AGRICULTURAL EXPERIMENT STATION. 213 



made as favorable as possible to the changes indicated by Andouard, 

 3'et the results obtained seem to show that the loss, if an}', was 

 trifling. 



METHOD FOR PHOSPHORIC ACID 



Isbert and Stuetzer (Zeit. Anal. Chemie. 26, 583) propose a 

 simplification of the ordinary method of determining phosphoric 

 acid. The abridgement is based upon the constant relation which 

 is said to exist between the ammonia and the phosphoric acid in the 

 ammonium phosphomolybdate precipitate. These writers also 

 claim that washing the yellow precipitate with cold water dissolves 

 out the ammonium silico-molybdate which may be present, without 

 removing any appreciable amount of phosphoric acid. 



The method proposed is briefly- this : The phosphoric acid is 

 precipitated in the usual manner with ammonium moljbdate, the 

 precipitation being hastened by heating the solution to 60-70° C. for 

 fifteen minutes. After cooling, the supernatant liquid is poured 

 upon a filter, the precipitate washed several times by decantation 

 with cold water, and the precipitate itself at last transferred to the 

 filter. The washing is continued until the washings amount to one- 

 fourth litre. The filter containing the precipitate is then transferred 

 to a distillation flask, soda solution added, the ammonia distilled 

 into standard sulphuric acid and determined by titration. The 

 relation of nitrogen to phosphoric acid in the ammonium phospho- 

 molybdate was found to be 1 : 1.654. 



In order to test the applicability of this method to general 

 laboratory use a number of trials were recentl}' made at this station. 

 The results obtained were not satisfactory. Moreover great diffi- 

 culty was experienced in washing from the filter the last traces of 

 ammonium nitrate contained in the molybdate solution. So much 

 time was consumed in this way that the method was abandoned as 

 being longer and less accurate than the gravimetric method in 

 ordinary use. 



As regards the use of cold water in washing the ammonium 

 phospho- molybdate precipitate, it was found that in a few cases 

 slightly lower results were obtained than when ammonium nitrate 

 was employed. The diflTerences were so slight, however, that it was 

 decided to make this substitution in fertilizer anah'ses of the coming 

 year. 



