14,1~ter Research Vol. 9. pp. 969 to 979. Pergamon Press I975. Printed in Great Britain. CHARACTERIZATION OF TRACE METAL SPECIES AND MEASUREMENT OF TRACE METAL STABILITY CONSTANTS BY ELECTROCHEMICAL TECHNIQUES R. ER,~s'r,* H. E. ALLEN'['~. and K. H. MANCY Environmental Chemistry Laboratory, Department of Environmental and Industrial Health. School of Public Health, The University of Michigan, Ann Arbor. MI 48104. U.S.A. (ReceiL'ed 23 May 1974) Abstract--Differential pulse polarography and differential pulse anodic stripping voltammetry were used to determine copper, lead, cadmium and zinc complexes of relevance to environmental conditions. The dependency of peak current on pH and alkalinity was interpreted in terms of the variation in the metal species present. Stability constants of copper and lead complexes with carbonate were deter- mined from the magnitude of the shift of peak potential using the method of Lingane. Inclusion of the transfer coefficient, calculated from the peak half-width for electrochemically irreversible systems, was used to determine the stability constant. By using both electroanalytieal techniques, the investigator validates the correctness of his results without needing to rely on prior determinations of the stability constant for verification. INTRODUCTION Present awareness of the importance of trace metals in aquatic systems has prompted increased activity in both the development of trace analytical techniques and the study of the physicochemical behavior of low levels of metals. Although the n~jority of water qua- lity standards specify, and environmental studies have measured only the total concentration of a trace metal, it has become apparent that the chemical form of the metal must be known to permit an accurate interpretation of both its biological effects and geo- chemical reactions. Prediction of trace metal environ- mental levels, forms, and transformations requires the development of mechanistic models which are based on equilibrium constants of metal complexes. Methods for the determination of stability con- stants have been reviewed by Martell and Calvin (1952), Rossotti and Rossotti (1961) and Beck (1970). These books deal with many analytical procedures and their underlying theoretical bases. However, the examples used are almost exclusively simple systems generally containing a single metal and a single well- characterized ligand, both present at levels much higher than those in the environment. Application of these techniques to the study of environmentally sig- nilicant complexation reactions has been carried out primarily by soil chemists concerned with metal- humic acid and metal-fulvic acid reactions~ The tech- niques which have received the most use are potentio- * Present address: Department of Chemistry, University of Lausanne, Lausanne, Switzerland. t Present address: Department of Environmental Engineering, Illinois Institute of Technology, Chicago, IL 60616, U.S.A. :[:To whom correspondence should be addressed. metric titration, Job's continuous variation technique and ion-exchange equilibrium (Schnitzer and Khan, 1972; Stevenson and Ardakani, 1972). Recent work by Cheng, Patterson and Minear (1975) has utilized the ion-exchange equilibrium method for determining the stability constant of various metals complexed with the soluble portion of sewage. Other investiga- tors (Bradford, 1973; Stumm and Bilinski, 1973) have employed anodic stripping voltammetry for the deter- mination of the stability constants of hydroxide and carbonate complexes of zinc and lead. These investi- gations were conducted to ascertain those complexes which predominate under environmental conditions. We have investigated the use of both differential pulse polarography (DPP) and differential pulse ano- dic stripping voltammetry (DPASV) for the assess- ment of trace metal complexation. The application of these techniques to environmental measurements is presented here while the electrochemical basis for these results will be discussed elsewhere (Ernst, Allen and Mancy, 1974). POLAROGRAPHIC DETERMINATION OF STABILITY CONSTAN"fS Although polarography has been widely used for the quantitative determination of metals in the aqua- tic system, utilization of this technique for the deter- mination of stability constants of these metals with naturally occurring ligands has been extremely 'limited. The detection limit for normal polarography does not permit analysis at the level at which metals normally occur in the environment. If samples are concentrated, shifts in chemical equilibria will result. Newer electroanalytical techniques such as pulse polarography and anodic stripping voltammetry have 969