Sulfite Stabilization and Reduction of the Aqueous Mercuric Ion:  Kinetic Determination of Sequential Formation Constants

Abstract

The aqueous sulfite ion reacts with Hg 2+ (aq) to form 1:1 and 2:1 coordination complexes. The Hg(SO 3 ) 2 2- complex is redox stable. However, dissociation of a sulfite ligand forms redox-unstable HgSO 3 . Under conditions where Hg(SO 3 ) 2 2- predominates, the rate of reduction of the mercuric ion to Hg 0 by coordinated sulfite depends inversely on the concentration of uncoordinated sulfite, while it is unaffected by the amount of sulfite liberated by dissociation. Analysis of the kinetics yields the sequential sulfite binding constants K 1 = 2.1 × 10 13 and K 2 = 1.0 × 10 10 at μ = 0.10 M. These values lead to the prediction that HgSO 3 is more abundant in clouds than is Hg(SO 3 ) 2 2- under virtually all atmospheric conditions. The product of the redox reaction appears to be a strongly bound Hg 0 ·SO 2 complex, which is at least 3 orders of magnitude more soluble than uncomplexed Hg 0 (aq) . This finding may have important implications for the partitioning of atmospheric mercury from the gas phase into atmospheric water droplets prior to its wet deposition.