Study of Anodic Stripping Voltammetry with Collection at Tubular Electrodes Gary W. Schieffer and Walter J. Blaedel’ Departmeni of Chemistry, University of Wisconsin, Madison, Wis. 53706
The design and construction of a pair of tubular electrodes In series for anodic stripping voltammetry with collection at thin mercury films are described. A typical cell consists of two glassy carbon tubular working electrodes separated by a thin Teflon spacer. Trace metal cations strlpped from the upstream electrode are collected at the downstream electrode at constant potential. Exploratoryexperiments have shown the dependence of the stripping and collection peaks on flow rate, deposltion time, scan rate, and concentration. The exploratory equipment used permittedthe measurement of subnanomolar concentrations of lead.
Conventional anodic stripping voltammetry (ASV) a t thin mercury films deposited on solid electrodes (usually some form of carbon) has been widely used for trace metal analysis (I). The trace is first accumulated in the mercury film by holding the working electrode a t a reducing potential for a fixed period of time called the deposition time. The concentrated metal is then reoxidized or “stripped” from the film by applying an anodic potential scan (usually linear). The resulting stripping peak height (current) or area (coulombs) is proportional to the bulk concentration of the analytical species. Charging current generally limits the detectability of ASV a t thin film electrodes by providing a large, changing background from which small signals have to be extracted. Johnson and Allen ( 2 ) developed a technique called anodic stripping voltammetry with collection (ASVWC) in which charging current was largely eliminated in the ASV analysis of silver by depositing the silver on the glassy carbon disk electrode of a rotated ring disk system, stripping the silver from the disk with a linear potential scan, and redepositing a constant fraction of the stripped silver ions at the constant potential platinum ring electrode. Since no charging current was passed a t the ring electrode, the relatively flat baseline permitted the M for relatively short determination of silver down to deposition times. Laser and Ariel ( 3 ) extended the technique to a mercury plated glassy carbon rotated ring disk electrode to determine copper, lead, zinc, and cadmium. They also demonstrated that increasing the rate of potential scan a t the disk electrode increased the faradaic collection current observed a t the ring electrode without increasing or otherwise affecting the ring background current. This further increased the sensitivity and decreased the analysis time. The purpose of the present research is to employ a similar technique for tubular electrodes. These electrodes are inherently simple in that rapid convection of species to the electrode surface is maintained with high reproducibility by the flowing solution. The need for a stirring motor with noisy brush contacts is avoided. The continuous replenishment of solution prevents concentration depletion during the deposition step and makes tubular electrodes amenable to automation and continuous or on-stream analysis. A single tubular mercury-covered graphite electrode has been used to determine thallium down to 2 X 10-9 M for 30-
min deposition times ( 4 ) . Very high flow rates (up to 900 ml/min) were used in a similar electrode to detect zinc in seawater a t concentrations of 1 X M for 5-min depositions (5).In the present work, a second glassy carbon tubular electrode has been placed downstream from the first to serve as a constant potential collection electrode for species stripped from the upstream electrode. The characteristics and potentialities of such a twin tubular electrode cell for ASVWC are explored in this paper.
THEORY Roe and Toni have derived a simple equation for the stripping peak current (iPJ obtained with a mercury film electrode (6).Assuming that the solution is stirred and that the film is so thin ( 4 0pm) that diffusion in the mercury does not limit the stripping current: where A is the electrode area (cm23,v is the potential scan rate (V/s), 1 is the mercury film thickness (cm), C# is the metal concentration in the mercury film after deposition (mol/cm3), and e is the base of Napierian logarithms. Although mercury is deposited on glassy carbon in the form of small droplets (ca. 1 pm in diameter) rather than a uniform thin film (7), Batley and Florence have found that Equation 1 is obeyed for very thin films (
