The thermodynamic formation constants for iron(III) thiocyanate complexes at zero ionic strength

Abstract The successive thermodynamically relevant standard formation constants for Fe(SCN) 2+ ( K 1 0 ) and Fe(SCN) 2 + ( K 2 0 ) have been determined at 25 °C using absorbance spectra at time zero to reduce the impact of kinetic instability. Average ionic strengths of 0.0581 M, 0.0955 M, 0.1756 M, 0.2506 M, 0.5004 M, and 1.0011 M were used in the study to enable extrapolation to zero ionic strength. Extrapolation to zero ionic strength was supported by the application of the Specific Ion Interaction theory (SIT) developed by Bronsted, Guggenheim and Scatchard. Application to the iron(III) thiocyanate system led to a log K 1 0 value of (2.85 ± 0.08) and a log K 2 0 value of (1.51 ± 0.13). The change in the empirical specific ion interaction coefficients associated with K 1 0 , Δ e 1 , is (−0.29 ± 0.16), and that associated with K 2 0 , Δ e 2 , is (−0.18 ± 0.25). The log K 1 0 value is lower than those published in the literature due to the fact that, for this study, the equilibrium data were based on initial spectra captured before kinetic instability significantly interfered.