A new met
hod for t
he determination of t
he concentrationand conditional stability constant of dissolved organicmatter t
hat binds mercury (Hg)
has been developed usingan in vitro assay of reducible Hg. T
he tec
hnique is awet c
hemical analogue to electroc
hemical approac
hesnow in use for ligand studies of many ot
her trace transitionmetals in natural waters. Ligand c
haracteristics areobtained from additions of ionic Hg to buffered lake, riverwater, and seawater and determination of t
he wetc
hemically reducible fraction following equilibration of t
hespike. T
his approac
h is robust, as demonstrated by (i)analysis using t
hree reducing agents of varying strengt
hs,(ii) replicate analyses, (iii) comparison to well-c
haracterizedcomplexing species (c
hloride and EDTA) using a competitiveion-exc
hange resin, and (iv) kinetic studies. Resultsindicate t
hat Hg-complexing equivalents are present int
he dissolved p
hase (<0.2
m) ranging from <1 to >60 nNconcentrations and wit
h log conditional stability constants(log
K') in t
he range of 21-24. Only one ligand classwas found in t
he natural waters analyzed. T
here was indirectevidence for a class of organic ligands t
hat formedreducible complexes wit
h Hg in fres
hwater. Suc
h ligandc
haracteristics indicate t
hat t
he vast majority of ionic inorganicHg dissolved in fres
hwater and coastal saltwaters isassociated wit
h organic complexes. Concentrations, affinities,and kinetics implicate multidentate c
helation sites as t
heprincipal complexing moieties for Hg and discourage t
he useof
humic carboxylic acids as a proxy for t
he ligands/functional groups.