Solubility of CaHPO4 2H2O in the system Ca(OH)2-H3PO4-H2O at 5, 15, 25, and 37.5 C

Abstract

Solubility isoth e rm s fo r Ca HPO ,· 2H 20, dical c ium phosphate dihydrate, DCPD , in th e tern ary sys te m Ca(OH ), -H"PO, -H2 0 were dete rmin ed at 5, 15, 25, a nd 37.5°C in th e pH range 3.5-7; th e relativ e pos iti ons of th e isotherm s indi cate th at DCI'D has a negati ve th e rmal coefficie nt of so lubility. Th e so lubility produ ct, K", of DCI'D an d th e stab ility co nstants Kx and Ky for th e ion pairs [Ca HPO~] and [Ca H2PO:t], res pectively, we re obtain ed as fun cti ons of te mpe ra ture by the use of a ge neralized leas t squares procedure subj ec t to three conditi on fun cti ons -constan cy of the solubili ty product, elec tri c al ne utralit y in the solution, and congru e nt di ssoluti on of the so lid. Th e equations ob tain ed areIn In Kx= 51090/7'-341.14 + 0.58807'The existe nce of a max imum in Ks in the neighborhood of 25°C is plau s ibl e on the basis of availabl e th e rmodynamic da ta for DCI'D. T he rmodynam ic fun cti ons are reported for th e solution of DCPD a nd for th e assoc iation of th e ion pairs. Ke y word s : Calcium phosp hate ; d icalcium phosp hate dihydrate; ion pa irs; solu bili ty ; solubility isothe rm s; solubility produ ct.

Solubility of Ca5{P04)3OH in the System Ca{OH)2-H3P04-H20 at 5, 15, 25, and 37 °C

“…Calcium, phosphorus, and pH were determined using th e methods and instrume nts described in earlier publications [11,15]. The estimated errors for Ca (1.5%) , P (1.0%), and pH (0.015) guided the assignment of weights (sec.…”

“…Details of the method used for the calculation of solubility products and ion pair association constants by generalized least squares have been reporte d previously [11]. The spec ific equations and conditions needed to describe equilibrium systems containing solid Ca5(P04hOH are given here.…”

“…The spec ific equations and conditions needed to describe equilibrium systems containing solid Ca5(P04hOH are given here. Curly brackets will be used to denote equation, sec tion and table numbers used in Reference [11]. The purpose of these calc ulations was to obtain the solubility product, Ks, of Ca5(P04hOH as a fun ction of temperature, allowing for the presence of ion pairs, and to compute the adjusted values of the observations.…”

“…The quantities gCa, k3 , and N are defin ed in the eq uations of [11] already referred to; (H +) = lO-PH , and T is the sum of ion pair concentrations, eq {8}. The apparent solubility product K' HA was calculated taking T to be zero.…”

“…In addition to the work re ported here, we have re-examined solubility data obta ined a t 25°C in other studies [8][9][10] of hydroxyapatites pre pared in a manne r similar to the one used he re. The adjustm ent procedure [5,[11][12][13][14][15] used in treating the d ata is unusually ri gorous and provides a superior assessme nt of the re i iability of the de rived constants .…”