130 AGRICULTURAL ANALYSIS 



116. Cladding's Modification. Gladding has proposed the 

 following modified method for the direct determination of phos- 

 phoric acid by weighing the yellow precipitate: 3 To the solu- 

 tion of phosphoric acid 25 cubic centimeters of strong ammonia, 

 0.90 specific gravity, is added, and concentrated nitric acid to 

 acidity. The beaker containing the solution is placed in a water 

 bath at 50 and the ordinary molybdate solution added at the rate 

 of three drops per second in excess and with constant stirring. 

 After standing 10 minutes the solution is filtered through a 

 weighed filter paper, washed six times with i :ioo nitric acid, 

 once with water and dried to constant weight at from iO5-io8 

 in a glycerol or salt water oven. Careful analysis of the dried 

 precipitate led to the formula: 24MoO 3 ,P 2 O 3 3(NH 4 ) 2 O+ 

 2 4 MoO 3 ,P 2 O 6 ,2(NH 4 ) 2 O.H 2 O+5H 2 O. 



Later Gladding recommended that the yellow precipitate be 

 given two final washings with alcohol to facilitate drying. 4 This 

 method has been examined by the Association of Official Agri- 

 cultural Chemists. With slight modifications, such as a gooch 

 crucible with a paper or a'sbestos felt for the filtrations, and more 

 thorough washing of the precipitate with water (total quantity 

 from 100 to 300 cubic centimeters) it has been found to give 

 very accurate and trustworthy results. 5 



VOLUMETRIC DETERMINATION OF PHOSPHORIC ACID. 



117. Classification of Methods. The time required for a gravi- 

 metric determination of phosphoric acid has led analysts to try 

 the speedier, if less accurate processes, depending on the use of 

 volumetric methods. The chief difficulty with these methods has 

 been in securing combinations of standard composition and some 

 sharp method of distinguishing the end reaction. In some cases, 

 as, for instance, in the uranium method, it has been found neces- 

 sary to remove a portion of the titrated solution and prepare it 

 for final testing by subsidence or filtration. As is well known, 

 this method of determining the end reaction is less accurate and 

 more time-consuming than those processes depending on a change 



3 Journal of the American Chemical Society, 1896, 18 : 23. 



4 Division of Chemistry, Bulletin 51, 1898 : 47. 



6 Division of Chemistry, Bulletin 49, 1897 : 60 ; Bulletin 51, 1898 : 47. 



