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 are In Ks=-8403.5/ T +41.863-0.096787' In Kx= 51090/7'-341.14 + 0.58807' In K y = 19373/7'-122 .81 + 0.19947'. 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.