Solubility of CaHPO4 2H2O and formation of ion pairs in the system Ca(OH)2 - H3PO4 - H2O at 37.5 C

Abstract

The solubility isoth erm for CaHP04 • 2H,O (DCPD) in the three-component system Ca(OH), -H3P04 -H,O was determined in the pH range 3.5 to 6.8 by leaching a thermostated column of DCPD with dilute phosphoric acid solutions. In confirmatory experiments, equilibrium was approached both from s uper-and under-saturation by shakin g DCPD with appropriate solutions. The calcu lated ioni c activity product (Ca + +) X (HPO ,), appeared to be a parabolic function of pH with a minimum near pH 5.0. The pH dependence of the ionic product could be accounted for by co nsid erin g th e ion pairs [CaHP04] O and [CaH,P04]+ as semi-empirical parameters, Under the co ndition of saturation with respect to DCPO , the activity of the pair [CaHPO,]O mu st be a consta nt. The act iv ity of th e species [CaH,PO,]+ was shown to vary direc tl y with hydrogen ion activity. The activiti es of the two ion pairs were adjusted to give a set of pH-independen t ionic activit y produ cts with a mean of 2.19 ± 0.011 X 10 -'. The stabilit y consta nts for [CaHP04]0 a nd [CaH,PO,] + are 5.88 ± 0.031 X 10' and 7.49 ± 0.039, respectively. Experiments were conducted to study the hydrolysis of DCPD to more basic calcium phosphates and the kin etics of these transformations is discussed. The significance of the ion pairs in hum an se rum is co nsidered.Key Words : Solubilit y of calcium monohydroge n ph osp hate dihydrate at 37.5 °C, calciumph os phate ion-pairs, solubility of di calcium phosphate dih ydrate , so lubilit y of dibasic calc ium phosphate dihydrate .

Preparation and solubility of hydroxyapatite

“…After two weeks, the suspension was transferred to a thermostated saturator, described in an earlier publication [9], in which the pH of the clear filtered solution was measured. Samples of this solution were taken for calcium and phosphorous analyses.…”

“…The solubility product constants , where parentheses denote ionic activities, were calculated by a procedure very similar to th e one described elsewhere [9]. The values are listed in tables 1, 2, and 5.…”

“…The iterative proc edure, starting with unitar~ activity coefficients, reported in previous publicatIOns [9,18] was us ed for these calculations. The activity coefficients Ix and IH po were taken to be unity, and it was assumed that +y3 = 4j, _. respectively, along with the values of KSfJ calculated from these results using the activity product that appears as the leading term in eq (2).…”

Preparation and solubility of hydroxyapatite:

“…Initially, 9 liters of N ammonium acetate solution were boiled in the reaction vessel for about 12 hours to remove C0 2 ? then ammonia gas was passed in until the pH reached a value between 8.5 and 9 , and the pumping of the phosphate and calcium solutions was started, maintaining vigorous boiling.…”

“…The pH was maintained between 8*5 and 9.5 (measured at room temperature) by an adequate flow of NHg until the total volume of the solutions was pumped into the reaction vessel; then the flow of NRj was discontinued but boiling was continued for about three hours. The solid was allowed to settle, the supernatant liquid was withdrawn, about 9 liters of boiled distilled water were introduced and the mixture was boiled for about two hours. This washing operation was repeated as ' …”