Thermodynamic Analysis of Methyl Orange Anion Association with α-Cyclodextrin Using a Conductometric Approach

Abstract

Abstract The electrical conductivities of aqueous solutions of the azo dye methyl orange (sodium salt) were measured at 25.0 °C in the concentration range between 2.103 × 10-4 and 9.255 × 10-4 mol L-1. Molar conductivity values fit the Debye-Hückel-Wager equation for a symmetric electrolyte. The value of the molar conductivity of the sodium salt of methyl orange at infinite dilution is 77.93 ± 0.38 S cm2 mol-1. For the anions of methyl orange, the ionic conductivity at infinite dilution was calculated to be 27.82 S cm2 mol-1. Using the same methodology, a thermodynamic analysis of the association between methyl orange anion and α-cyclodextrin was conducted at 20.0, 25.0, 32.0, and 40.0 °C. The measured molar conductivities decreased as the mole ratio of α-cyclodextrin to methyl orange went below 3. The conductivity measurements were analysed using a model using 1:1 stoichiometry at the four different temperatures. The values of AHo and ASo are determined, as well as a discussion of their relevance.