A new approach to the equilibrium study of iron(III) thiocyanates which accounts for the kinetic instability of the complexes particularly observable under high thiocyanate concentrations

• FeSCN2+ and Fe(SCN)2+ kinetically unstable.
• Determine initial spectrum at time zero for different SCN− concentrations.
• Global analysis produces refined equilibrium constants and molar absorbance data.
• Equilibrium constants are lower and molar absorbance data higher than literature values.

The kinetic instability of iron(III) thiocyanate complexes in aqueous solution, evidenced by the loss of solution colour with time, has made it difficult to determine reliable equilibrium data. A technique, which measures the absorbance of these solutions as a function of time within milliseconds of mixing Fe3+ with SCN− and which extrapolates to an initial spectrum at time zero, has been designed to overcome these difficulties and is described in this paper. Two complexes, Fe(SCN)2+ and Fe(SCN)2+, have been identified upon analysing the absorbance data for thiocyanate concentrations up to 0.25 M at 25 °C and an ionic strength of 0.5 M. Equilibrium constants, K1 = (98±1)/M-1(98±1)/M-1 and K2 = (7±1)/M-1(7±1)/M-1 have been determined and comparisons made with the literature.

The graphic shows initial absorbance data as a function of wavelength for different thiocyanate concentrations and a total iron concentration of 1.5 × 10−4 M and ionic strength of 0.5 M. Modelling is consistent with the presence of FeSCN2+ and Fe(SCN)2+.Figure optionsDownload as PowerPoint slide