140 Analysis and Valuation of Manures [227 



required approximately, the superphosphate, after drying, 

 may be transferred to a weighed platinum dish, ignited over an 

 Argand in a draught cupboard until it has lost its dark colour, 

 then heated over the blow-pipe until it ceases to fume. It is 

 then cooled in a desiccator and weighed. The loss is due to 

 organic matter, sulphuric acid, and changes in the composition 

 of the phosphates. In almost all cases the loss due to sul- 

 phuric acid and the changes in the phosphates amounts to half 

 the weight of soluble phosphate calculated as CaP 2 6 . If this 

 amount, therefore, be subtracted from the total loss, a very 

 close approximation to the amount of organic matter will be 

 obtained. l 



227. Soluble Phosphoric Acid.— A portion of the 

 powdery sample is selected as described in the chapter on 

 sampling, Part IV., paragraph 119, and placed in an iron mortar. 

 It is banged with the pestle until a smooth, pasty mass is 

 obtained. This takes two or three minutes. Of course it may 

 happen that a very dry sample has been obtained, in which 

 case it will not become pasty. In such a case it must be 

 remembered that the object of triturating in an iron mortar is to 

 break down all hard lumps and to render the mass as homo- 

 geneous as possible. When the mass is thoroughly mixed, a 

 portion is taken out with a spatula. It takes some little practice 

 to enable the operator to take off about the right quantity, but 



1 When soluble phosphate of lime is strongly heated with calcium 

 sulphate the following reaction takes place : 



CaH 4 (P0 4 ) 2 + CaS0 4 = Ca 2 H 2 (P0 4 ) 2 + H 2 S0 4 , 



of which the H 2 S0 4 is volatilised — i.e., one molecule of CaH 4 (P0 4 ) 2 

 which is calculated in the analytical result as CaP 2 6 [CaH 4 (P0 4 ) 2 — 2H0O] 

 will liberate one molecule of H 2 S0 4 . Or, in figures, 198 parts by weight 

 of CaP 2 6 will liberate 98 parts of H 2 S0 4 . Hence the change in the 

 phosphates and loss of H 2 S0 4 on heating a superphosphate amounts to 

 ^, or about half the weight of the soluble phosphate calculated as CaP 2 6 . 



