DESIRABILITY OF METHODS 179 



the quantity required for the neutralization of the 10 cubic centi- 

 meters is added, less one-half cubic centimeter. By this treatment 

 the phosphoric acid in the presence of sodium acetate is completely 

 precipitated as silver phosphate. The excess of silver is determined 

 by diluting the mixture to 200 cubic centimeters, filtering, and 

 titrating 100 cubic centimeters of the filtrate with ammonium thio- 

 cyanate, using a ferric salt (ferric-potassium-alum) as indicator. 

 The presence of sulfuric and nitric acids does not interfere with 

 the reaction, but, of course, hydrochloric acid must be absent. 

 Alkalies and alkaline earth metals may be present, but not the 

 heavy metals. 



When iron and aluminum are present 100 cubic centimeters of 

 the solution are precipitated with 30 cubic centimeters of nor- 

 mal sodium acetate, the phosphoric acid is determined in 50 cubic 

 centimeters of the filtrate, and the precipitate of iron and alumi- 

 num phosphates is ignited and weighed, and its weight multiplied 

 by 2.225 i s added to the phosphoric anhydrid found volumetric- 

 ally. If ammonia be present it must be removed by boiling, as 

 otherwise it affects the titration with phenolphthalein. 



For agricultural purposes this method can have but little value, 

 inasmuch as the phosphates to be examined almost always have 

 a certain proportion of iron and aluminum. Moreover, since the 

 amount of these bases has to be determined gravimetrically, there 

 would be no gain in time and no simplification of the processes 

 by the use of the volumetric method as proposed. 



TECHNICAL DETERMINATION OF PHOSPHORIC ACID 



156. Desirability of Methods. In the preceding paragraphs, has 

 been given a statement of the principal methods now in use by 

 chemists and others connected with fertilizer control for the 

 scientific and agronomic determinations of phosphoric acid, and 

 its agricultural value. 



A resume of the important methods, in a form suited to use 

 in a factory for preparing phosphatic fertilizers for the market, 

 seems desirable. In these factories the chemists have been accus- 

 tomed to use their own, or private methods, and there has not 

 been a general disposition among them to publish their methods 

 and experience for the common benefit. For factory processes, 



