ZNO SOLUBILITY AND ZN2-DEGREES-C WITH IN-SITU PH MEASUREMENT( COMPLEXATION BY CHLORIDE AND SULFATE IN ACIDIC SOLUTIONS TO 290)

The solubility of zincite in mildly to strongly acidic aqueous solutio ns, according to the reaction ZnO + 2H(+) double left right arrow Zn2 + H2O, has been measured at ionic strengths of 0.03-1.0 (stoichiometr ic molal basis) from 50 to 290 degrees C at saturation vapor pressure in sodium triffuoromethanesulfonate solutions (NaTriflate, a noncomple xing 1:1 electrolyte). The hydrogen-electrode concentration cells empl oyed in this study permit continuous and highly accurate pH measuremen t at elevated temperatures, and periodic sampling to determine the dis solved metal content of the experimental solution. The solubility of z incite is shown to be reversible at 200 degrees C by addition of acidi c and basic titrants, at constant ionic strength. The equilibrium cons tant is precisely described (+/-0.05 log units) by the function log K = -4.0168 + 4527.66/T. One additional adjustable parameter, together w ith an extended Debye-Huckel function, is sufficient to model the ioni c strength dependence of the reaction. The solubility product at infin ite dilution obtained from this study is in quantitative agreement wit h the thermodynamic model of Ziemniak (1992). This experimental approa ch is demonstrated to be advantageous in studying the complexation of Zn2+ with Cl- and SO42-, by titrations involving the appropriate anion into NaTriflate solutions pre-equilibrated with zincite at constant t emperature and ionic strength. Formation constants in 0.1 molal NaTrif late for the reaction Zn2+ + yL(z-) double left right arrow Zn(L)(y)(2 -yz) are reported for ZnCl+, ZnCl2 degrees and ZnSO4 degrees at 200 de grees C (log Q = 1.7 +/- 0.1, 3.0 +/- 0.1, and 2.6 +/- 0.1, respective ly). Estimates of the equilibrium constants for the chloride species a t infinite dilution and 200 degrees C are log K = 2.5 +/- 0.1 (ZnCl+), and 4.2 +/- 0.1 (ZnCl2 degrees). This value for the dichlorozinc comp lex 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 (19 87) (log K = 3.1 +/- 0.3). Copyrights (C) 1998 Elsevier Science Ltd.