60 AGRICULTURAL ANALYSIS 



tion of density 0.90. Let stand for some time ; two hours is usual- 

 ly enough. Filter, wash with 2.5 per cent. NH 3 until practically 

 free from chlorids, ignite to whiteness or to a grayish white, and 

 weigh. 



65. Influence of Insoluble Silica. It is assumed in the above 

 methods that there is no more than a mere trace of soluble silica 

 in the solutions of the phosphate with which the operations are 

 conducted. Silica in solution (silicic acid) has also the property 

 of forming yellow compounds with molybdate of ammonia and 

 thus when present in any quantity would contaminate the precip- 

 itate produced. This trouble is avoided if the acid solution of 

 the phosphate is evaporated to dryness, rubbed to a fine powder 

 before becoming perfectly dry, moistened with hydrochloric acid 

 and taken up with water. The pasty state of the phosphoric acid 

 and large quantities of soluble salts present in these cases make 

 this a tedious process to be practiced only when necessary. 



66. Use of Tartaric Acid in Phosphoric Acid Estimation. In 

 the presence of iron the molybdate mixture is likely to carry 

 down some ferric oxid with the yellow precipitate. To prevent 

 this, and also hinder the separation of molybdic acid in the solu- 

 tion on long standing, tartaric acid has been recommended. 



Jiiptner has found that the presence of tartaric acid does not 

 interfere with the separation of the yellow precipitate, as some 

 authorities assert. 30 Even 100 grams of the acid in one liter of 

 molybdate solution produce no disturbing effect. Molybdate 

 solution treated with tartaric acid does not show any separation 

 of molybdic acid when kept for a year at room temperatures. 

 The presence of tartaric acid, therefore, is highly useful in pre- 

 venting the danger of obtaining both ferric oxid and molybdic 

 acid with the yellow precipitate. 



67. Water-Soluble Phosphoric Acid. The method of procedure 

 recommended by the Association of Official Agricultural Chemists 

 is as follows: 31 Place two grams of the sample in a nine centi- 

 meter filter ; wash with successive small portions of cold water, al- 

 lowing each portion to pass through before adding more, 



30 Chemisches Central-Blatt, 1894, 2 : 813. 

 sl Bureau of Chemistry, Bulletin 107, 1907 : 3. 



