Electrical conductivity of acidic sulfate solution
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INTRODUCTION
ELECTRICAL conductivity is undoubtedly an important parameter for characterizing the usefulness of practical electrolysis solutions. It is a simple matter to write an electrical conductivity equation that is a linear combination of ion concentrations and their equivalent conductances; i.e.,
K = E Ic, z,X,I
[1]
i
where Ci, zi, and ki are the concentration in mol cm -3, charge number, and equivalent conductivity in S cm -~ (ohm -I cm-l), respectively. For concentrated electrolytes which are common in electrowinning and refining of metals, Eq. [1] is not useful, because (a) Xi values are unknown except at infinite dilution (they are ionic strength dependent), and (b) Ci values are also unknown, because of the formation of complex ions or ion pairs such as HSO4 and MSO4. Tozawa et al.l'2 have plotted measured electrical conductivities against the parameter 2 [ H 2 8 0 4 ] - [SO4]T because H § ions have a much higher equivalent conductance than any other ions, and this parameter is proportional to the H § concentration/f HSO4 is weakly ionized (as at infinite dilution) and/fother sulfate complexes or ion pairs are strongly ionized. The problem is that Tozawa's measurements show electrical conductivities to be very high when his parameter has zero or negative values. For example, for zinc tankhouse electrolytes, the electrical conductivity is 0.302 S cm -1 when 2[H2SO4] [SO4]T = - 0 . 3 6 4 , and 0.536 S cm -1 when 2[H2SO4] [SO4]T = + 1 . 1 0 9 . Since H § has an equivalent electrical HIROSHI MAJIMA, Professor, YASUHIRO AWAKURA, Lecturer, and SUNG KOOK PARK, Graduate Student, are with the Department of Metallurgy, Kyoto University, Kyoto 606, Japan. ERNEST PETERS is Professor, Department of Metallurgical Engineering, The University of British Columbia, 309-6350 Stores Road, Vancouver, BC V6T IW5, Canada. Manuscript submitted February 25, 1986. METALLURGICALTRANSACTIONS B
conductance value of 5 to 10 times that of other inorganic ions present in these acid solutions, it is impossible to account for the high electrical conductivity values of this parameter if 2[H2SO4] - [SO4]r is proportional, even approximately, to [H+]. In fact, Baes 3 gave an ionic strengthdependent equation for HSO~ ionization that shows this ion to be about 60 pct ionized in zinc plant electrolytes, even when Tozawa's parameter is zero or negative. Thus, the assumptions made for establishing Tozawa's parameter are invalid. An alternative way of explaining the high conductivity of these solutions, and of formulating a useful predictive equation, is to consider the mechanism of H § conduction, which involves a proton jump mechanism 4 (Figure 1). This jump mechanism involves free water molecules. Water that is tied up in solvating ions would not participate as readily in this proton jump mechanism. Thus, the impact of neutral salts can be seen as lowering the activity of water through solvation effects, which should lower the equivalent electrical conductance of H § ions. In this study, the electrical conductivity data reported by Tozaw
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