Solubility of ettringite (Ca_6[Al(OH)_6]_2(SO_4)_3 centre dot 26H_2O) at 5-75 deg C

摘要

The solubility of ettringite (Ca_6[Al(OH)_6]_2(SO_4)_3 centre dot 26H_2O) was measured in a series of dissolution and precipitation experiments at 5-75 deg C and at pH between 10.5 and 13.0 using synthesized material. Equilibrium was established within 4 to 6 days, with samples collected between 10 and 36 days. The log K_(SP) for the reaction Ca_6[Al(OH)_6]_2(SO_4)_3 centre dot 26H_2O mutually implies 6Ca~(2+) + 2Al(OH)_4~-+3SO_4~(2-) + 4OH~-+26H_2O at 25 deg C calculated for dissolution experiments (-45.0+-0.2) is not significantly different from the log K_(SP) calculated for precipitation experiments (-44.8+-0.4) at the 95% confidence level. There is no apparent trend in log K_(SP) with pH and the mean log K_(SP.298) is -44.9+-0.3. The solubility product decreased linearly with the inverse of temperature indicating a constant enthalpy of reaction from 5 to 75 deg C. The enthalpy and entropy of reaction #DELTA#H_r~0 and #DELTA#S_r~0, were determined from the linear regression to be 204.6+-0.6 kJ mol~(-1) and 170+-38 J mol~(-1) K~(-1). Using our values for log K_(SP), #DELTA#H_r~0, and #DELTA#S_r~0 and published partial molal quantities for the constituent ions, we calculated the free energy of formation #DELTA#_(f, 298)~0, the enthalpy of formation #DELTA#H_(f, 298)~0, and the entropy of formation #DELTA#S_(f, 298)~0 to be -15211+-20, -17550+-16 kJ mol~(-1), and 1867+-59 J mol~(-1) K~(-1). Assuming #DELTA#C_(P, r) is zero, the heat capacity of ettringite is 590+-140 J mol~(-1) K~(-1).