ANODIC STRIPPING VOLTAMMETRY Voltammetry • An electrochemical technique, used in analytical chemistry. • Information about an analyte is obtained by monitoring the current at an electrode as a function of potential applied to that electrode. • The plot between the measured current and the applied potential is known as Voltammogram. • Application - quantitative analysis of trace of metals (μg/L levels or less) Voltammetry • Measurement of current under conditions of complete concentration polarization. • The rate of oxidation or reduction of the analyte is limited by the rate of mass transfer of the analyte to the electrode surface. • To enhance the polarization, the working electrodes are relatively small with surface areas of a few square millimetres at the most and, in some applications, only a few square micrometres. • A minimal consumption of analyte. Voltammetry • For analytical purposes several types of voltammetry techniques are available. • Stripping voltammetry is very significant these days particularly for the determination of metals in the environment. • Linear Sweep Voltammetry is the simplest technique. • The electrode potential is varied at a constant rate (20 – 100 mV/s) throughout the scan and the resulting current is measured. • The scanning starts before the discharging potential and stops afterwards. • Detection limits range at mg/l levels. Anodic Stripping Voltammetry • Voltammetric method for quantitative determination of specific ionic species. • It is one of the most sensitive, convenient, and cost effective analytical method for detection and quantitation of metal contaminants in rivers, lakes, process streams and drinking water. • Several metals such as Cu, Pb and Cd can be analyzed simultaneously. • ASV can be thought of as a small scale electroplating experiment. Anodic Stripping Voltammetry • Incorporates three electrodes: – a working electrode, – an auxiliary electrode (sometimes called the counter electrode), and – a reference electrode. Working Electrode - ASV • It is the indicating electrode in an electrochemical system on which the reaction of interest is occurring. • Common working electrodes can consist of inert metals such as gold, silver or platinum, inert carbon such as glassy carbon or pyrolytic carbon, and mercury drop and film electrodes. • For most standard tests, the working electrode is a mercury film electrode. Working Electrode - ASV • The mercury film is formed over a glassy carbon electrode. • This film forms an amalgam with the analyte of interest, which upon oxidation results in a sharp peak. • In cases where the analyte of interest has an oxidizing potential above that of mercury, or where a mercury electrode would be otherwise unsuitable, a solid, inert metal such as silver, gold, or platinum may also be used. Auxiliary Electrode - ASV • Also called the counter electrode. • The auxiliary electrode's potential is opposite in sign to that of the working electrode, but its current and potential are not measured. • Rather, it is used to ensure that current does not run through the reference electrode (three electrode system), which would disturb the reference electrode's potential. Auxiliary Electrode - ASV • The auxiliary electrode passes all the current needed to balance the current observed at the working electrode. • It serves as a source or sink for electrons so that current can be passed from the external circuit through the cell. In general, neither its true potential nor current is ever measured or known. • Auxiliary electrodes are often fabricated from electrochemically inert materials such as gold, platinum, or carbon. Reference Electrode - ASV • Has a stable and well-known electrode potential. • It can be taken as the reference standard against which the potentials of the other electrodes present in the cell can be measured. • Its only role is to act as reference in measuring and controlling the working electrodes potential and at no point does it pass any current. • Generally Ag/AgCl is used. Anodic Stripping Voltammetry • Concentrate metal ions by reduction: – The metals, as ions in solution, are plated onto an electrode by applying a negative potential (deposition potential) for a specific period of time. – The deposition serves to concentrate the metal ions from the solution onto the electrode in the metallic form. – If the electrode is Hg, the metals often form an amalgam. – This pre-concentration leads to low detection limits. Anodic Stripping Voltammetry • Stripping – After deposition, the potential is scanned toward positive potentials. Current peaks appear at potentials corresponding to the oxidation of metals as they are oxidized (stripped) from the electrode back into the solution. – The current is measured during the stripping step. – The peak height or area can be correlated with the concentration of the metal ions in the solution. It is necessary to calibrate the procedure with standard solutions containing known quantities of the metal ions. Anodic Stripping Voltammetry • Consider a sample containing copper and cadmium (the two analytes). • First concentrated in a mercury working electrode and then sequentially reduced back to their metal ions and the current time curves reported. • The first stage is to reduce the metal ion to the metal that will form an amalgam with the mercury working-electrode. • This can be depicted by the following equation. Mn+ + ne- + Hg M(Hg) Anodic Stripping Voltammetry • If a mixture of cations are present, the working electrode should be maintained at a potential 0.3 - 0.5 V more negative than the standard potential of that metal most difficult to reduce. • The deposition of all the metal components of the mixture will be determined by the magnitude of the respective mass transport of each. • The speed of deposition can be accelerated by electrode rotation or by vigorously stirring the electrolyte. Anodic Stripping Voltammetry • The concentration of the reduced metal(s) in the mercury will be given by C Hg il t d nFVHg • il: limiting current for the deposition (reduction) of the metal • td: deposition time period • n: the number of moles of electrons transferred in the half reaction • F: the Faraday constant (96,487 coulombs/mole of e-) • VHg: volume of the mercury electrode Anodic Stripping Voltammetry • The time of the deposition will vary with a number of factors including the analyte concentration and the electronic parameters, but the conditions should be adjusted so that the deposition takes from one to ten minutes. • The pre-concentration step can then be followed by a change in the electrolyte composition to make it more amenable to the stripping procedure and this process takes place in the rest period. • Electrode rotation or stirring should be arrested during this procedure. Anodic Stripping Voltammetry • In the third stage the metals are stripped from the mercury by applying an appropriate voltage/time program. When the applied working electrode potential reaches the standard metal-metal ion redox potential the metal is stripped from the amalgam according to the following equation. M(Hg) Mn+ + ne- + Hg • In the current time curve, peaks are observed for the analytes, cadmium and copper. The peak potential of each metal is characteristic of the analyte and can be used to identify the metal. The height of each metal peak (ip) is proportional to the concentration of the metal in the test solution. Anodic Stripping Voltammetry • If a thin-film mercury electrode is employed, under which circumstances the metal is rapidly depleted from the amalgam, then the expression for the peak current (ip) is given by, ip n 2 F2 v1/2AlCHg • A: area of the film • l: length of the film • v: potential scan rate 2.7RT Differential Pulse ASV - Several metals in scan - Extremely sensitive - Hydrogen wave - Mercury oxidation wave Anodic Stripping Voltammetry • Very sensitive and reproducible method for trace metal ion analysis. • Useful for the analysis of very dilute solutions, 10-11 M. • Concentration limits of detection for many metals are in the low ppb to high ppt range • Approximately 12-15 metal ions can be analyzed for by this method. • The stripping peak currents and peak widths are a function of the size, coverage and distribution of the metal phase on the electrode surface (Hg or alternate).
