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 Hg2++nH2O=Hg(OH)(2−n)+n+nH+βn Hg2++nCl−=HgCl(2−n)+nβi have been used to calculate the activity coefficients for Hg(OH) n (2–n)+ and HgCl n (2–n)+ complexes using the Pitzer specific interaction model. These values have been used to determine the Pitzer parameters for the hydroxide and chloro complexes (β(0)ML,β(1)ML and C ML). The values of λij and ζijk have been determined for the neutral complexes (Hg(OH)2 and HgCl2). The resultant parameters yield calculated values for the measured values of log β∗ to ±0.01 from I = 0.1 to 3 m at 25°C. Since the activity coefficients of Hg(OH)(2−n)+n and HgCl(2−n)+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 PbCl(2−n)+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.