Effect of the (K +, CL −) ions on the order parameters and on the Lorentz–Lorenz relation in the isobutyric acid–water critical mixture

Abstract

The density ( ρ) and the refractive index ( n) in each phase of the critical binary fluid mixture “isobutyric acid–water” (I–W) with added (K +, Cl −) ions at various concentrations, were measured in the vicinity and far from the critical temperature. The obtained data were used to test the Lorentz–Lorenz (L–L) relation in this electrolyte system. The density and refractive index coexistence curve data were analyzed to determine the critical exponent β. The results show that the density ( ρ) and refractive index ( n) constitute good order parameters as valid as the volume fraction ( ϕ). Furthermore, the critical exponent β obtained for the pure mixture is in good agreement with theoretical predictions. The shift of the transition phase's area depends on the ion concentration. The Lorentz–Lorenz (L–L) is verified for all systems (I–W) and (I–W) + (K +, Cl −) ions. The relative deviation of the (L–L) relation is generally in the order of 7%, as well for the case of the pure system without ions, and in the presence of the ions with weak concentration.