Solubility of KFe(CrO 4) 2·2H 2O at 4–75°C

Abstract

The solubility of KFe(CrO 4) 2·2H 2O, 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(CrO 4) 2·2H 2O. Four precipitation experiments were conducted at 25°C with final pH values between 0.16 and 1.39. The log K SP for the reaction KFe(CrO 4) 2·2 H 2 O⇄ K ++ Fe 3++2 CrO 2− 4+2 H 2 O at 25°C is −19.39±0.13. From this measured solubility product, the free energy of formation of KFe(CrO 4) 2·2H 2O, Δ G 0 f,298 , 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°C, Δ H 0 r,298 , is 27.54±1.38 kJ mol −1, the entropy of reaction, Δ S 0 r,298 is −280.8±4.8 J mol −1 K −1, and the heat capacity of the reaction, Δ C p, r , over the temperature range of the experiments is −989±115 J mol −1 K −1. The measured solubility indicates that KFe(CrO 4) 2·2H 2O 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.