164 AGRICULTURAL ANALYSIS 



15 minutes and which should be determined by testing with 

 moist litmus paper. After complete cooling, from six to eight 

 drops of the phenolphthalein solution are added and the excess 

 of alkali titrated with one-half normal sulfuric acid. 



Calculation. The number of added cubic centimeters of one- 

 half normal soda-lye, after the subtraction of the number of cubic 

 centimeters of one-half normal acid used, multiplied by 1.268, 

 gives the quantity of P 2 O 5 in milligrams. 



145. Comparison of the Methods of Weighing and Titrating 

 the Yellow Precipitate. Baxter gives the result of his experi- 

 ments upon the estimation of phosphoric acid by weighing the 

 yellow precipitate after heating to 300 and also by titration with 

 standard alkali. 21 



In securing the yellow precipitate in a form sufficiently pure 

 for analytical purposes he states that it can not be washed with 

 water owing to decomposition. Ten per cent, ammonium nitrate 

 solution is therefore used as a wash, but no data are given to 

 prove that washing was carried to the complete removal of the 

 molybdate. Indeed, all the results show that the precipitate still 

 contains considerable quantities of ammonium molybdate or 

 molybdenum oxid. It is stated that the formula of the precipi- 

 tate washed with the ammonium nitrate solution and dried at 300 

 is (NH 4 ) 3 PO 4 .i2MoO 3 , but owing to the occlusion of molyb- 

 dic acid the theoretical percentage of P 2 O 5 (3.783) is never ob- 

 tained. The precipitate only contains 3.742 per cent, of phos- 

 phoric acid. 



In the second article evidence is given to show that the washed 

 unheated yellow precipitate has the formula (NH 4 ) 2 HPO 4 .~ 

 i2MoO 3 , and that the quantity of hydroxid required in titration 

 corresponds much more nearly to 48 molecules than to 46 mol- 

 ecules for each molecule of phosphorus pentoxid. Owing to the 

 occlusion of some molybdic acid the author gives the following 

 exact formula as that of the unheated washed yellow precipitate 

 actually obtained in analytical work (NH 4 ) 2 HPO 4 .i2.i43MoO 3 . 



Attention is also called to the fact that the composition of the 

 precipitate is more constant and the results more reliable when 

 11 American Chemical Journal, 1902, 28 : 298 ; 1905, 34 : 204. 



