ESTIMATION OF PHOSPHORIC ACID 165 



the phosphate solution is added to the molybdic solution than 

 the reverse. 



There is a number of other minor well known discoveries in 

 this article which need not be mentioned here. The author seems 

 to be entirely unaware of the work of the Association of Official 

 Agricultural Chemists on this subject. 



The conclusions seem to be applicable to the conditions under 

 which the author worked, but criticism might be made that the 

 precipitate was not sufficiently washed in any of the experiments, 

 and further, that the quantity of precipitate is very much larger 

 than it is customary to use when working this method, weighing 

 as it does almost three grams. The value of all the work de- 

 pends on the washing of the yellow precipitate, which, as has been 

 said, was incomplete, and no evidence is given to show that im- 

 purities could not have been entirely removed. 



146. Estimation of Phosphoric Acid as a Lead Compound. 

 In the volumetric lead method, as described by Wavelet, the 

 phosphoric acid is precipitated by the magnesium citrate solu- 

 tion as in the uranium method of Joulie, as practiced by the 

 French chemists, and the washing of the precipitate and its solu- 

 tion of nitric acid are also conducted as in that method. 22 After 

 solution in nitric acid, ammonia is added to neutrality and the 

 solution is then made acid with acetic. The phosphoric acid is 

 precipitated in the acid solution by a standard solution of lead 

 nitrate, the precipitate having the formula P 2 O 5 3PbO. 



The end reaction is determined by placing a drop of the 

 titrated mixture on a white greased dish in contact with a drop 

 of a five per cent, solution of potassium iodid. When all the 

 phosphoric acid is precipitated, the least excess of the lead salt 

 is revealed by the characteristic yellow precipitate of lead iodid. 



The author of the process claims that the lead phosphate is in- 

 soluble in the excess of acetic acid, and that the phosphate itself 

 does not give any yellow coloration with potassium iodid. The 

 process is quite as exact as the uranium method and the end 

 reaction is far sharper ; the standard reagents are easily made 

 and preserved. 23 The method described merits, at least, a com- 



21 Repertoire de Pharmacie, 1893 [3], 5 : 153. 



M Revue de Chimie analytique applique"e, 1893, 1 : 113. 



