104 AGRICULTURAL ANALYSIS 



each one is conducted with the proper precautions peculiar to it. 73 

 In the latter part of 1892, at the general meeting of the associa- 

 tion, it was declared that the citrate method, after having been 

 subjected to repeated tests, was found to be satisfactory, changing 

 the composition of the solution so that it might have noo instead 

 of looo grams of citric acid and four liters of 24 per 

 cent, ammonia to each 10 liters. The data afforded by the citrate 

 method, when applied to an artificial mixture of known composi- 

 tion, were more satisfactory than those obtained by the molyb- 

 dic process. 



In the laboratory of the Bureau of Chemistry the citrate method 

 has been found to give nearly agreeing results with the old pro- 

 cess. It is much shorter and less expensive, and is recommended 

 most favorably for practical use, with the suggestion, however, 

 that, with every new kind of phosphate or phosphatic fertilizer 

 varying notably in composition from the standard, the work should 

 be checked at first by comparison with the molybdate method. The 

 later investigations carried on by the Association of Official Agri- 

 cultural Chemists have served only to confirm the superiority of 

 the molybdate gravimetric method for all purposes where great ac- 

 curacy is demanded and have shown that for speed and con- 

 venience the volumetric method already outlined and later to be 

 described in full, is to be preferred to all other quick processes. 



96. The Citrate Precipitate Purity. Jorgensen recommends the 

 following as the safest form of citrate precipitation. The phos- 

 phoric acid solution is treated with 25 cubic centimeters of 

 neutral ammonium citrate solution or, in case it contains a large 

 quantity of calcium salts, with 30 cubic centimeters, and then 

 25 cubic centimeters of a 10 per cent, ammonia solution is 

 added, and the mixture, in a covered dish, heated to the boil- 

 ing point, and, according to the quantity of the phosphate pre- 

 cipitate, treated with 30 or 40 cubic centimeters of the neutral 

 magnesium chlorid solution. By vigorous stirring or shaking 

 the precipitate crystallizes, and after standing at least four 

 hours is filtered. If very small quantities of phosphoric acid 

 are present the mixture should be left at least 24 hours 

 " Die landwirtschaftlichen Versuchs-Stationen, 1892, 41 : 329. 



