DIRECT AND MOLYBDATE METHODS 213 



liminary treatment by the citric acid test for silicic acid should 

 not be omitted, since it is a safe means of discovering those 

 samples which must be treated with particular care. 



186. Comparison of the Direct and the Molybdate Method for the 

 Estimation of the Total Phosphoric Acid in Basic Slag, Bone 

 Meal, Etc. 152 V. Schenke has stated that the direct precipitation 

 of total phosphoric acid in bone meal and basic slags, according 

 to the method of the Association of Agricultural Experiment 

 Stations of Germany, that is, direct precipitation of magnesia 

 mixture, gives from 0.3 to 0.4 per cent, less phosphoric acid than 

 the molybdate method. 63 He, therefore, considers it necessary that 

 the strongly acid phosphate solution before precipitation with 

 magnesia mixture should be almost neutralized and only half the 

 quantity, namely, 50 cubic centimeters instead of 100 cubic centi- 

 meters, should be treated with the ammonium citrate solution. 

 It has, however, already been shown by the earlier data of 

 Maercker and Halenke, as well as by the latest researches of 

 Mach 64 that this view of Schenke is not correct, and also the 

 analyses of Bottcher indicate the same fact, and that neutraliza- 

 tion of the solution before the precipitation with the citrate solu- 

 tion and the magnesia mixture uther by aqua regia or by sulfuric 

 acid is not necessary. 65 Bottcher says it is claimed by many ana- 

 lysts that by solution of bone meal, etc., with aqua regia and subse- 

 quent direct precipitation with citrate solution and magnesia mix- 

 ture, incorrect and, indeed, higher results are obtained than 

 when sulfuric acid is used for the solution. As a reason for 

 this it is said that the compensation for the errors which take 

 place in the aqua regia solutions is irregular, according to the kind 

 and quantity of the bases which are present in the solution. It 

 is also supposed that in bone meals and other organic substances 

 by reason of the incomplete oxidation with aqua regia, organic 

 acids are formed whose lime salts are equally soluble in the am- 

 monium citrate solution and are, therefore, carried down by the 

 precipitate. In the solutions by sulfuric acid the proportions re- 



(W Chemiker-Zeitung, 1905, 29 : 1294. 



63 Die landwirtschaftlichen Versuchs-Stationen, 1905, 62 : 3. 



64 Die landwirtschaftlichen Versuchs-Stationen, 1905-6, 63 :8i. 



65 Chemiker-Zeitung, 1905, 29 : 1294- 



