§ Gl. PHOSPHORIC ACID. 85 



place ; according to Kubel ( Versuchs Stationen, 10, 123) 

 there is a lo3s of magnesia when the i^recipitated plios- 

 phate is dissolved and re-precipitated. 



b. In the presence of alkaline earths, alumina, ferric 

 oxide, and manganous oxide, phosphoric acid is best de- 

 termined indirectly, by precipitation as aynmoniG phospho- 

 molyhdate. If silica is present, it must first be removed 

 by evaporation to dryness in the usual manner (§ 58, «, 1). 



. To the solution, free from silicic acid, add the solution 

 of ammonic molybdate containing an excess of nitric 

 acid, whose preparation is described in § 3, /, and 

 which, if made as there directed, contains 5° !„ of molyb- 

 dic acid, in such a quantity that the amount of molybdic 

 acid added shall be from 40 to 60 times as great as that 

 of the phosphoric acid supposed to be in the solution ; 

 since the molybdic acid must be so largely in excess, it is 

 well to take a quantity of the solution of phosphate that 

 contains not over 0.1 grra. of the acid, and the solution 

 should be tolerably concentrated. Digest the mixture 

 from 12 to 24 hours at a temperature of about 40° C. ; then 

 take out a small sample of the clear liquid Avith a pipette, 

 mix it in a test-tube with its volume of ammonic molyb- 

 date, and heat the mixture gently for an hour or more. 

 If more of the precipitate appears, rinse the test-tube into 

 the beaker again, add more ammonic molybdate, digest 

 12 hours longer, and repeat the test. Not until the mix- 

 ture remains perfectly clear in this test may the precipita- 

 tion be considered as finished. 



Collect the precipitate on a small filter, rinse the beaker 

 out with portions of the filtrate, and wash the contents 

 of the filter with a mixture of 100 parts of the solution of 

 ammonic molybdate, 20 parts of nitric acid (Sp. Gr. = 1.2), 

 and 80 parts of water {Fres. Zeitschrift 6, 405), until, in 

 case lime was present, the filtrate gives no turbidity in 

 strong alcohol to which sulphuric acid has been added. 

 Dissolve the precipitate in the smallest quantity of am- 



