742 Proceedings of the Royal Irish Academy. 



LIX. — The Detectiojst and Precipitation- of Phosphoric Acid by 

 Ammonic Molybdatb. By Archibald Inichol M'Alpine, B. So. 

 ^ (Lond.), Eoyal Exhibitioner, Eoyal College of Science. 



[Eead June 26, 1876.] 



The detection and precipitation of phosphoric acid by amnionic 

 molybdate has occupied the attention of several chemists. 



Bichters'" has found that the test is rendered more delicate, and 

 precipitation more rapid, if ammonic nitrate is added to the solution 

 to be examined for the phosphoric acid prior to adding the molybdic 

 test. 



Prom a series of experiments I have made, I find that the test can 

 be rendered still more delicate, and the precipitation still more rapid, 

 if an excess of ammonic molybdate, some strong nitric acid, and 

 finally, strong ammonia, be added to the solution, until it is nearly 

 neutral. In tliis way I found phosphoric acid in waters which yielded 

 no trace with the usual molybdic solution after standing at a gentle 

 heat for a considerable time. 



In making my experiments I noticed that molybdic acid is easily 

 separated from the larger portions of the ammonia with which it is 

 combined, under certain conditions ; these conditions are that the 

 solution should be hot, and saturated with ammonic nitrate, and that 

 free nitric acid should not be present in excess. It may be that the 

 greater delicacy of the test, in a phosphate solution containing nitric 

 acid to which ammonia has been addecl to near the neutral point, is 

 due to the formation of this acid molybdate, which combines with the 

 phosphoric acid to form the ammonio-phospho-molybdate precipitate. 



I then made some experiments to determine the effect of nitric 

 acid on the precipitation. I found that beyond a certain point nitric acid 

 hindered the precipitation in very dilute phosphate solution, and 

 generally it retarded the formation of the precipitate. This is contrary 

 to the statement made in some of the works on chemical analysis. 



To determine good proportions for the ammonia and nitric acid, 

 I made various experiments with 10 cubic centimetre portions of a 

 phosphate solution containing "010 grammes of phosphoric acid per 

 litre, varying the quantities of nitric acid in the different experiments. 

 I added ammonia to the solution until the precipitate formed most 

 favourably, and finally I determined the amount of free acid remaining 

 in the solution. I found the following proportions made the test the 

 most sensitive : — 



* See Dingl. Polyt., J. cxeix. 183; also vol. 24 Chem. Soc. Journal. 



