Transference numbers for NaCl in a 60 wt.% ethanol–water mixture at 25 °C

Abstract

Transference number values for both ions of NaCl in a 60 wt.% ethanol–water mixture were determined by the moving boundary method at 25 °C. These values, after the volume and the solvent corrections had been applied, were optimized by dividing them by their sum at each concentration studied and the T_best values obtained. From the analysis of these best values against C(NaCl) three different regions of behaviour were clearly defined. In region I only the presence of free ions and ionic pairs were considered and the transference number values were fitted by using the classical Fuoss–Onsager and Pitts equations and Barthel and Perie's equations, based on a chemical model. From these treatments a limiting value (〈T⁰₊〉= 0.4361₃) was obtained. Region II starts with an abnormal and sudden rising of the transference number value of Na⁺ ion constituent which was justified by considering the formation of the triple ions Na₂Cl⁺ and NaCl^–₂. An estimation of the concentration of these species was made. When C(NaCl) is made sufficiently large, this abnormal behaviour ends (and also region II) which was interpreted as a consequence of the formation of the Na₂Cl₂ species. Finally, ionic limiting conductance values [λ⁰(Na⁺)= 18.98 and λ⁰(Cl^–)= 24.53 Ω^–1 cm² mol^–1] were determined.