THE SOLUBILITY OF CALCIUM PHOSPHATE I. THE EFFECT OF pH AND OF AMOUNT OF SOLID PHASE BY ISIDOR GREENWALD

In 1937, Logan and Taylor (1) came to the conclusion “that the bone salts cannot precipitate spontaneously from the bone plasma unless the ion product is increased. Once formed, the bone salts cannot dissolve unless the concentrations of the ions composing them are decreased below the concentrations found in the blood plasma.” This conclusion was based upon some observations which seemed to Logan and Taylor to prove that the apparent solubility product, [Ca++13[P04-12, depended on the amount of the solid phase, being greatest with small amounts and decreasing with larger amounts. The solid phases they employed were precipitated calcium phosphate and powdered bone. Such a change of ion product, at equilibrium, with the amount of solid phase is, of course, quite remarkable and, in order to explain their results, Logan and Taylor resorted to as remarkable an explanation. We will discuss their experiments and their explanation later. A recent paper by Klement and Weber (2) leads to a quite different conclusion from that of Logan and Taylor. Klement and Weber prepared a hydroxyapatite by mixing dilute solutions of CaiOHlz and of H3P04 in the equivalent proportions. Their preparation was, as they stated, assuredly free of foreign ions, but there is no evidence that it was a single chemical individual. No such claim was made for it. The precipitate was filtered and dried. Weighed quantities of from 100 to 1200 mg. were added to 250 ml. of conductivity water in paraffined flasks and equilibrated for 24 hours at 25”. The amounts of Ca dissolved varied from 3.6 to 11.0 mg. per liter and those of P from 8.6 to 32.2 mg., increasing regularly with the amount of solid phase. Upon the basis of this experiment, Klement and Weber deny the significance of the solubility product [Ca++13 [POa]“. However, it may well be objected that 24 hours may not have been sufficient for the attainment of equilibrium. We have been engaged in a study of the influence of various substances upon the solubility of calcium phosphate. In the course of this work we found that this solubility, whether it be expressed as [Ca++13 [POh12 or, as we prefer, as [Ca++13 [HPO,=]z/[H+]“, is not constant,’ but varies with the