nation of Phosphoric Acid by Ammonium Molyhdate. 205 



phospho-molybdate, and Presenius (Quantitative Analyse, 5tli 

 edition, page 33-4) advises the separation of silicic acid as a 

 preliminary. W. Knop has observed (Chemisches Centralblatt, 

 1857, page 691) that ammonium molybdate, added to a solu- 

 tion containing silicic acid and a large quantity of ammonium 

 chloride, produces a lemon yellow precipitate much like ammo- 

 ' ' ' Without this excess of ammonium 



L the cold or after 

 heating to boiling. " 



It might readily be supposed, however, that, though not 

 precipitated by itself with ammonium molybdate, silicic acid 

 could come down in a precipitate of ammonium phospho- 

 molybdate and introduce an error. To ascertain whether this 

 actually happens the following experiments have been made. 

 A solution of potassium silicate was employed, made by boil- 

 ing pure silicic acid with potassium hydrate, and acidifying 

 slightly with nitric acid. It contained in 50 c,c. •2055 grs. of 

 silicic acid, and bare traces of phosphoric acid. A solution of 

 pure hydrodisodic phosphate was prepared, and the phosphoric 

 acid estimated by the ammonium molybdate method. 25 c.c. 

 gave 



(1) -0844 MgaPgO, = -05398 PgO^ 



Estimations of phosphoric acid were made in varied quanti- 

 ties of tbe solution of sodium phosphate with the addition of 

 potassium silicate, by the molybdic method. 



The amounts of the solutions employed and the results ob- 

 tained are given below. 

 •0492 SiO^ -f-lcc. sodic phosphates -0022 P2O5 gave -0023 PgOj 

 •0492 " -)-5 « =-0108 '' " -0114 " 



■0492 " -I-12-5 " —-0270 " " '0267 " 



•0123 « 4-25 " =-0540 " " '0540 " 



•0246 " +25 " =-0540 " " '0545 " 



•0492 " -1-25 " == -0540 " " '0544 " 



•2055 « -1-25 " z=:-0540 " " '0538 " 



•2055 " ^ 



•5000 CaSO^ I 



•2000 MgO i +25 " == -0540 " " ^0544 " 



•5000 Fe^O, I 

 1-0000 Alum J 



A solution of tricalcic phosphate in nitric acid containing in 

 50 c.c. -0379 grs. P^O, gave with -310.0 SiO, '0381 grs. P^O^ 



The ammonium molybdate and magnesium chloride solu- 

 tions used in these determinations were made as recommended 



