Topics in..
.
Edited by GALEN W. EWING, Seton Holl University, So. Orange, N. 1. 07079
These articles are intended to serve the renders o f ~ ~JOURNAL rs by calling attention lo new developments i n the theory, design, or availability of chemical laboratory instrumenlation, or by presenting useful insights and ezplanations of topics that are of practical imporlance to those who use, or leach the use of, modem instrumentation and instrumental techniques. The editor invites correspondence from prospective contributors.
LXIX.
Anodic Stripping Voltammetry William D. Ellis, Honeywell Corporate Research Center, 10701 Lyndale Avenue South, Bloomington, Minn. 55420
INTRODUCTION Electroanalytical techniques comprise one of the major families of instrumental methods, with their general application to the qualitative and quantitative analysis of species in solution. The family is made up of three primary branches: potentiometry, voltammetry, and coulometry. Of these, potentiometry enjoys the most widespread use, with a pH meter in nearly every laboratory and a variety of ion-selecLive electrodes available. Voltammetry is the youngest of the group, getting its start with the development of polarography by Heyrovsky in the 1920's. From that work a variety of techniques evolved, including solid electrode voltammetry, potential sweep chronoamperometry, stripping voltammetry, and the more sophisticated forms of polarography, including the use of ae, square wave and pulsed waveforms. Although polarography is the best known of the voltammetric techniques, stripping voltammetry is rapidly growing in importance. It is the purpose of this review to help make anodic stripping voltammetry (ASV) a laboratory, if not a household, ward. The topic is being confined to the anodic form of stripping voltammetry, since the vast majority of the work in the field is in that area. The cathodic version has been covered in the general review of stripping voltammetry by Barendrecht (1). The strength of ASV is in its application to the analysis of trace metals. Its sensitivity is achieved through a pre-caneentration process in which a metal ion in solution is reduced by a controlled potential more negative than the reduction potential of the species, and the reduced metal is plated onto a solid electrode or forms a n amalgam with a mercury electrode. The concentration step is of such a duration as
toproduce the required sensitivity, and in some instances is continued until the species is exhausted from the solution, although practical considerations generally dictate the use of a nanexhaustive deposition. Fallowing the concentration step, the potential is scanned anodically and the current is measured. A peak appears in the i-E curve with its peak potential a qualitative indication of the identity of the metal ion and the peak height a quantitative measure of its concentration in the solution. ASV was first used by Zhinden in 1931 (2). He was attempting to determine capper electrogravimetrically by plating it onto a platinum electrode. However, the amount plated proved to he too small to weigh accurately, so he devised the technique of reversing the current and stripping the copper from the electrode and made the quantitative determination by measuring the current consumed during the process. Little further work was done in this area until the 1950's when the use of mercury electrodes became common. During the 1960's. the theory of ASV for mercury film electrodes (MFE) and hanging mercury drop electrodes (HMDE) was developed, and in the past few years, a variety of techniques have been introduced which have enhanced the method's capability.
THEORY The purpose of this section is to provide the reader with a working knowledge of the relationship between the experimentally controlled parameters and the resulting stripping peak parameters. Those interested in a more detailed discussion of the theory should consult the references (1. 3-51.
Dr. William D. Ellis is a Senior Principal Kesearch Scientist a t the Honeyweil Corporate Research Center. He received his B.A. degree from Occidental College, Los Angeles, in 1965 and a Ph.D. in physical chemistry from the University of Alberta in 1968. His doctoral thesis was on fast reaction kinetics and physical measurements of biachemical systems. Since joining Honeywell in 1968, his research has centered on the development of analytical methods and instrumentation for biomedical uses and far air and water pollution measurements. His other research interests include interfacing analytical instruments with minicomputers or dedicated digital logic and a study of the fundamental mechanisms of olfaction.
Consideration will first be eiven to the concentration step. For this treatment, it is assumed that a constant current is maintained during the cathodic deposition. This will be the case if the concentration in the bulk solution is not appreciably changed during the course of the electrodeposition and if the solution is stirred a t a constant rate. Far a HMDE or a MFE the concentration of the reduced metal in the mercury is given by Faraday's law:
it
C, = -
nFV
(1)
where C, is the concentration of metal in the amalgam, i is the reduction current, t is the duration of the concentration step, n is the number of electrons in the reduction, F is Faraday's constant and V is the volume of mercury in the film or drop. The reduction current is analogous to the limiting current in a diffusion-controlled process:
. = nFDChA -
1
6 (2) where D is the diffusion coefficient in (Continued on page A1351
Volume 50, Number 3,March 1973 / A131
Chemical Instrumentation cm2/sec, C b is the concentration of the metal ion in the bulk solution being analyzed, A is the electrode area in cm2 and 6 is the thickness of the diffusion layer. Since the solution is stirred d will be affected bv the stirrine rate. cell wametrv and elecirode design."~otake thirinta a'. count, equation 2 can be rewritten:
where m, the mass transport coefficient is approximately proportional to the square root of the stirring rate (1, 6). By substituting equation 3 into equation 1 and simplifying, the final equation for C. is obtained:
and
where 1 is the thickness of the mercury film and r is the radius of the mercury drop. In the second step the metal is oxidized and stripped out of the mercury. Roe and Toni have derived equations for the peak potential and peak current of the stripping process using a MFE (51. They assumed that there is no concentration gradient in the film, a condition which can be met by thin films ( < l o pm thick) and law patential scan rates (
