Role of electro-analytical techniques

Electro-analytical techniques have been used extensively in studies of natural waters. For example, ion-selective electrodes allow measurement of the activity of free hydrated ions in solution (species highly relevant in toxicity studies) and voltammetric methods (polarography and ASV) exhibit a high degree of selectivity (for highly labile species). Electro-chemical techniques also facilitate identification of the valency state of elements such as Fe, Cr, Tl, Sn, Mn, Sb, As, Se, Y U and I. For several other elements only one state is electro-chemically active, and redox state speciation becomes a special case of labile/inert species discrimination. The toxicity of an ion can vary with valency, for example, Cr is more toxic than Crm, while for As, Sb and Tl, the lower valency form is more toxic. 

Up to 30 elements can be determined by voltammetric procedures, but when the metals of interest are present at 10-9 to 10 12 mol dm-3 levels, the introduction of a pre-concentration step becomes essential. The concentration process may disturb the distribution pattern of labile molecular species but the status of stable species should change little. Some pre-concentration procedures can be selective in respect to the species retained, and this can limit the value of total element content analyses (i.e. procedures need to be validated). 

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