84 § 61. BASES AND ACIDS WITH KEAGENTS. 



When a very small quantity of a solution of a phos- 

 phate is added to a considerable quantity of a solution of 

 amnionic molybdate, containing an excess of nitric acid, 

 a lemon-yellow, pulverulent precipitate is formed, at once 

 or after long standing ; a portion of the precipitate ad- 

 heres strongly to the sides of the tube. This precipitate 

 is soluble in a solution of a phosphate and in ammonia, 

 but is insoluble in dilute nitric acid in the presence of 

 excess of the molybdate. The reaction is exceedingly 

 delicate. 



Quantitatire estimation. — a. In cases where the acid 

 is free or combined with an alkaline metal only, the deter- 

 mination of it may be made as magnesic phosphate, 

 Mo-PO. 



•^-^&2-^ 2^7* 



Neutralize a quantity of the solution of the substance 

 containing not more than 0.2 grm. of the acid with am- 

 monia, if it is acid, and add magnesia mixture {§ 18, h) 

 as long as a precipitate is formed ; 12-15 c.c. of the re- 

 agent will be required for 0.2 grm. of P^O^. ; then add 

 diluted amnionic hydrate containing one part of ammonia- 

 water of 0.96 Sp. Gr. and three of water, imtil the vol- 

 ume of the mixture is about 110 c.c, and proceed further 

 as directed for the treatment of the same precipitate un- 

 der magnesium (§50, a). It contains 63.96° |„ of phos- 

 phoric anhydride, PjO^. 



If in any case the precipitate has a somewhat suspi- 

 cious flocculent appearance, and does not become crystal- 

 line after long digestion, it had better be dissolved in 

 dilute hydrochloric acid on the filter ; evaporate the solu- 

 tion to dryness on the water-bath, treat the residue with 

 dilute hydrochloric acid, and precipitate the phosphoric 

 acid again with magnesia mixture as before. Neverthe- 

 less it is best to avoid the necessity of this re-solution and 

 re-precipitation if possible, by careful attention to the di- 

 rections for removing silicic acid and other substances 

 from the solution at the proper time and in the proper 



