Dissociation constants of the monoprotic ferrate( VI ) ion in NaCl media
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Fe(VI) is a powerful oxidant that can oxidize a variety of inorganic and organic substrates. The values of dissociation constants for protonated Fe(VI) species are critical for the interpretation of kinetic data of oxidation reactions at different pH. The constants (pK3*) for the dissociation of HFeO4- (HFeO4- ⇔ H+ + FeO42- have been determined in NaCl solutions from I = 0.0 to 2.5 mol (kg H2O)-1 at 5,25 and 45°C for the first time from spectral measurements obtained by a stopped-flow kinetic technique. The results were fitted to the equation (σ = 0.074 in pK3*) pK3* = 4.247 + 888.5/T + 0.8058I1/2 + 0.5144I - 529.43I1/2/T where I is the ionic strength and T is the absolute temperature (K). This equation gives a value of 7.227 ± 0.074 for pK3 at 25°C. The enthalpy of dissociation for HFeO4- is given by δH3* (kJ mol-1) = 17.0 - 28.9 I1/2. The pK3* results were used to estimate Pitzer parameters (β0, β1 and Cφ) for the interaction of Na+ with HFeO4- and FeO42-. The values of B0, B1 and C for the difference in the Pitzer coefficients of NaHFeO4 and Na2FeO4 as a function of temperature (t, °C) are given by (σ = 0.07 in pK3*) B0 = 2.10948 - 4.462 × 10-2 t; B1 = -6.76369 + 8.309 × 10-2 t; C = -0.51051 + 9.86769 × 10-3 t. These coefficients can be used to estimate the values of pK3 in natural waters composed mostly of NaCl.