45 



Ferrous sulphate did not increase the citric acid solubility of 

 the phosphates under investigation. In fact with the tri-calcium 

 the effect was to lessen the solubility. 



The remarkable effect of ferrous sulphate when used with 

 superphosphate as was the case in the field tests made by Grif- 

 fiths, indicated that the substance converted the phosphoric acid 

 into forms more available than the reverted phosphate formed 

 from ferric hydroxide. 



The precipitate formed on the addition of superphosphate to 

 ferrous sulphate was found to be easily soluble in i per cent citric 

 acid. In order to determine whether this compound is formed in 

 the presence of ferric hydroxide, 2 grams of the latter were 

 mixed with 0.5 gram of ferrous sulphate and 50 c.c. of super- 

 phosphate solution. Of the reverted phosphate of iron formed 

 from ferric hydroxide without the sulphate, 17.3 per cent of the 

 phosphoric acid was soluble in i per cent citric acid, while the 

 compound formed when ferrous sulphate was present showed 

 19.0 per cent citric acid solubility. 



It is improbable therefore that ferrous sulphate can be of 

 any benefit as a retentive agent for phosphoric acid, especially 

 when there is sufficient ferric hydroxide present. 



