Dibasic Potassium Phosphate
K2HPO4 174.18

Phosphoric acid, dipotassium salt.
Dipotassium hydrogen phosphate [7758-11-4].
» Dibasic Potassium Phosphate contains not less than 98.0 percent and not more than 100.5 percent of K2HPO4, calculated on the dried basis.
Packaging and storage— Preserve in well-closed containers.
Identification— A solution (1 in 20) responds to the tests for Potassium 191 and for Phosphate 191.
pH 791: between 8.5 and 9.6, in a solution (1 in 20).
Loss on drying 731 Dry it at 105 to constant weight: it loses not more than 1.0% of its weight.
Insoluble substances— Dissolve 10 g in 100 mL of hot water, filter through a tared filtering crucible, wash the insoluble residue with hot water, and dry at 105 for 2 hours: the weight of the residue so obtained does not exceed 20 mg (0.2%).
Carbonate— To 1 g add 3 mL of water and 2 mL of 3 N hydrochloric acid: not more than a few bubbles are evolved.
Chloride 221 A 1.0-g portion shows no more chloride than corresponds to 0.40 mL of 0.020 N hydrochloric acid (0.03%).
Sulfate 221 A 0.20-g portion shows no more sulfate than corresponds to 0.20 mL of 0.020 N sulfuric acid (0.1%).
Iron— Dissolve 0.33 g in 10 mL of water, add 6 mL of hydroxylamine hydrochloride solution (1 in 10) and 4 mL of orthophenanthroline solution prepared by dissolving 1 g of orthophenanthroline in 1000 mL of water containing 1 mL of 3 N hydrochloric acid, and dilute with water to 25 mL: any red color produced within 1 hour is not darker than that of a control prepared from 1 mL of Standard Iron Solution (see Iron 241): the limit is 0.003%.
Sodium 191 A solution (1 in 10) tested on a platinum wire imparts no pronounced yellow color to a nonluminous flame.
Heavy metals, Method I 231: Dissolve a portion equivalent to 4.2 g of K2HPO4 in enough water to make 50 mL of stock solution. Transfer 12 mL of this stock solution to a 50-mL color-comparison tube (Test Preparation). Transfer 11 mL of the stock solution to a second color-comparison tube containing 1.0 mL of Standard Lead Solution (Monitor Preparation). Transfer 1.0 mL of Standard Lead Solution and 11 mL of water to a third color-comparison tube (Standard Preparation). Proceed as directed for Procedure, omitting the dilution to 50 mL: the limit is 0.001%.
Limit of fluoride— Proceed as directed in the test for Fluoride under Dibasic Calcium Phosphate. The limit is 0.001%.
Limit of monobasic or tribasic salt— Dissolve 3 g in 30 mL of water, cool to 20, and add 3 drops of thymol blue TS: a blue color is produced, which is changed to yellow (with a greenish tinge) by the addition of not more than 0.4 mL of 1 N hydrochloric acid.
Assay— Transfer 40.0 mL of 1 N hydrochloric acid to a 250-mL beaker, add 50 mL of water, and titrate potentiometrically with 1 N sodium hydroxide VS to the endpoint. Record the volume of 1 N sodium hydroxide VS consumed as the blank. Transfer about 5.2 g of Dibasic Potassium Phosphate, accurately weighed, to a 250-mL beaker, add 50 mL of water and 40.0 mL of 1 N hydrochloric acid VS, stir until dissolved, and proceed as directed in the Assay under Dibasic Sodium Phosphate, beginning with “Titrate the excess acid.” Where A is equal to or less than B, each mL of the volume A of 1 N sodium hydroxide is equivalent to 174.2 mg of K2HPO4. Where A is greater than B, each mL of the volume 2B A of 1 N sodium hydroxide is equivalent to 174.2 mg of K2HPO4.
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