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

Abstract

The aqueous sulfite ion reacts with Hg2+(aq) to form 1:1 and 2:1 coordination complexes. The Hg(SO3)22- complex is redox stable. However, dissociation of a sulfite ligand forms redox-unstable HgSO3. Under conditions where Hg(SO3)22- predominates, the rate of reduction of the mercuric ion to Hg0 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 K1 = 2.1 × 1013 and K2 = 1.0 × 1010 at μ = 0.10 M. These values lead to the prediction that HgSO3 is more abundant in clouds than is Hg(SO3)22- under virtually all atmospheric conditions. The product of the redox reaction appears to be a strongly bound Hg0·SO2 complex, which is at least 3 orders of magnitude more soluble than uncomplexed Hg0(aq). This finding may have important implications for the partitioning of atmospheric mercury from the gas phase into atmospheric water droplets...