Chapter 8
Determination of Organic Complexation
C.M.G. van den Berg
8.1
Introduction
It has been known for several decades that trace metals can become complexed by
organic matter in natural waters. Only relatively recently has it become known that
some interesting metals such as copper, zinc, iron, nickel, cobalt and probably more,
participate in these reactions. This complexation changes the geochemistry of
the metals by preventing them from being scavenged and thus increases their residence time in estuaries and the upper water column. The complexation also changes
the availability to organisms (algae, mollusks or fish). It is therefore important to determine the chemical speciation of metals in addition to their dissolved concen
tration in sea water. The organic complexation has been determined in the past by
several methods, including fluorescence quenching (Berger et al. 1984), solubility increases (Campbell et al. 1977), anodic stripping voltammetry (Chau et al. 1974), competitive adsorption on Mn02 (van den Berg and Kramer 1979) and preconcentra
tion on C18 cartridges (Mills and Quinn 1981). The most important methods to determine the chemical speciation of metals in the marine system now use voltammetry
with and without ligand competition. These methods will be explained in this chapter.
8.2
Metal Species in Solution
The metal species in seawater, and their relationships, are shown in Table 8.1. The main
species are M' (all inorganic metal ions), ML (the metal complexed by the natural organic matter) and L' (the natural organic ligands not associated with M). The metalorganic speciation in seawater would be known exactly if M' and ML could be determined directly. A problem is that the measurement itself usually removes a fraction
of the detected species, thus causing a shift in the equilibrium. The metal speciation
can also be calculated from the total metal and ligand concentrations ([Mtl and [LtD
and the conditional stability constant (KML or K ML): in this case the actual species do
not need to be determined, but are evaluated from ligand concentrations and conditional stability constants which have to be determined for each sample separately; with
this information the concentration of the free metal ion can be calculated. The two
methods are discussed below.
Determination of Organic Complexation
C.M.G. van den Berg
8.1
Introduction
It has been known for several decades that trace metals can become complexed by
organic matter in natural waters. Only relatively recently has it become known that
some interesting metals such as copper, zinc, iron, nickel, cobalt and probably more,
participate in these reactions. This complexation changes the geochemistry of
the metals by preventing them from being scavenged and thus increases their residence time in estuaries and the upper water column. The complexation also changes
the availability to organisms (algae, mollusks or fish). It is therefore important to determine the chemical speciation of metals in addition to their dissolved concen
tration in sea water. The organic complexation has been determined in the past by
several methods, including fluorescence quenching (Berger et al. 1984), solubility increases (Campbell et al. 1977), anodic stripping voltammetry (Chau et al. 1974), competitive adsorption on Mn02 (van den Berg and Kramer 1979) and preconcentra
tion on C18 cartridges (Mills and Quinn 1981). The most important methods to determine the chemical speciation of metals in the marine system now use voltammetry
with and without ligand competition. These methods will be explained in this chapter.
8.2
Metal Species in Solution
The metal species in seawater, and their relationships, are shown in Table 8.1. The main
species are M' (all inorganic metal ions), ML (the metal complexed by the natural organic matter) and L' (the natural organic ligands not associated with M). The metalorganic speciation in seawater would be known exactly if M' and ML could be determined directly. A problem is that the measurement itself usually removes a fraction
of the detected species, thus causing a shift in the equilibrium. The metal speciation
can also be calculated from the total metal and ligand concentrations ([Mtl and [LtD
and the conditional stability constant (KML or K ML): in this case the actual species do
not need to be determined, but are evaluated from ligand concentrations and conditional stability constants which have to be determined for each sample separately; with
this information the concentration of the free metal ion can be calculated. The two
methods are discussed below.