Use of Pitzer Equations to Examine the Formation of Mercury(II) Hydroxide and Chloride Complexes in NaClO4 Media

Abstract

The thermodynamic stability constants for the hydrolysis and formation of mercury (Hg2+) chloride complexes $$ \hbox{Hg}^{2+} + n\hbox{H}_{2}\hbox{O} = \hbox{Hg(OH)}_n^{(2-n)+} + n\hbox{H}^{+ } \quad\quad \beta_n $$ $$ \hbox{Hg}^{2+} + n\hbox{Cl}^{-} = \hbox{HgCl}_n^{(2-n)+ } \quad\quad \beta_i $$ have been used to calculate the activity coefficients for Hg(OH) (2–)+ and HgCl (2–)+ complexes using the Pitzer specific interaction model. These values have been used to determine the Pitzer parameters for the hydroxide and chloro complexes $$(\beta_{\rm ML}^{(0)}, \,\beta_{\rm ML}^{(1)}$$ and C ML). The values of $$\lambda_{ij}$$ and $$\zeta_{ijk}$$ have been determined for the neutral complexes (Hg(OH)2 and HgCl2). The resultant parameters yield calculated values for the measured values of log $$\beta^{\ast}$$ to ±0.01 from I = 0.1 to 3 m at 25°C. Since the activity coefficients of $$\hbox{Hg(OH)}_n^{(2-n)+}$$ and $$\hbox{HgCl}_n^{(2-n)+}$$ are in reasonable agreement with the values for Pb(II), we have estimated the effect of temperature on the chloride constants for Hg(II) from 0 to 300°C and I = 0–6 m using the Pitzer parameters for $$\hbox{PbCl}_n^{(2-n)+}$$ complexes. The resulting parameters can be used to examine the speciation of Hg(II) with Cl− in natural waters over a wide range of conditions.