PEMBERTON'S VOLUMETRIC METHOD 153 



precipitate is dissolved in ammonia, and the solution treated as 

 above. 



The crystals of ammonio-manganous phosphate are dissolved 

 in dilute hydrochloric acid, the solution, diluted to 300 cubic centi- 

 meters, treated with from one to three cubic centimeters of hydro- 

 gen peroxid, 20 cubic centimeters of a 10 per cent, solution of 

 potassium hydroxid added and boiled for some time to expel an 

 excess of the peroxid. 



After cooling, 20 cubic centimeters of 20 per cent, hydrochloric 

 acid are added and the solution allowed to stand for some time in 

 order to destroy any traces of an alkaline peroxid. After the ad- 

 dition of 20 cubic centimeters of a 10 per cent, solution of potas- 

 sium iodid, the liberated iodin is titrated by a set solution of 

 sodium hyposulfite. The reactions which take place in these 

 operations are illustrated by the following equations : The hydro- 

 gen peroxid converts the manganous salt into the compound 

 MngOi^sMnO-jMnO. The ammonio-manganous phosphate has 

 the composition 6NH 4 MnPO 4 . When oxidized by hydrogen 

 peroxid, it yields three molecules of phosphoric anhydrid, -3P 2 O 5 ,- 

 and one molecule of the compound 5MnO 2 MnO. When the 

 manganic peroxid is treated with potassium iodid the reaction is : 

 5MnO 2 +2OHCl+ ioKI=5MnCl,+ ioKCl+ioI, and ioI+ioNa a 

 S,O 3 = sNa 2 S 4 O 6 + loNal. Then ioNa,S 2 O 3 = 10! = 3P, 2 O 5 . 

 From these data the percentage of P 2 O 5 is readily calculated. 



The method is of interest from the principle involved, but is of 

 little practical value because of the necessity of separating the 

 phosphoric acid from all insoluble phosphates by the molybdate 

 method. The authors have endeavored to avoid this separation 

 and express the hope that a direct way may be found. 



THE DETERMINATION OF PHOSPHORIC ACID BY TITRA- 

 TION OF THE YELLOW PRECIPITATE 



140. Pemberton's Volumetric Method. In order to shorten the 

 work of determining the phosphoric acid, numerous attempts have 

 been made to execute the final determination directly on the yel- 

 low precipitate obtained by treating a solution of a phosphate 

 with ammonium molybdate in nitric acid. The composition of 

 this precipitate appears to be somewhat variable, and this fact 



