THE TEMPERATURE-DEPENDENCE OF THE SOLUBILITY PRODUCT CONSTANT OF VIVIANITE

Vivianite Fe-3(PO4)(2).8H(2)O is an important phosphate mineral in many natural and environmental aquatic systems. Yet, the solubility product constant of vivianite has been determined only at room temperature. A new apparatus has been designed to facilitate the study of redox sensitive elements. Using this apparatus the solubility product constant of vivianite has been determined from 5 to 90 degrees C. The equilibrium aqueous model MINTEQA2 used for data interpretation assumes the formation of all possible aqueous complexes. The temperature dependence of the negative logarithm of vivianite solubility (pKsp) product can be described between 5 to 90 degrees C by pK(sp) = -234.205 + 12,242.6/T + 92.510 log(10)T, where T is temperature in Kelvin, yielding log K-sp = -35.767 +/- 0.076 at 25 degrees C. Using this expression, the calculated Values of Delta G degrees, Delta H degrees, Delta S degrees, and Delta C-p degrees, at 25 degrees C and 0.969 atm total pressure are 204.1 kJ mol(-1), 5.05 kJ mol(-1), -667.8 J K-1 mol(-1), and -769 J K-1 mol(-1), respectively.