SOLUBILITY OF KFE(CRO4)2·2H2O AT 4-75°C

Abstract

The solubility of KFe(CrO4)2·2H2O, a precipitate recently identified in a Cr(VI)-contaminated soil, was studied in dissolution and precipitation experiments. Ten dissolution experiments were conducted at 4–75°C and initial pH values between 0.8 and 1.2 using synthetic KFe(CrO4)2·2H2O. Four precipitation experiments were conducted at 25°C with final pH values between 0.16 and 1.39. The log KSP for the reactionat 25°C is −19.39±0.13. From this measured solubility product, the free energy of formation of KFe(CrO4)2·2H2O, ΔG0f,298, is −2348.0±5.6 kJ mol−1. For a constant heat capacity for the reaction, the log KSP over the range of temperature of 277 to 348 K is given by log KSP=331.476−16847.6/T−119.002 log T, where T is the temperature in K. Based on this temperature dependence of the solubility product, the enthalpy of reaction at 25°C, ΔH0r,298, is 27.54±1.38 kJ mol−1, the entropy of reaction, ΔS0r,298 is −280.8±4.8 J mol−1 K−1, and the heat capacity of the reaction, ΔCp,r, over the temperature range of the experiments is −989±115 J mol−1 K−1. The measured solubility indicates that KFe(CrO4)2·2H2O is stable under conditions of low pH and high Cr(VI) and K+ concentrations. Such conditions are likely to be found in the immediate vicinity of a release of acidic, chromate-rich solutions.