70 PRACTICAL AGRICULTURAL CHEMISTRY [237~239a 



237. TABLE III.-FOR THE EXAMINATION OF 

 THE PHOSPHATE PRECIPITATE 



The AmHO precipitate may contain FeP0 4) AlP0 4 , 

 Ca 3 P 2 8 , MgNH 4 P 2 8 



Dissolve the precipitate in hot dilute HC1. To the solution add 

 Am^COjj solution, drop by drop, until the precipitate first formed re- 

 dissolves with difficulty (238). Add a fair quantity of a mixture of 

 acetic acid and ammonium acetate solution, and then add FeCl 3 until 

 the liquid becomes reddish ; warm and filter. 



Filtrate may contain 

 Ca and Mg- 



Add AmHO in excess, filter 

 (239) and add Am. 2 C 2 4 solu- 

 tion ; warm and filter 



Precipi- 

 tate will 

 be CaC,0 4 



showing 



presence 



of Ca 



Filtrate may 

 contain Mg- 

 Add Na 2 HP0 4 

 solution and 



shake ; a white 

 crystalline precipi- 

 tate shows 

 Presence of Mg 



Precipitate will contain FeP0 4 

 and possibly A1P0 4 



Heat precipitate with K.HO solu- 

 tion ; filter 



Filtrate may 

 contain A1P0 4 

 dissolved in 

 KHO 



Add HC1 in ex- 

 cess, then AmHO 

 in very slight 

 excess ; a white 

 gelatinous precipi- 

 tate shows 

 Presence of Al 



Precipitate will 

 contain FeP0 4 , 



which reject, since 

 iron will always 

 be found at this 

 stage owing to 

 FeCL; having been 

 added (239a). 



EXPLANATION OF THE PHOSPHATE TABLE 



238. — The separation of the above phosphates depends on the 

 following facts : — 



The phosphates of iron and aluminium are insoluble in acetic acid, 

 whereas those of calcium and magnesium are soluble. Hence the 

 HO solution of the phosphates is altered by the addition of HA and 

 AmA into an acetic acid solution, when A1P0 4 and FeP0 4 precipitate. 

 The FeCL, is added for the purpose of precipitating the remainder (if 

 any) of the phosphoric acid as FeP0 4 , and so leaves the Ca and Mg as 

 acetates, which are then tested for by the usual reagents. 



239. — It is usually necessary to filter at this stage, since a little 

 iron may be left in the solution. The AmOH would then cause a 

 brown precipitate, which is neglected. 



239a. — This precipitate of FeP0 4 is neglected here, since FeCL, has 

 been added. Iron should be tested for in the original solution by 

 tests (165 and 171). It should be noted that the metals Ca and Mg 



