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Anodic Stripping Voltammetry Introduction

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Anodic Stripping Voltammetry Introduction Department of Environmental Science M.Sc. (Environmental Science) 2nd Semester MSESC 2006E04 Analytical Methods Course Instructor: Dr. Rajesh Kumar Ranjan ANODIC STRIPPING VOLTAMMETRY INTRODUCTION • Stripping analysis is an analytical technique that involves (i) pre concentration of a metal phase onto a solid electrode surface or into Hg (liquid) at negative potentials and (ii) selective oxidation of each metal phase species during an anodic potential sweep. • Excellent technique for trace metal ion analysis. • Pre concentration leads to low detection limits. • Very sensitive and reproducible method for trace metal ion analysis. • Useful for the analysis of very dilute solutions, 10-10 M. • Concentration limits of detection for many metals are in the low ppb to ppt. • Approximately 12-15 metal ions can be analysed 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) HISTORY • Electroanalytical techniques- Major families of instrumental method- applicable to qualitative & quantitative analysis- have 3 branches: I. Potentiometry II. Voltametry III.Coulometry • Voltammetry started with the development of polarography- Heyrovsky 1920. • ASV first used by Zbinden in 1931. • 1950s – mercury electrode become common. • 1960s – MFEs & HMDEs was developed. (Ellis, 1973) PRINCIPLE • Applicable to metal ion detection at an electrode. • Accumulated by potentiostatic deposition onto a small-volume Hg electrode. • Cathodic deposition at a controlled time and potential. • Stripping analysis. FIG 1: Principle of ASV. Values shown are typical one ; potential and Ep are typical of Cu 2+ analysis. (a) preelctrolysisi at E; stirred solution. (b) rest period; stirrer off. (c) anodic scan (v=10-100 mV/s). • 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 anamalgam. • This pre-concentration leads to low detection limits • 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. INSTRUMENTATION FIG 2 : Showing different electrodes arrangement. Fig 3: a typical cell. WORKING ELECTRODE • 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. • 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 • 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 • 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 ELECTROLE • 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 REFERENCES • Brezonik, P. L., Brauner, P., & Stumm, W. (1976). Trace metal analysis by anodic stripping voltammetry: effect of sorption by natural and model organic compounds. Water Research, 10(7), 605-612. • Copeland, T. R., & Skogerboe, R. K. (1974). Anodic stripping voltammetry. Analytical Chemistry, 46(14), 1257A-1268a. • Ellis, W. D. (1973). Anodic stripping voltammetry. Journal of Chemical Education, 50(3), A131. • https://en.wikipedia.org/wiki/Electrochemical_stripping_analysis • https://www2.chemistry.msu.edu/courses/cem837/Anodic%20Stripping%20Voltammetry.pdf • Skoog, D. A., West, D. M., Holler, F. J., & Crouch, S. (2013). Fundamentals of analytical chemistry. Nelson Education. • Wang, J. (2007). Stripping analysis. Encyclopedia of Electrochemistry: Online. • Whitnack, G. C., & Sasselli, R. (1969). Application of anodic-stripping voltammetry to the determination of some trace elements in sea water. Analytica Chimica Acta, 47(2), 267-274..
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