DETEEMIXATION OF PHOSniORIC OXIDE . 



265 



2. Solutions of dicalcium pliospliate in nitric acid were 

 mixed with varying amoiuits of a solution of water glass, 

 and also in most cases with varying quantities of ferric 

 snlphate. The results, given below, show that dissolved 

 silica and iron have no eifect on the lesults, even when present 

 in excessive amounts : — 



Influence of Amouxt of Phosphoric Oxide. 



It is well known that the composition of the ammonium 

 phospho-molybdate is liable to vary according to the nature 

 (jf the solution in which precipitation takes place. For this 

 reason the conditions of precipitation must be carefully 

 controlled, in order that concordant residts may be obtained 

 by any method in wliicli the precipitate is weighed or its 

 amount detei mined volumetrically. In the process under 

 discussion the concentiation of molybdic acid is constant, the 

 concentration of suli)huiic acid and of ammonium sulphate is 

 practically constant, and the concentration of nitric acid is) 

 also nearly the same in all cases. The variable factors are 

 the concentration of phosphoric acid and of the other con- 

 stituents of the substance under examination. The data given 

 beloAv show that for the useful range of a volumetric process 

 the relation between phosphoric oxide and sodium hydroxide 

 is constant. — that is, that the composition of the precipitate 

 is independent of the concentration of phosphoiic acid in the 

 solution in which precipitation takes ])lace. Herein lies one 

 of the greatest merits of the method. Wlien once the solution 

 containing the phosphoric oxide lias been prepared — no matter 

 how — no further treatment is necessary beyond the addition 

 of the requisite amount of the sulphuric-nitric acid mixture 

 (or of nitric acid alone) before proceeding Avitli the pre- 

 cipitation. It is just this virtue which diiferentiates the 

 Lorenz method of precipitation from others: — 



