Aluminum Phosphate Gel

Phosphoric acid, aluminum salt (1:1).
Aluminum phosphate (1:1) [7784-30-7].
» Aluminum Phosphate Gel is a water suspension containing not less than 4.0 percent and not more than 5.0 percent (w/w) of aluminum phosphate (AlPO4). It may contain sodium benzoate, benzoic acid, or other suitable agent, in an amount not exceeding 0.5 percent, as a preservative.
Packaging and storage— Preserve in tight containers.
Identification—
A: A solution of it in hydrochloric acid meets the requirements of the tests for Aluminum 191.
B: A solution of it in 2 N nitric acid meets the requirements of the tests for Phosphate 191.
pH 791: between 6.0 and 7.2.
Soluble phosphate— Filter 20 g, and wash the residue with 30 mL of water. Add to the filtrate 2 mL of nitric acid, heat to 60, and add 20 mL of ammonium molybdate TS. Heat at 50 for 30 minutes, filter, wash the precipitate with dilute nitric acid (1 in 36), then wash with potassium nitrate solution (1 in 100) until the last portion of the filtrate is not acid to litmus paper. Dissolve the precipitate in 50.0 mL of 0.5 N sodium hydroxide VS, add phenolphthalein TS, and titrate the excess alkali with 0.5 N hydrochloric acid VS. Each mL of 0.5 N sodium hydroxide is equivalent to 2.065 mg of PO4. The soluble phosphate, calculated as PO4, does not exceed 0.30%.
Sulfate 221 Add 10 mL of 3 N hydrochloric acid to 10 g of Gel, and heat to boiling. Cool, dilute with water to 250 mL, and filter, if necessary. A 10-mL portion of the solution shows no more sulfate than corresponds to 0.20 mL of 0.020 N sulfuric acid: not more than 0.05% is found.
Arsenic, Method I 211 Prepare the Test Preparation by dissolving 5.0 g of Gel in the smallest necessary volume of 3 N hydrochloric acid. The limit is 0.6 ppm.
Heavy metals, Method I 231 Proceed as directed in the chapter, except to make the following modifications.
Standard Preparation— Into a 50-mL color-comparison tube pipet 4.0 mL of Standard Lead Solution, dilute with water to 25 mL, adjust with 6 N ammonium hydroxide to a pH between 1.9 and 2.1, dilute with water to 40 mL, and mix.
Test Preparation— Dissolve 8.0 g in 5 mL of 3 N hydrochloric acid, warming if necessary, dilute with water to 25 mL, and adjust with 6 N ammonium hydroxide to a pH between 1.9 and 2.1. Transfer to a 50-mL color-comparison tube, dilute with water to 40 mL, and mix.
Monitor Preparation— Into a 50-mL color-comparison tube place 25 mL of the Test Preparation, add 4.0 mL of Standard Lead Solution, adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 1.9 and 2.1, dilute with water to 40 mL, and mix.
Procedure— Proceed as directed in the chapter, except to omit the addition of 2 mL of pH 3.5 Acetate Buffer. Not more than 5 µg per g is found.
Chloride— Transfer 25 g to a beaker with the aid of about 50 mL of water, add 5 mL of nitric acid, mix, then add, with stirring, 30.0 mL of 0.1 N silver nitrate VS. Warm on a steam bath for 30 minutes, filter, and wash the precipitate with water acidified with nitric acid. To the filtrate add ferric ammonium sulfate TS, and titrate the excess silver nitrate with 0.1 N ammonium thiocyanate VS. Each mL of 0.1 N silver nitrate is equivalent to 3.545 mg of Cl. Not more than 0.16% of chloride is found.
Assay— To about 20 g of Gel, accurately weighed, in a 100-mL volumetric flask, add nitric acid to effect solution, dilute with water to volume, and mix. Transfer 10.0 mL of this solution to a 400-mL beaker, dilute with water to 100 mL, heat to 60, add an excess of ammonium molybdate TS, and maintain at 50 for 30 minutes. Filter, and wash the precipitate with dilute nitric acid (1 in 36), then with potassium nitrate solution (1 in 100) until the last portion of the filtrate is not acid to litmus paper. Dissolve the precipitate in 50.0 mL of 0.5 N sodium hydroxide VS, add phenolphthalein TS, and titrate the excess sodium hydroxide with 0.5 N sulfuric acid VS. Each mL of 0.5 N sodium hydroxide is equivalent to 2.651 mg of AlPO4.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Elena Gonikberg, Ph.D.
Senior Scientist
1-301-816-8251
(MDGRE05) Monograph Development-Gastrointestinal Renal and Endocrine
USP32–NF27 Page 1485
Pharmacopeial Forum: Volume No. 27(6) Page 3242