PII S0016-7037(98)00039-8 ZnO solubility and Zn 2 complexation by chloride and sulfate in acidic solutions to 290°C with in-situ pH measurement DAVID J. WESOLOWSKI,* PASCALE B´ EN´ EZETH, and DONALD A. PALMER Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6110, USA Abstract—The solubility of zincite in mildly to strongly acidic aqueous solutions, according to the reaction ZnO + 2H + N Zn 2+ + H 2 O, has been measured at ionic strengths of 0.03–1.0 (stoichiometric molal basis) from 50 to 290°C at saturation vapor pressure in sodium trifluoromethanesulfonate solutions (NaTriflate, a noncomplexing 1:1 electrolyte). The hydrogen-electrode concentration cells employed in this study permit continuous and highly accurate pH measurement at elevated temperatures, and periodic sampling to determine the dissolved metal content of the experimental solution. The solubility of zincite is shown to be reversible at 200°C by addition of acidic and basic titrants, at constant ionic strength. The equilibrium constant is precisely described (0.05 log units) by the function log K =-4.0168 + 4527.66/T. One additional adjustable parameter, together with an extended Debye-Hu ¨ckel function, is sufficient to model the ionic strength dependence of the reaction. The solubility product at infinite dilution obtained from this study is in quantitative agreement with the thermodynamic model of Ziemniak (1992). This experimental approach is demonstrated to be advantageous in studying the complexation of Zn 2+ with Cl - and SO 4 2- , by titrations involving the appropriate anion into NaTriflate solutions pre-equilibrated with zincite at constant temperature and ionic strength. Formation constants in 0.1 molal NaTriflate for the reaction Zn 2+ + yL z- N Zn(L) y 2-yz are reported for ZnCl + , ZnCl 2 ° and ZnSO 4 ° at 200°C (log Q = 1.7  0.1, 3.0  0.1, and 2.6  0.1, respectively). Estimates of the equilibrium constants for the chloride species at infinite dilution and 200°C are log K = 2.5  0.1 (ZnCl + ), and 4.2  0.1 (ZnCl 2 °). This value for the dichlorozinc complex agrees quantitatively with values reported by Bourcier and Barnes (1987) and Ruaya and Seward (1986). However, the latter authors give a value for the monochlorozinc complex (log K = 4.01  0.02) that is markedly different from our result and that of Bourcier and Barnes (1987) (log K = 3.1  0.3). Copyright © 1998 Elsevier Science Ltd 1. INTRODUCTION 1.1. Background Zincite (ZnO) occurs in only a limited number of natural environments, such as the metamorphic ore deposit at Franklin, New Jersey, USA. However, ZnO is the principal corrosion product of zinc in industrial systems involving hydrothermal solutions. Furthermore, studies of zinc oxide solubility can be and have been used (e.g., Bourcier and Barnes, 1987; Shock et al., 1997) to extract the thermodynamic properties of aqueous zinc ions and complexes. These can then be used in modeling the transport and deposition of this element during diagenesis, meteoric/metamorphic/magmatic fluid-rock interactions, and formation of the very abundant and economically important zinc sulfide ore deposits, including Mississippi Valley Type deposits, massive sulfide ores of the Sullivan and Kuroko types, and various skarn and manto ores, the formation conditions of which span a very wide range of temperature and fluid salinity. Potentiometric measurements using hydrogen electrode con- centration cells to 300°C have become a routine experimental method in our laboratories for studying protolytic hydrothermal reactions in dilute to concentrated brines (e.g., Wesolowski et al., 1995; Mesmer et al., 1995). Such reactions include the hydrolysis of metal ions, the dissociation constants of organic and inorganic acids and bases, and the complexation of ions in solution by inorganic and organic ligands. The very accurate and precise results obtainable by this method are needed for detailed modeling of a wide range of geochemical and indus- trial processes, as well as in the development of computational algorithms for the estimation of speciation and reaction ther- modynamics in aqueous systems (e.g., Shock and Helgeson, 1988; Sverjensky et al., 1997; Shock et al., 1997). Recently, we have begun to apply this approach in the study of heteroge- neous reactions, including adsorption of H + /OH - on rutile surfaces to 295°C (Machesky et al., 1994, 1998), and the solubilities of boehmite to 290°C (Be ´ne ´zeth et al., 1997) and brucite to 200°C (Brown et al., 1996). Details of the method- ology and modifications of the instrumentation which facilitate such measurements are presented in Palmer et al. (1998), who also present a more detailed discourse on our studies of boeh- mite solubility over a wide range of pH, temperature, and ionic strength. 1.2. Previous Studies The work of Schindler et al. (1964, 1965) gives a direct measure of the equilibrium constant of the zincite dissolution reaction ZnO cr + 2H + N Zn 2+ + H 2 O (l) (1) log Q s0 = [Zn 2+ ]/[H + ] 2 = log K s0 - log ( Zn / H 2 ) - log (a w ) (2) at 25°C. Their very careful studies involved reversed solubility measurements of zincite over a wide range of pH in 0.2M potassium nitrate and sodium perchlorate, using a potentiomet- * Author to whom correspondence should be addressed (dqw@ornl.gov). Pergamon Geochimica et Cosmochimica Acta, Vol. 62, No. 6, pp. 971–984, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 + .00 971