254 AGRICULTURAL ANALYSIS 



minum phosphates, the solubility of aluminum phosphate in cold 

 acetate solutions, and the solubility and dissociation of iron and 

 aluminum phosphates in water. 11 Also when the phosphates are 

 fused with sodium carbonate, and the iron determined by pre- 

 cipitation with ammonia, the contamination of the iron with 

 calcium phosphate, which is not always entirely decomposed by 

 fusion, is a source of error. Of these the most serious are the 

 first and last mentioned. 



3. In the caustic alkali methods there is danger of some of 

 the aluminum being held by the voluminous precipitate pro- 

 duced by the alkali ; there is also danger of alumina being pre- 

 cipitated if much carbon dioxid is absorbed. Lasne and Licht- 

 schlag have shown that while the method is long, it gives accu- 

 rate results if properly conducted. Blattner and Brasseur have 

 investigated the more important methods and conclude : 12 



The acetate method should be discontinued; figures for alum- 

 ina are nearly always too low. 



The Glaser method (alcohol) gives accurate results in the 

 absence of manganese. 



The caustic soda method, as carried out by Lasne, gives exact 

 results. 



In view of these many sources of error in the conventional 

 methods, considerable time has been devoted to the study of a 

 method that, it is hoped, is free from most of the above men- 

 tioned objections. It is an adaptation, so far as possible, of the 

 good points of the present best methods. From the precipita- 

 ting reagent used, it may be designated the thiosulfate method. 



The use of a soluble thiosulfate for the separation of alumina 

 from iron and aluminum from several other metals seems to be 

 due to Chancel. 13 Later it was used by Stead and by Carnot; 

 by the latter for the separation of aluminum as phosphate, in- 

 the presence of ammonium acetate, from iron. 14 Lasne also uses 

 it to precipitate aluminum phosphate in the presence of am- 



11 Chemiker-Zeitung, 1897, 21 : 264. 



11 Bulletin de la Socit chimique de Paris, 1897, [3], 17 : 760. 



18 Comptes rendus, 1858, 46 : 987. 



14 Blair, Chemical Analysis of Iron, 6th Edition, 1903, : 196. 



