130 The Citric SolnhiUty of Mineral Phosphates 



It is seeu from these results that the hi^'hcr the dilution of citric 

 acid, with mjin.^ kept constant, the greater is the proportion of citric 

 solubility of tho jihosphato expressed as a percentafze of the weifiht of 

 the sample (coluiim 5. Tal)le III). Bassett' considers that the compound 

 usually present in mineral phosphates is hydroxyapatite which may be 

 written |Ca3(P04)2l3C'a(()H)^. Ho also thinks it probable that hydroxy- 

 apatite is the only calcium phosphate that can permanently exist under 

 normal soil conditions. It forms the stable solid phase over a ran^'c of 

 acidity of great practical importance, as it can exist in contact with 

 faintly acid, neutral or alkaline solutions. An attempt has been made to 

 fit a theoretical curve to this series of experiments on the assumptions 

 that there is equilibrium at the end of the experiment and that the 

 following equation represents the reaction : 



Ca03Ca3(P04)2+ .'^HaCeHsO,, H20i^CaH,(PO,)2+ SCaHCgHsO, 



+ 2Ca3(POA+H,p (1) 



Of course other equilibrium equations including dicalcium phosphate can 

 be written from which the same mass action equation can be deduced. 

 The above equation is merely given as a suggestion. 



The sample of mineral phosphate contained a proportion of COj 

 equivalent to 0-G5 gram of calcium carbonate in 5 grams of the sample. 

 Hence 1-36 grams of citric acid would be used up in the formation of 

 citrates, leaving 8-64: grams of acid available to attack the phosphatic 

 compound. If the original acid concentration is taken to be proportional - 

 to the concentration at ecpiilibrium, a constant should be obtained on 

 applying the law of mass action. A good agreement between theory and 

 observation is obtained on this hypothesis. If w = molecular concentra- 

 tion of acid after alkaline lime has been neutrahsed ; m = molecular con- 

 centration of phosphate (expressed as tricalcium phosphate) at the end 

 of 30 minutes agitation then we should have y^jic^ = k. The last two 

 columns (columns 8 and 9, Table III) show the observed molecular con- 

 centrations and the theoretical values on the basis of above equation. 

 A good fit is also obtained using the equation u*l{w — u)\w — 2m) wIumi 

 the theoretical values are found by Horner's method. Diagrams 1 and 

 2 show in graphical form the results of Table III. 



' Trans. Chem. Soc. 1017, 111. 



- The values of u from the equation m*'(wi - 3«)' show ureater cliver<;ence.s from observa- 

 tional values than the values obtained from either of the equations civen in the text. Tho 



ratio ^ varies from -72 to -SO, showint; that the original concentration is nearlv 



w 



proportional to concentration at the end of 30 minutes shaking. Whatever the reason, the 

 formula u'jiv^ gives by far the best fit to the results. 



