96 CHEMISTR V FOR A GRICUL TURAL STUDENTS 



calcic, dicalciCj and tricalcic phosphate. The two first are add 

 salts^ i.e. compounds intermediate between a salt and an acid, 

 and either can be produced by mixing the other with the acid 

 or the neutral salt, as the case may be, in the right proportion. 

 If the solution of monocalcic phosphate be mixed with the 

 liquid containing the precipitate of tricalcic phosphate, and 

 allowed to stand, the dicalcic phosphate will in time be formed. 



Many other acids are able to convert the insoluble phos- 

 phates into the soluble monocalcic phosphate. To one portion 

 of the milky liquid add a few drops of dilute sulphuric acid ; 

 note that the precipitate dissolves. Add potash or lime water ; 

 note that it is reprecipitated. Through the other pass a stream 

 of carbonic acid gas (supplied) ; again note that the precipitate 

 dissolves, and is reprecipitated by potash. Write an equation 

 to represent the action of sulphuric acid on phosphate gf 

 calcium. 



Phosphoric acid is distinguished by two characteristic in- 

 soluble compounds : the phospho-molybdate of ammonium 

 and the double phosphate of ammonium and magnesium. 

 Take a few drops of one of the liquids containing phosphate of 

 calcium, dissolve the precipitate by a drop of nitric acid, and 

 add the liquid to a solution of molybdate of ammonium (the 

 ammonium salt of the acid derived from the higher oxide of 

 the metal molybdenum) contained in a test-tube. On warming, 

 a characteristic yellow precipitate will be formed. To a small 

 portion of the solution of phosphoric acid add a mixture of 

 sulphate of magnesium, chloride of ammonium, and ammonia 

 solutions. A characteristic white crystaUine precipitate of the 

 double phosphate will be deposited. 



In the latter test ammonia is added to neutralise the acid, as otherwise no 

 precipitate would be formed ; the ammonium chloride must then be also 

 added in order to prevent the precipitation of magnesia from the magnesium 

 sulphate, a double salt of ammonium and magnesium being produced, from 

 which magnesia is not precipitated by ammonia. This reaction is also used 

 for the detection of magnesium ; for this purpose, ammonia, ammonium 



