KPO3 118.07

Metaphosphoric acid (HPO3),potassium salt.
Potassium metaphosphate [7790-53-6].

Packaging and storage— Preserve in well-closed containers.

Identification—

Viscosity á911ñ— Mix 300mg with 200mLof sodium pyrophosphate solution (3.5in 1000),using a magnetic stirrer.Determine the viscosity of the clear solution obtained,or of the liquid phase of the mixture obtained after 30minutes of continuous stirring:the viscosity is between 6.5and 15centipoises.

Lead— Asolution of 1g in 10mLof 3Nhydrochloric acid contains not more than 5µg of lead (corresponding to not more than 5ppm of Pb)when tested as directed in the test for Lead á251ñ.

Heavy metals,Method Iá231ñ— Warm 1g with 10mLof 3Nhydrochloric acid until no more dissolves.Add 15mLof water,mix,and filter:the limit is 0.002%.

Limit of fluoride— Place 5.0g of Potassium Metaphosphate,25mLof water,50mLof perchloric acid,5drops of silver nitrate solution (1in 2),and a few glass beads in a 250-mLdistilling 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-mLvolumetric flask until the temperature reaches 135.Add water from the funnel or introduce steam through the capillary to maintain the temperature between 135and 140.Continue the distillation until 225mLto 240mLhas been collected,then dilute with water to volume,and mix.Transfer 50.0mLof this solution to a 100-mLcolor-comparison tube,and transfer 50.0mLof water to a similar tube to serve as a control.Add to each tube 0.1mLof a filtered solution of sodium alizarin-sulfonate TSand 1mLof freshly prepared hydroxylamine hydrochloride solution (1in 4000),and mix.Add,dropwise,and with stirring,0.05Nsodium 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 1.0mLof 0.1Nhydrochloric acid,and mix.From a buret,graduated in 0.05-mLincrements,add slowly to the tube containing the distillate enough thorium nitrate solution (1in 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(10µg of Fper 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 1or 2drops of sodium fluoride TSto the control.Adistinct change in color appears.The volume of sodium fluoride TSrequired for the control solution does not exceed 1.0mL(0.001%).

Assay— Mix about 200mg of Potassium Metaphosphate,accurately weighed,with 15mLof nitric acid and 30mLof water,boil for 30minutes,cool,and dilute with water to about 100mL.Heat to 60,add an excess of ammonium molybdate TS,and heat at 50for 30minutes.Filter,and wash the precipitate,first with 0.5Nnitric acid,and then potassium nitrate solution (1in 100)until the filtrate is no longer acid to litmus.Add 25mLof water to the precipitate,dissolve it in 50.0mLof 1Nsodium hydroxide VS,add phenolphthalein TS,and titrate the excess sodium hydroxide with 1Nsulfuric acid VS.Each mLof 1Nsodium hydroxide is equivalent to 3.086mg of P2O5.

Expert Committee:(EMC)Excipients:Monograph Content

USP28–NF23Page 3065

Phone Number:1-301-816-8379