20O AGRICULTURAL ANALYSIS 



that many difficulties beset the separation of iron and alumina 

 from the other substances occurring in phosphatic deposits. Veitch 

 has called especial attention to these difficulties in view of the 

 persistence with which ordinary precautions are disregarded. 20 

 The important points to be kept in view are that all the iron 

 should be in solution in the ferric state, the solution should be free 

 of silica, and should contain no more than 0.5 gram substance in 

 300 cubic centimeters. In these conditions the precipi- 

 tation of the phosphates of iron and alumina is made in a five 

 per cent, solution of ammonium chlorid, and the precipitate after 

 washing several times with hot water, is dissolved in hydrochloric 

 acid diluted to about 300 cubic centimeters and reprecipitated as 

 before, with care to have always an excess of phosphoric acid 

 over the amount required to unite with all the iron and alumina 

 present. The precipitate is washed free of chlorid with a five 

 per cent, ammonium nitrate solution and ignited to constant 

 weight. With the precautions noted, concordant results may be 

 obtained by precipitating with ammonium acetate and determining 

 the phosphates thrown out. The aluminum may be separated 

 from the iron with thiosulfate of ammonium or sodium, as pointed 

 out by Veitch. 21 The iron may afterwards be determined by 

 titration. 



The iron may be determined separately by reduction to the 

 ferrous state and oxidizing by potassium permanganate or bi- 

 chromate. 



The solutions must in all cases be free of organic matter. If 

 manganese titanium or vanadium be present the accuracy of the 

 iron determination will be affected, and these elements must be 

 separately determined where extreme accuracy is desired. The 

 small quantities of these bodies usually found, although they deport 

 themselves in the presence of phosphoric acid much in the same 

 way as iron, do not introduce any material error into the per- 

 centage determination when weighed as iron phosphate, because 

 they have approximately the same atomic weight as iron itself. 



224. Estimation of Sulfuric Acid. As a rule, sulfates are not 

 20 Proceedings of the Fifth International Congress of Applied Chemis- 

 try, Berlin, 1903, 1 : 492. 



.* l Journal of the American Chemical Society, 1900, 22 : 246. 



