§ 115. COMMERCIAL CONCEXTRATED MANURES. 225 



In case a large quantity of phosphoric acid is present, 

 or much free acid, 10 c.c. of the acetate may not be suf- 

 ficient to saturate all this acid with soda. The brown 

 color will appear, then, before all the phosphoric acid is 

 precipitated; but it is diffused through the entire drop, 

 and does not present a well-defined brown zone where the 

 drop of the test solution and of the ferrocynnide come to- 

 gether; in this case add 5 c.c. more of sodic acetate, and 

 it will be found that, in case free mineral acid was present, 

 more of- the uranic solution must be added before the 

 brown color appears. In order to be sure in regard to 

 this matter, it will always be well, after having reached 

 the point of saturation, to add 5 c.c. more of sodic acetate, 

 heat the mixture to boiling, and test a drop of the solu- 

 tion with the ferrocyanide. If a brown color appears, the 

 result first obtained was correct. Even 30 c.c. of sodic 

 acetate may be added without sensibly impairing the 

 accuracy of the work. 



h. The examination for potassa may be conducted ac- 

 cording to either of the methods described in § 93, G. 

 The third method, in which platinic chloride is used to 

 separate potassa from the alkaline earths, is more particu- 

 larly applicable in the examination of the native potash 

 salts, of which such extensive beds have been discovered 

 in Germany, and from which large quantities are taken for 

 agricultural purposes. 



c. The determination of the nitrogen is always made by 

 combustion with soda-lime (§ 85). 



d. Fresenius {Quantitative Chemische Analyse^ 894) 

 gives the following good plan for stating the results of the 

 analysis of a superphosphate ; a similar plan can be fol- 

 lowed, with such variations as may be necessary in each 

 particular case, in stating the result of the analysis of any 

 commercial fertilizer. 



10* 



