Densities and Apparent Molar Volumes of Aqueous Solutions of K₄Fe(CN)₆, K₃Fe(CN)₆, K₃Co(CN)₆, K₂Ni(CN)₄, and KAg(CN)₂ at 293 to 343 K

Densities of aqueous solutions of K₄Fe­(CN)₆, K₃Fe­(CN)₆, K₃Co­(CN)₆, K₂Ni­(CN)₄, and KAg­(CN)₂ have been measured with a vibrating-tube densimeter at temperatures from 293.15 to 343.15 at 5 K intervals over the approximate concentration range 0.02 to 0.4 mol·kg^–1. Apparent molar volumes, V_ϕ, calculated from these data were fitted with an extended form of the Redlich–Rosenfeld–Meyer equation to obtain the standard partial molar quantities, V°. The V_ϕ and V° values for all the salts are in reasonable agreement with literature data at all temperatures. Deviations of the V_ϕ values from the Debye–Hückel limiting law were found to increase considerably with increasing anion charge. Values of V° for the cyanometallate anions were obtained using an appropriate extrathermodynamic assumption and were found to increase with increasing crystallographic size or ionic charge except for Fe­(CN)₆^4–, which shows a remarkable contraction of more than 35 cm³·mol^–1 relative to Fe­(CN)₆^3–. The origins of this anomaly are unclear. Isobaric expansivities, α, of all the salt solutions show the greatest increase with solute concentration, but the smallest dependence on the nature of the anion, at lower temperatures. These effects are consistent with the dissolved ions changing from water-structure breaking to net structure making with increasing temperature.