Monobasic Potassium Phosphate
Phosphoric acid, monopotassium salt.
Monopotassium phosphate [7778-77-0].
» Monobasic Potassium Phosphate, dried at 105 for 4 hours, contains not less than 98.0 percent and not more than 100.5 percent of KH2PO4.
Packaging and storage Preserve in tight containers.
Loss on drying 731 Dry it at 105 for 4 hours: 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 residue does not exceed 20 mg (0.2%).
Arsenic, Method I 211: 3 ppm.
Lead A solution of 1 g in 20 mL of water contains not more than 5 µg of lead (corresponding to not more than 5 ppm of Pb) when tested as directed in the test for Lead 251.
Heavy metals, Method I 231 Dissolve 1 g in 25 mL of water: the limit is 0.002%.
Limit of fluoride Proceed as directed in the test for Fluoride under Dibasic Calcium Phosphate. The limit is 0.001%.
Assay Transfer about 5 g of Monobasic Potassium Phosphate, previously dried and accurately weighed, to a 250-mL beaker, add 100 mL of water and 5.0 mL of 1 N hydrochloric acid VS, and stir until the assay specimen is completely dissolved. Place the electrodes of a suitable pH meter in the solution, and slowly titrate the excess acid, stirring constantly, with 1 N sodium hydroxide VS to the inflection point occurring at about pH 4. Record the buret reading, and calculate the volume (A), if any, of 1 N hydrochloric acid consumed by the assay specimen. Continue the titration with 1 N sodium hydroxide VS until the inflection point occurring at about pH 8.8 is reached, record the buret reading, and calculate the volume (B) of 1 N sodium hydroxide required in the titration between the two inflection points (pH 4 and 8.8). Each mL of the volume (B A) of 1 N sodium hydroxide is equivalent to 136.1 mg of KH2PO4.