Characterization of Complexing Agents in Natural Waters by Copper(II)/Copper(I) Amperometry

A convenient amperometric titration procedure for the characterization of organic copper complexing ligands is developed and applied to a variety of synthetic and naturally occurring organic compounds. The procedure is based upon the ability, in solutions of high chloride content, to obtain a sensitive and reproducible amperometric measurement of reducible Cu(II) at positive voltages up to about 100 mV relative to an Ag/AgCl reference electrode. Cu(II) is reduced to Cu(I) which is stabilized by chloride despite the presence of oxygen. Application of the titration technique to a high chloride content electrolyte containing various concentrations of nitrilotriacetic acid confirms that copper-ligand reduction and dissociation are not major problems provided that a sufficiently positive working electrode potential is chosen and that the concentration of the organic ligand is low. Application of the procedure to a variety of naturally occurring organic agents including a fulvic acid, freshwater algal exudates, and a sample of Sargasso seawater produces results that are consistent with those found by alternative methods.