DETERMINATION OF PHOSPHORIC ACID AND NITROGEN 87 



ments have shown that there is no appreciable increase in the 

 weight of the precipitate when colored in this way. 



When the precipitation is carried on according to the citrate 

 method, Neubauer proposes to eliminate this coloration by the 

 use of ammonium sulfate. 59 About seven cubic centimeters of a 

 saturated solution of ammonium sulfate should be added to the 

 solution before the precipitation by the magnesium mixture. 

 With this precaution it is possible to obtain a perfectly white 

 precipitate after five minutes of ignition. The lively glowing of 

 the precipitate throughout the whole mass at the time of changing 

 into pyrophosphate is much more easily observed by this treat- 

 ment than when the mass is gray or black. Even should the 

 addition of the ammonium sulfate solution to one containing a 

 large amount of lime produce a precipitate of crystalline calcium 

 sulfate, it is of no importance, inasmuch as the ammonium citrate 

 immediately dissolves large quantities of the calcium salt. 



A white pyrophosphate is easily obtained by treating the pre- 

 cipitate on the gooch after washing free from chlorids with a drop 

 or two of ammonium nitrate. The ignition is commenced very 

 gently at first and afterwards, when the mass is white, the blast 

 is used. 



If the ignited residue be gray it may sometimes be whitened by 

 moistening with a drop or two of nitric acid, burning at a very 

 low temperature, followed by the blast. There is no appreciable 

 difference in weight between a gray and white pyrophosphate. 



88. Determination of Phosphoric Acid and Nitrogen in the Same 

 Solution by Treatment with Sulfuric Acid and Mercury. Fertiliz- 

 ing materials which contain organic nitrogen and phosphoric acid, 

 such as bones, are of such a nature that it is often difficult to ob- 

 tain a fair sample of them in quantities suited to the direct deter- 

 mination ; viz., about one gram. Thus it often becomes important 

 to take a much larger quantity of the material, to bring it into 

 solution and to take an aliquot part thereof. It may also often 

 happen that it is important to determine the phosphoric acid in 

 the same sample which has been used for the determination of 

 the nitrogen by moist combustion with sulfuric acid and mercury. 

 59 Zeitschrift fur angewandte Chetnie, 1894, 7 : 678. 



