Linear behaviour in isothermal equilibria involving saturated electrolyte solutions: The H2O–Co(NO3)2–Ni(NO3)2 ternary system at low temperature

• The solid–liquid equilibria in the ternary system H2O–Co(NO3)2–Ni(NO3)2 were studied at 248.15 K and 258.15 K.
• A linear equation of the ice liquidus curve was established based on the quasi-ideal model.
• Two widespread solid solutions were highlighted: Ni(1−ω)Coω(NO3)2·6H2O and Co(1−σ)Niσ(NO3)2·6H2O.
• The solubility curve of these solid solutions also presented a linear behaviour.

The solid–liquid equilibria of the H2O–Co(NO3)2–Ni(NO3)2 ternary system were studied at 248.15 K and 258.15 K, using a synthetic method based on conductivity measurements. The obtained results were complemented by chemical and powder X-ray diffraction analyses. The solid phases are ice and two large substitutional solid solutions: Co(1−σ)Niσ(NO3)2·6H2O, which has a monoclinic structure, and Ni(1−ω)Coω(NO3)2·6H2O, which has a triclinic structure. The formation of a Ni(1−ε)Coε(NO3)2·9H2O solid solution is also highly probable.The ice liquidus curves were modelled in a truly remarkable way, using the quasi-ideal model taking into account ionic dissociation and solvation. The liquidus of the solid solutions were analysed without regard to cation solvation, as a first approximation. Linear behaviour was observed in all cases. On the basis of simple, representative equations, the experimental values were evaluated critically, refined and extrapolated.