142 The Citric Sohihility of Mineral PJioxphnteii 



phosphate were dissolved (i.e., percentages of mineral phosphate taken). 

 The molecular concentration on the other hand (i.e. number of gram- 

 molecules per litre) decreased with increasing dilution. It is not held 

 that the equations describing the results are valid for concentrations or 

 dilutions outside those used in these experiments. 



(2) If a constant weight of sample and a constant volume are used 

 in a series of e.xperiments, the concentration of citric acid being varied 

 from 1 gram to 10 grams, the citric solubilities varied from 4-4 to 19-4 

 per cent, of weight of sample taken. That is to say, citric solubility in- 

 creased, as we should e.\pe(t, with increased acid concentration at con- 

 stant volume and with a constant weight of hydroxyapatite. 



(3) If a constant weight of citric acid and a constant volume are used 

 in a series of experiments, the quantities of sample being varied from 

 experiment to experiment, the citric solubilities decreased from 19 to 

 1 per cent, of weight of sample taken. These decreases are due (1) to the 

 presence of Ca(0H)2 in the molecule of hydroxyapatite, {'!) to the presence 

 of Ca(0H)2, CaO or CaCOg in the free condition and (3) to the fact that 

 the results are expressed in terms of mineral phosphate taken for analysis. 



(4) It is seen that, other conditions being constant, citric solubility 

 depends upon an unlimited choice of constant values of any two factors, 

 together with an unlimited number of values of the third varying factor. 

 If, therefore, a citric solubility test has to be adopted, the theoretical 

 condition determining the relative quantities of sample, acid and volume 

 must be found, otherwise the test has no practical value. Wagner has 

 not supplied any theoretical basis for selecting the specified constant 

 quantities of the three factors. We can, therefore, at will select for slags 

 suitable values of sample, acid and volume to secure high citric solubility 

 values. We can, however, select at will quite different values of sample, 

 acid and volume which will give equally high citric solubility values for 

 mineral phosphates. Further, we could select for both slags and mineral 

 phosphates suitable values of sample, acid and volume which would 

 give, on the one hand, perfect citric solubihty (100 per cent.) or, on the 

 other hand, no citric solubihty at all. 



(5) The citric solubility of mineral phosphate has been contrasted 

 with the solubility of a pure substance, dicalcium phosphate, in hydro- 

 chloric acid. It is shown that with the dilutions used and with an excess 

 of dicalcium phosphate the reaction is practically complete. Since there 

 is an excess of dicalcium phosphate it has been assumed that the main 

 substance present at the end of the reaction is monocalcium phosphate. 



(6) Citric solubility, if apphed to fertilisers may in a certain degree 



