132 The Citric Solxbility of Mineral P/iosphates 



Experiment 6 of Table IV is an experiment identical in character with 

 the private test already mentioned (see Table I). The results of experi- 

 ments parallel to this experiment were used by certain sellers to describe 

 the citric solubility of the mineral phosphate put on the market. It 

 will be seen that Exp. 6 falls naturally into its place among the experi- 

 ments given in Table IV. The amount of phos])hate dissolved amounts 

 to (52-2 per cent, of the weight of mineral pliospliatc tested while the 

 same phosphate, analysed in accordance with the official test (Table III, 

 Ex]). 1) shows a citric solubility of 19-24 per cent. If we consider the 

 proportion dissolved in relation to the total amount of phosphate present 

 expressed as tricalcic phosphate it is found that, by the official method 

 29-83 per cent, of the total Ca^P.^Og content is dissolved, while by the 

 private test (at great dilutions, see Table IV) 95-8 per cent, of the total 

 CagP-^Og content was dissolved. These results show what might naturally 

 be expected, namely, a much higher solubility of mineral phosphate in 

 the experiments where the constant ratio m^jni^ = 10 (Table IV) was 

 used than where mjm.^ = 2 (Table III) was used. In other words if 

 mjm2 is nuide large enough we should reach the limit of 100 j)er cent, 

 citric solubility for all very high dilutions. 



The results in Table II show that, using the official test on both slags 

 and iniueral phosphates, slags generally show a higher citric solubility 

 tliaii mineral phosphates. If the uiiodicial test (Exp. (1. Tal>le IV) was 

 universally applied to slags we should have similar liigh solubility figures, 

 in other words the citric solubility at high dilution would be practically 

 100 per cent, and the only item of information whicii would be valuable 

 to the purchaser would be the actual proportion of phosphate, expressed 

 as tricalcium phosphate, present in the fertihser. In some slags the citric 

 solubility in terms of total phosphate content is as much as 90 per cent. 

 (see Table I) and therefore the actual increase on dilution must be 

 necessarily small compared with the increase in citric solubility on 

 dilution of any mineral phosphate. 



The undernoted table (Table V) shows the average composition of 

 five commercial mineral pho.sphates^. The sixth (Egyptian) was analysed 

 in my laboratory. 



Since calcium carbonate is present in varying ))roportions in com- 

 mercial mineral phosphate it is clear that with a constant initial molecular 

 concentration of citric acid, varying quantities of citric acid will be 

 available to attack the insoluble phosphate. For example, suppose we 

 selected two dift'erent varieties of mineral phosphate, ground to the same 

 1 Robertson, J.S.C. I. ,35, p. 218. 



