220 SOILS AND MANURES 



This change is known to occur when a solution of 

 monocalcic phosphate is heated, and also at the ordinary 

 temperature on prolonged standing, and it is quite pos- 

 sible that it may occur in superphosphates if allowed to 

 become damp or heated. 



Keversion may also be caused by the interaction of 

 monocalcic phosphate with salts of iron and aluminium 

 chiefly sulphates formed during the process of manufacture 

 of the superphosphates, thus 



CaH 4 (P0 4 ) 2 + A1 2 (S0 4 ) 3 = 2A1P0 4 + CaS0 4 + 2H 2 S0 4 



Monocalcic Aluminium Aluminium Calcium Sulphuric 

 phosphate. sulphate. phosphate, sulphate. acid. 



Pyrites and silicates of iron and aluminium, if present 

 in the raw material, are not attacked by the sulphuric 

 acid or subsequently acted upon by the monocalcic phos- 

 phate in the superphosphate, and are therefore harmless. 

 But oxides and phosphates of iron and aluminium, if 

 present in the raw phosphates, are converted into sul- 

 phates by the action of the sulphuric acid and afterwards 

 cause the soluble phosphate to revert as above described. 



This is probably the principal cause of reversion. It 

 is for this reason that native phosphates which contain 

 much iron and alumina, e.g., Bedonda phosphate, are 

 deemed unsuitable for the manufacture of superphos- 

 phates. Samples which are destitute of compounds of 

 iron and alumina or which contain only the silicates of 

 these bases, or pyrites, if dry, can be kept for long periods 

 without reversion taking place. The reversion of the 

 soluble phosphate to the insoluble state greatly reduces 

 the value of the superphosphates. The reverted phosphate 

 is, however, more readily soluble than the original tricalcic 

 compound, and has consequently a higher agricultural 

 value. It is largely soluble in a neutral solution of ammo- 

 nium citrate, and is often called the citrate soluble 

 phosphate. 



