Thermodynamics of the In|In+3 Electrode in HCl + InCl3 Solutions

Electromotive force measurements have been made using the cell $$\mbox{In(s)}|\mbox{HCl }(m_{\mathrm{A}}),\mbox{InCl}_{3}(m_{\mathrm{B}}),\mbox{H}_{2}\mbox{O}|\mbox{AgCl, Ag}$$ in the ionic strength range of I=0.05, 0.1, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol?kg^?1 at 25?°C. The value of E ^o, the standard potential of the In/In³⁺ electrode, has been determined at 25?°C. Our value of E ^o (?0.3371 V) at 25?°C obtained from our measurements is in good agreement with ?0.336 (Hakomori, J. Am. Chem. Soc. 52: 2372–2376, 1930) and ?0.3382 V (Covington et al., J. Chem. Soc. 4394–4401, 1963). The activity coefficients of InCl₃ as well as Harned interaction coefficients have been determined at 25?°C for each of the experimental ionic strengths at ionic strength fractions of 0.1, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 of HCl. Harned’s rule for the salt is obeyed at I=0.05,0.1 and 0.25 mol?kg^?1 but the quadratic terms are needed for higher ionic strengths. These data, together with others for the activity coefficient of HCl in the same solutions, have been treated by the ion-interaction (Pitzer, Activity Coefficients in Electrolyte Solutions, CRC Press, 1991) equations in a previous publication.