METHOD OF KRUG AND MCELROY 241 



212. Suggestion of Ogilvie. When a phosphate is dissolved in 

 a mineral acid preparatory to the separation of the various sub- 

 stances which pass into solution, most authorities advise that the 

 solutions be brought to dryness before proceeding to separate the 

 calcium. Some analysts, however, neglect this part of the process, 

 and, as Ogilvie has shown, with a chance of error. 98 It appears 

 from this fact that in the analyses of the majority of the phos- 

 phatic products in use for manurial purposes, special care must be 

 exercised to procure a pure and perfectly granular precipitate of 

 magnesium ammonio-phosphate, either by evaporating the first 

 solution to dryness, or by separating the precipitate which forms 

 on the addition of ammonia to the solution containing citric acid. 



213. Method of Krug and McElroy. Krug and McElroy show 

 that when sufficient alcohol is added to precipitate all of the cal- 

 cium sulfate in the Glaser method, it will also cause a precipitation 

 of a considerable quantity of iron, by means of which the calcium 

 sulfate will be colored." The presence of potassium and ammo- 

 nium salts also affects very notably the precipitation of calcium. 

 The procedure suggested, in order to avoid these sources of error, 

 is based on the separation of the phosphoric acid by the molyb- 

 date method and is as follows : 



One hundred cubic centimeters, equivalent to one gram of the 

 substance, in a nitric acid solution, are placed in a half-liter flask 

 and a solution of ammonium molybdate added until all the phos- 

 phoric acid has been precipitated. The addition of ammonium 

 nitrate will hasten the separation of the ammonium phospho- 

 molybdate. The liquid should be allowed to stand for 12 

 hours. The flask is filled to the mark, the contents well 

 shaken, filtered through a dry filter, and duplicate samples of 

 200 cubic centimeters each of the filtrate subjected to examination. 



A small quantity of ammonium nitrate is dissolved in the 

 liquid, and ammonia cautiously added, keeping the solution as 

 cool as possible. The iron and alumina are precipitated as 



98 Crookes, Select Methods in Chemical Analysis, 4th Edition, 1905 : 499. 



99 Journal of Analytical and Applied Chemistry, 1891, 5 : 671. 

 Zeitschrift fur angewandte Chemie, 1891, 4 : 170, 243, 357. 

 Zeitschrift fur analytische Chemie, 1891, 30 : 206. 

 Chemical News, 1891, 63 : 251. 



