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–n)+ _n and HgCl ^(2–n)+ _n 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.