Critical concentration in binary mixtures of limited miscibility

For decades, the Cailletet-Mathias (CM) law of rectilinear diameter was the base for determining the critical density for gas liquid-liquid critical point or the critical consolute concentration in mixtures of limited miscibility. Almost four decades ago, the decisive evidence showing the critical anomaly of the binodal's diameter questioned the CM law reliability. This report presents the new method, fast & simple in use, to determine the coexistence curve (binodal) and the critical concentration. Notable is the experimental simplicity of the method: for the preliminary estimation it is enough to prepare two samples with different total concentrations or densities and observed relative volumes occupied by coexisting phases (or fractional meniscus heights) for two different temperatures. The method is illustrated basing on experimental studies in nitrobenzene - hexane mixtures of limited miscibility. Notable is the importance of this result for a variety of practical implementations, ranging from chemical physics to foods and pharmaceuticals technologies. Additionally, the manifestations of various stages of the critical opalescence, from dark-navy-blue to bluish and whitish, in the broad range of temperatures and concentrations, are shown and discussed. (C) 2021 Published by Elsevier B.V.