make 500 mL, and standardize as directed in the Assay under Ammonium Alum.

Procedure— Transfer about 300 mg of undried Dihydroxyaluminum Sodium Carbonate, accurately weighed, to a 250-mL beaker, add 10 mL of 2 N sulfuric acid, cover the beaker, heat to 80 for 5 minutes, and boil for 1 minute. Add 30.0 mL of 0.1 M edetate disodium, again boil for 1 minute, cool, and then add 10 mL of acetic acid–ammonium acetate buffer TS, 50 mL of acetone, and 2 mL of dithizone TS. Using a pH meter, adjust with the addition of ammonium hydroxide or dilute sulfuric acid to a pH of 4.5, and titrate with 0.05 M zinc sulfate VS, maintaining the pH at 4.5 by the addition of ammonium hydroxide as necessary, to an orange-pink color. Perform a blank determination, and make any necessary correction. Each mL of 0.1 M Edetate disodium titrant is equivalent to 14.40 mg of CH2AlNaO5.