88 AGRICULTURAL ANALYSIS 



In this connection, however, it is somewhat difficult to avoid the 

 precipitation of some of the mercury with the phosphoric acid. 



The mercuric sulfate which is produced by the Kjeldahl method 

 is not precipitated in the presence of ammoniacal solution of am- 

 monium citrate, but there may be small quantities of mercurous 

 salts present or some finely divided metallic mercury which may 

 contaminate mechanically the phosphate precipitate. These dis- 

 turbing influences may be removed by previous treatment with 

 sodium chlorid. If from 50 to 60 cubic centimeters of sul- 

 furic acid have been used for the solution and oxidation and this 

 be made up to half a liter, it will be sufficiently dilute to permit 

 an almost quantitative separation of the mercurous chlorid pro- 

 duced by treatment with sodium chlorid. 



Neubauer, who has proposed this method, finds that when 

 sodium chlorid is used previous to the precipitation of the phos- 

 phoric acid, a precipitate of ordinary size contains, at most, only 

 one milligram of mercury, while without the use of sodium chlorid 

 as much as four milligrams may be found. The details of the 

 method employed by Neubauer are as follows : 60 



Ten grams of the fertilizing material are placed in a half liter 

 flask with from 50 to 60 cubic centimeters of strong sulfuric 

 acid, two grams of mercury, and a little paraffin to prevent foam- 

 ing. The oxidation is carried on as usual in the Kjeldahl method. 

 The liquid, after cooling, is diluted with water and one cubic 

 centimeter of a citrate solution of sodium chlorid added, cooled, 

 filled to the mark, filtered, and 50 cubic centimeters taken for 

 the determination of the phosphoric acid, according to the citrate 

 method, and the same quantity for the determination of the am- 

 monia by distillation. The methods of digestion of soils with 

 sulfuric acid described in Volume I, are also applicable to 

 fertilizing materials. 



THE CITRATE METHOD 



89. General Principles. It has been seen that in the molybdate 



method there is introduced a process at considerable cost, both 



of reagents and time, having for its object the separation of the 



phosphoric acid from all the other acids and bases which may 



80 Zeitschrift fur angewandte Chemie, 1894, 7 :678. 



