Solubility of KFe(CrO4)2•2H2O at 4-75°C

The solubility of KFe(CrO4)(2) . 2H(2)O, 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 degrees C and initial pH values between 0.8 and 1.2 using synthetic KFe(CrO4)(2) . 2H(2)O. Four precipitation experiments were conducted at 25 degrees C with final pH values between 0.16 and 1.39. The log K-SP for the reaction. KFe(CrO4)(2) . 2H(2)O reversible arrow K+ + Fe3+ + 2CrO(4)(2-) + 2H(2)O at 25 degrees C is -19.39 +/- 0.13. From this measured solubility product, the free energy of formation of KFe(CrO4)2 . 2H2O, Delta G(f298)(0), is -2348.0 +/- 5.6 kJ mol(-1) For a constant heat capacity for the reaction, the log K-SP over the range of temperature of 277 to 348 K is given by log K-SP = 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 degrees C, Delta H-r298(0), is 27.54 +/- 1.38 kJ mol(-1), the entropy of reaction, Delta S-r298(0) is -280.8 +/- 4.8 J mol(-1) K-1, and the heat capacity of, the reaction, Delta C-p,C-r, over the temperature range of the experiments is -989 +/- 115 J mol(-1) K-1. The measured solubility indicates that KFe(CrO4)(2) . 2H(2)O 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. (C) 1998 Elsevier Science Ltd. All rights reserved.