232 AGRICULTURAL ANALYSIS 



no free chlorin and is treated with a few drops of methyl orange 

 solution. Ammonia is added until nearly neutral, but the acid 

 reaction is retained, as shown by the indicator. A few cubic cen- 

 timeters of ammonium acetate are added, which produce a yellow 

 coloration of the liquid and also a complete precipitation of the 

 iron and aluminum phosphates when warmed to 70. At this 

 temperature the precipitation of any calcium phosphate is avoided. 

 A small quantity of the lime may be carried down mechanically 

 and therefore the precipitate should be dissolved in hydrochloric 

 acid and the precipitation again made as above after the addi- 

 tion of some sodium phosphate. If the original solution contains 

 any free chlorin, as may be the case when aqua regia is employed 

 as solvent, before beginning the separation, ammonia should be 

 added in slight excess and the acidity restored by hydrochloric 

 after adding the indicator. In washing the precipitates, water of 

 not over 70 must be used. As has been shown by Hess in the 

 work cited in the next paragraph, the statement that the precipi- 

 tates obtained as above are free of lime has not been proved to 

 be strictly correct. The process, however, is a distinct improve- 

 ment over the older methods and forms the basis of the amended 

 process given below, which appears to be sufficiently accurate to 

 entitle the acetate method to favorable consideration. 



205. Method of Hess. Hess has made an investigation of the 

 standard methods of determining iron and aluminum oxids in the 

 presence of phosphoric acid and has shown that the assumption, 

 that the composition of the precipitate is represented by the form- 

 ula Al 2 (PO 4 ) 2 +Fe 2 (PO 4 ) 2 , is erroneous. 90 



In the washing of the precipitated iron and aluminum phos- 

 phates, there is a progressive decomposition of the compound 

 with the production of the basic salt. The composition of the 

 precipitate at the end is dependent chiefly upon the way in 

 which the washing takes place. It is quite difficult to always 

 secure a washing in exactly the same way, and the final composi- 

 tion of the precipitate varies with almost every determination. 

 It is not, therefore, an accurate proceeding to take half the 

 weight of the precipitate as phosphoric acid or as iron oxid and 

 90 Zeitschrift fur angewandte Chemie, 1894, 7 : 679, 701. 



