1 62 AGRICULTURAL ANALYSIS 



out this method the substance is incinerated by a mixture of sul- 

 furic and nitric acids. From the ash solution obtained by the 

 method described the phosphoric acid is precipitated in the usual 

 manner as phosphomolybdate. The washed precipitate is imme- 

 diately dissolved in an excess of one-half normal soda-lye and the 

 excess of alkali titrated with one-half normal sulfuric acid, after 

 driving off the ammonia by boiling and allowing the solution to 

 become completely cool. 



Since each molecule of the phosphoric pentoxid (P 2 O 5 ) of the 

 yellow precipitate obtained by this method of treatment requires 

 for its complete neutralization, with the use of phenolphthalein 

 as an indicator, 56 molecules of NaOH, so each cubic centimeter 

 of one-half normal soda-lye used corresponds to 1.268 milligrams 

 of P 2 5 . 



The solutions used by Neumann are as follows : 



1 i ) 50 per cent, ammonium nitrate solution. 



(2) 10 per cent, of ammonium molybdate solution (dissolved, 

 cooled, and filtered). 



(3) One-half normal potash lye and one-half normal sulfuric 

 acid. 



(4) One per cent, alcoholic phenolphthalein solution. 



The substance is incinerated according to the method described 

 and the dilution with water, which is recommended at the close of 

 the method, as well as the boiling of the ash solution, are prop- 

 erly modified for the special estimation of the phosphoric acid. 



Under the assumption that for the incineration not more than 

 40 cubic centimeters of the acid mixture have been used, about 140 

 cubic centimeters of water are added to the ash solution, so that in 

 all the analyst has in amount from 150 to 160 cubic centimeters. 

 It should be remembered in this case that about one-half of the 

 acid mixture has been volatilized during the incineration. After 

 the addition of 50 cubic centimeters of ammonium nitrate, the 

 mixture is heated to from 70 to 80 until bubbles begin to 

 ascend through the liquid. Thereupon, 40 cubic centimeters of 

 ammonium molybdate are added. This quantity is sufficient for 

 at least 60 milligrams of P 2 O . It is recommended that the quan- 



