SOURCES OF ERROR IN THE MOLYBDATE METHOD 79 



Another method of avoiding any trouble from silica consists in 

 using sulfuric and a little nitric acid as the solvent for the orig- 

 inal substance. Silica is not soluble in hot concentrated sulfuric 

 acid. The volume of the sulfuric should be about ten times that 

 of the nitric acid used, and the boiling be continued until sulfuric 

 vapors are evolved. 



2. Error Due to Arsenic -Only in rare cases will arsenic be 

 found in phosphatic fertilizing materials. In case of pyritic phos- 

 phates, the iron disulfid may carry arsenic. The solution in such 

 a case is best accomplished in hydrochloric acid. If aqua regia 

 be used, all nitric acid should be removed by repeated evapora- 

 tion with hydrochloric acid. The arsenic can then be precipitated 

 in the hot dilute hydrochloric acid solution by hydrogen sulfid. 



3. Error Due to Occluded Magnesia. The danger of contami- 

 nation of the final precipitate with magnesium oxid has been 

 pointed out by some authors. The re-solution of the precipitate 

 followed by a second precipitation is the usual remedy proposed. 

 Lorenz states that this source of error may be entirely avoided by 

 the addition of two per cent, of citric acid to the solution. 54 

 Without the addition of citric acid the precipitation of some 

 magnesia with the phosphate, at least in strongly ammoniacal solu- 

 tions, cannot be avoided even by the slowest and most careful 

 addition of the magnesia mixture. The citric acid is used in the 

 common form of ammonium citrate solution. 



4. Error Due to Volatility of Phosphoric Acid. This source of 

 error has been made the subject of a special study by Neubauer. 55 

 From the results a table has been constructed, the use of which 

 is recommended for phosphoric acid determinations. The source 

 of error in the method where neutralization is omitted lies exclu- 

 sively in the loss of phosphoric acid by volatilization. The mag- 

 nesia-covered crucible lid offers a very good control of this error, 

 and its use is recommended to the analyst. Of course, the pres- 

 ence of sulfur in the gas used for ignition is liable to disturb this 

 check. 



54 Zeitschrift fur analytische Chetnie, 1893, 82 : 64. 



55 Journal of the American Chemical Society, 1894, 16 : 289. Trans- 

 lated and Abridged by K. P. McElroy. 



