There exist two upper critical points and one lower critical point near the DCP (double critical point) and CDP (critical double point) in the ternary mixtures of ethanol, water and potassium carbonate. The DCP and CDP were avoided by changing the mass ratio of ethanol to water to reach the regions where only one upper critical point exists. In this simple region, the coexistence curves of temperature-refractive index were measured for the mass ratios of ethanol to water being 0.3699 and 0.5042 to examine the crossover critical behaviour. It was found that the crossover is nonmonotonic and consistent with Sengers’ crossover theory.

Key words: electrolyte solution, single upper critical point, critical crossover