Potassium Metaphosphate
KPO3 118.07

Metaphosphoric acid (HPO3), potassium salt.
Potassium metaphosphate [7790-53-6].
» Potassium Metaphosphate is a straight-chain polyphosphate, having a high degree of polymerization. It contains the equivalent of not less than 59.0 percent and not more than 61.0 percent of P2O5.
Packaging and storage— Preserve in well-closed containers.
A: Add 1 g of finely powdered Potassium Metaphosphate, slowly and with vigorous stirring, to 100 mL of sodium chloride solution (1 in 50): a gelatinous mass is formed.
B: Boil a mixture of 0.5 g of Potassium Metaphosphate, 10 mL of nitric acid, and 50 mL of water for 30 minutes, and cool: the resulting solution responds to the tests for Potassium 191 and for Phosphate 191.
Viscosity 911 Mix 300 mg with 200 mL of sodium pyrophosphate solution (3.5 in 1000), using a magnetic stirrer. Determine the viscosity of the clear solution obtained, or of the liquid phase of the mixture obtained after 30 minutes of continuous stirring: the viscosity is between 6.5 and 15 centipoises.
Lead— A solution of 1 g in 10 mL of 3 N hydrochloric acid 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 Warm 1 g with 10 mL of 3 N hydrochloric acid until no more dissolves. Add 15 mL of water, mix, and filter: the limit is 0.002%.
Limit of fluoride— Place 5.0 g of Potassium Metaphosphate, 25 mL of water, 50 mL of perchloric acid, 5 drops of silver nitrate solution (1 in 2), and a few glass beads in a 250-mL distilling flask connected with a condenser and carrying a thermometer and a capillary tube, both of which extend into the liquid. Connect a small dropping funnel, filled with water, or a steam generator to the capillary tube. Support the flask on a distillation shield with a hole that exposes about one-third of the bottom of the flask to the flame. Distill into a 250-mL volumetric flask until the temperature reaches 135. Add water from the funnel or introduce steam through the capillary to maintain the temperature between 135 and 140. Continue the distillation until 225 mL to 240 mL has been collected, then dilute with water to volume, and mix. Transfer 50.0 mL of this solution to a 100-mL color-comparison tube, and transfer 50.0 mL of water to a similar tube to serve as a control. Add to each tube 0.1 mL of a filtered solution of sodium alizarin-sulfonate TS and 1 mL of freshly prepared hydroxylamine hydrochloride solution (1 in 4000), and mix. Add, dropwise, and with stirring, 0.05 N sodium hydroxide to the tube containing the distillate until its color just matches that of the control, which is faintly pink. Then add to each tube 1.0 mL of 0.1 N hydrochloric acid, and mix. From a buret, graduated in 0.05-mL increments, add slowly to the tube containing the distillate enough thorium nitrate solution (1 in 4000) so that, after mixing, the color of the liquid just changes to a faint pink. Note the volume of the solution added, add the same volume, accurately measured, to the control, and mix. Then add to the control sodium fluoride TS (10 µg of F per mL) from a buret to make the colors of the two tubes match after dilution to the same volume. Mix, and allow all air bubbles to escape before making the final color comparison. Check the endpoint by adding 1 or 2 drops of sodium fluoride TS to the control. A distinct change in color appears. The volume of sodium fluoride TS required for the control solution does not exceed 1.0 mL (0.001%).
Assay— Mix about 200 mg of Potassium Metaphosphate, accurately weighed, with 15 mL of nitric acid and 30 mL of water, boil for 30 minutes, cool, and dilute with water to about 100 mL. Heat to 60, add an excess of ammonium molybdate TS, and heat at 50 for 30 minutes. Filter, and wash the precipitate, first with 0.5 N nitric acid, and then potassium nitrate solution (1 in 100) until the filtrate is no longer acid to litmus. Add 25 mL of water to the precipitate, dissolve it in 50.0 mL of 1 N sodium hydroxide VS, add phenolphthalein TS, and titrate the excess sodium hydroxide with 1 N sulfuric acid VS. Each mL of 1 N sodium hydroxide is equivalent to 3.086 mg of P2O5.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Robert H. Lafaver, B.A.
(EM105) Excipient Monographs 1
USP32–NF27 Page 1322
Chromatographic Column—
Chromatographic columns text is not derived from, and not part of, USP 32 or NF 27.