Examples EDTA species distribution

Examples showing that metal speciation is important to metal toxicity include arsenic, copper, selenium, and chromium. While ionic copper (Cu2+) and CuClj are highly toxic, Q1CO3 and Cu-EDTA have low toxicity (Morrison et al, 1989). Toxicity tests show that As(III) is about 50 times more toxic than As(VI). Trivalent chromium is much less toxic than hexavalent chromium, probably because Cr(VI) is much smaller and the chemical structure of chromate is similar to sulfate. A special channel already exists in biomembranes for sulfate transport. While modeling metal speciation is not always possible, and redox equilibrium is not achieved in all natural waters, geochemical modeling of equilibrium species distribution remains one of the methods of discerning metal speciation. 

In order to do elemental speciation analysis, the original distribution of chemical spedes must be carefully preserved until the analysis is done. It can be very difficult to keep the sample stable up to the point where it is injected into the instrument. Since the analytes can be oxidized or sometimes reduced to related species within the sample, just handhng the sample may result in changes in the relative concentrations of the analytes. Preconcentration of the samples is not advised, and in many cases the samples must be treated to preserve the original state. Eor example, EDTA may be added to complex the metals in a sample, protecting them from oxidation or reduction reactions. Nitric add may not be used for speciation because nitric add is an oxidizing agent. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

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