Activity coefficients from the emf of liquid membrane cells V. alkaline earth hexacyanoferrates (III) in aqueous solutions at 25°C

The relative mean activity coefficients of the M₃Fe(CN)₆]₂ salts, M=Mg, Ca, Sr, Ba, are measured down to about 5×10^–6 mol-kg^–1 using the liquid membrane cell method. In the dilute region these salts display negative instead of positive deviations from the limiting law, contrary to Debye-Hückel's theory predictions. An indirect method based on auxiliary emf measurements in MCl₂, K₃Fe(CN)₆ and KCl, rather than a theory-assisted direct extrapolation to zero of the relative activity coefficients, is used to identify the actual values of the activity coefficients. The data are compared with Mayer's theory, ion-pair theory and numerical integration of the Poisson-Boltzmann equation. Best-fit coefficients of Pitzer's equation which meet the activity coefficients of the M₃Fe(CN)₆]₂ salts to be reproduced, are reported.