A thermodynamic study of the aqueous (sodium chloride + sodium hydroxide) electrolyte using sodium amalgam and thallous chloride electrode cells

The aqueous mixed (sodium chloride + sodium hydroxide) system has been studied using e.m.f. measurements at T = 298.15 K of the combined cells Pt|Na-Hg|NaCl(m1) + NaOH (m2)|HgO|Hg|Pt and Pt|Tl-Hg|TlCl|NaCl(m1) + NaOH(m2)|HgO|Hg|Pt at constant total molalities (m1 + m2) of (1, 2, 3, 4, 5, 6, 7, 9, and 11)mol · kg-1. The mean molal activity coefficients γ of each single component have been determined as a function of the respective molalities. For both components, the behaviour can, with reasonable approximation, be represented by Harned's rule (linear dependence of lgγ on m), although a quadratic dependence is more appropriate. In terms of the mixing fraction X = m2/mTOT, the lgγ against X curves for NaCl and NaOH have slopes in opposite directions, contrary to (KCl + KOH)aq and most other (MX + MY)aq. The respective trace activity coefficients are unequal, and both of them are lower than the corresponding pure-component activity coefficients.