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The behavior of mercury under conditions similar to a wet flue gas cleaning process has been discussed in terms of thermodynamic equilibrium. The spectrophotometric technique developed in our laboratory has been used to study the systems HgCl2 + HCl + H2O, HgCl2 + CaCl2 + H2O, and SO2 + HgCl2 + HCl + H2O at 50 °C and atmospheric pressure in the absence of oxygen. A thermodynamic model presented in earlier communications is developed further. The substance-specific data required were extracted from literature and used without any further adjustment. The experimental results show that the soluble bivalent mercury (HgCl2) is completely reduced to elemental insoluble form through sulfur dioxide, even in the presence of 0.1 M HCl (i.e., also in strongly acidic solutions). This is also confirmed by the calculations done with the thermodynamic model.

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In 1996 we published a paper dealing with the description of a new apparatus, which allows the study of phase and chemical equilibria in aqueous systems of volatile weak electrolytes. (1) The apparatus was used to study (sulfur dioxide + water) at T = 293 K. The molality of the dissolved sulfur dioxide and its partial pressure were calculated from the measured initial gas phase concentration c 1 (SO 2 ) and the thermodynamic equilibrium concentration c 2 (SO 2 ). The molality of the dissolved SO 2 is correct, as confirmed by independent ion chromatographic (i.c.) measurements, but there is an error in the evaluation of the partial pressures, which were wrongly converted using a standard pressure of 101.325 kPa. Equation (23) should be replaced bywhere p(SO 2 ) is the partial pressure of SO 2 ; c 2 is the gas phase equilibrium concentration of SO 2 and T 2 is the equilibrium temperature. The partial pressures reported in reference 1 and in a subsequent communication (2) should be corrected by multiplying the previous values with the pressure ratio ( p 2 /101.325), where p 2 is the equilibrium pressureThe published partial pressures are about 4 per cent low. The correct partial pressures are listed in table 1 of the present note. Consequently, the correlation of (vapour + liquid) equilibrium changes as well. Henry's constant was fitted to reproduce our own experimental results for p(SO 2 ) at T = 293 K to T = 333 K; however, all the other equilibrium constants remain unchanged. Because the measurements were performed at high dilution we used Pitzer's method (3) for the Debye-Hückel term, which required no further substance-specific parameters. The new values for the thermodynamic quantities using p o = 100 kPa for the reaction: SO 2 (g) = SO 2 (aq)