SOME FACTORS INFLUEXCINi i rill': S( )LLi;iLITV OF 

 PHOSPHORIC OXIDE IX MIXI-.I) Fl^RTILISEkS 



coxT Aixixc suPERPHosiMi \ri-:s. 



By Edmund Victor Flack. 



Sui)erplK)S])hates are produced l)y treating bones or ph-is- 

 l)liatic minerals with sul])huric acid. The reactions wliicli take 

 ])lace may be summarised as follows: 



*Ca,P.,0, -h 2H2SO, =- 2CaSO, + H,Ca P/). 

 Tricalcium Sulphuric Gyi)sum. Super])hos])hate. 



phosphate. acid. ( Alonocalcium 



phosphate. ) 



It is well known that the water-soluble phosphoric oxide 

 ccntent of su])er])hosphate j^radually decreases in amount, and 

 this decrease is known as " reversion " or " retrosrression." The 

 longer the super])hosphate is kept, the greater will be the amount 

 of reversion, and the rate of reversion varies with the process of 

 manufacture. In some superphosphates there may be practically 

 no reversion, whereas in a badly-made article, in which unsuit- 

 able materials have been employed, it may be considerable. Thus 

 a mineral containing more than 10 per cent calcium carbonate, 

 3 i)er cent of oxide of iron and fluorides, would be very unsuit- 

 able.! Insufficiency of acid in the material used for preparation 

 also causes C(^nsiderable reversion. Such reversion may be due 

 to any undecom]3osed tricalcium phosphate, or to calcium car- 

 bonate, iron oxide or alumina in the mixture acting on the 

 water-soluble phosphate. In the case of the first-mentioned com- 

 pound the reaction may possibly be very slight, and would be 

 represented by the following equation : 



CagPoOs + H,Ca P,Os = 2HoCa,, P,0,. 

 Tricalcium Alonocalcium Bicalcium phosphate, 



phosphate. phosphate. Reverted or retrograde 



phosphate. 



When iron is present as pyrites or ferrous silicate in the 

 mineral used, there is no reversion, but when present as ferric or 

 ferrous oxide, the reversion is great. What actually takes place 

 is not so clear as when tricalcium j^hosphate is present. If 

 the acid used had dissolved a quantity of iron, the reaction of 

 the resulting ferric sulphate with the superphosphate would pro- 

 bablv be as under : 



Fe,{SO,), -f H,Ca P,0. = 2Fe,P,0, + CaSO.. + 2ih,S(), 

 Ferric Super- Ferric Calcium Sulphuric 



sulphate. phosphate. phosphate, sulphate. acid. 



*H. W. Wiley: "Principles of Agricultural Analysis." 2, 277. 

 ■f Bulletin Imperial Institute, 9 fil. 



