Dissociation constants of the monoprotic ferrate(VI) ion in NaCl media

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 (pK(3)*) for the dissociation of HFeO4- (HFeO(4)(-)double left right arrowH(+)+FeO42-) have been determined in NaCl solutions from I=0.0 to 2.5 mol (kg H2O)(-1) at 5, 25 and 45 degreesC for the first time from spectral measurements obtained by a stopped-flow kinetic technique. The results were fitted to the equation (sigma =0.074 in pK(3)*) pK(3)* = 4.247 + 888.5/T + 0.8058I(1/2) + 0.5144I - 529.43I(1/2)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 pK(3) at 25 degreesC. The enthalpy of dissociation for HFeO4- is given by DeltaH(3)* (kJ mol(-1))=17.0-28.9 I-1/2. The pK(3)* results were used to estimate Pitzer parameters (beta (0), beta (1) and C-phi) for the interaction of Na+ with HFeO4- and FeO42-. The values of B-0, B-1 and C for the difference in the Pitzer coefficients of NaHFeO4 and Na2FeO4 as a function of temperature (t, degreesC) are given by (sigma =0.07 in pK(3)*) B-0=2.10948-4.462x10(-2) t; B-1=-6.76369+8.309x10(-2) t; C=-0.51051+9.86769x10(-3) t. These coefficients can be used to estimate the values of pK(3) in natural waters composed mostly of NaCl.