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^*)

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 I^1/2. The pK₃^* results were used to estimate Pitzer parameters (β⁰, β¹ and C^ϕ) 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 × 10⁻²t; B¹ = − 6.76369 + 8.309 × 10⁻²t; C = − 0.51051 + 9.86769 × 10⁻³t. These coefficients can be used to estimate the values of pK₃ in natural waters composed mostly of NaCl.