anodic stripping voltammetry

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ANODIC STRIPPING VOLTAMMETRY BY: CHITRAKSHI GOEL

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Page 1: Anodic Stripping Voltammetry

ANODIC STRIPPING VOLTAMMETRY

BY:CHITRAKSHI GOEL

Page 2: 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)

Page 3: Anodic Stripping Voltammetry

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.

Page 4: Anodic Stripping Voltammetry

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.

Page 5: Anodic Stripping Voltammetry

Linear Sweep Voltammetry

Linear potential sweep

Page 6: Anodic Stripping Voltammetry

Linear Sweep Voltammetry

Page 7: Anodic Stripping Voltammetry

Linear Sweep Voltammetry

For potentials from 0 to -1 V, the current is small and constant. This small current is due to the electron flow necessary to maintain a charged double layer at the electrode surface. This current is often called the charging current.

As the potential is varied beyond -1 V, an electrochemical reaction occurs at the surface of the electrode—that is, electrons transfer from the electrode surface to the hydrogen ions in solution, causing a reduction of H+ to H2 and a rapid increase in current.

Page 8: Anodic Stripping Voltammetry

Linear Sweep Voltammetry

Page 9: Anodic Stripping Voltammetry

Linear Sweep Voltammetry

With Cd2+ added to the HCl solution, the current does not remain constant from 0 to -1 V.

As the potential goes more negative, the reductive power of the system increases. Thus, at potentials where Cd2+ is reduced, the current increases.

As the scan continues in a negative direction, cadmium continues to reduce and the current continues to increase.

The current measured is a function of the concentration of the Cd2+ at the surface of the electrode.

Page 10: Anodic Stripping Voltammetry

Linear Sweep Voltammetry

Page 11: Anodic Stripping Voltammetry

Linear Sweep Voltammetry

The addition of a second electro-active species to the solution (in this case cadmium and lead) produces multiple peaks.

Each peak occurs at a potential characteristic of the reacting species, the type of supporting electrolyte and the working electrode type.

Page 12: Anodic Stripping Voltammetry

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.

Page 13: Anodic Stripping Voltammetry

Anodic Stripping Voltammetry Incorporates three electrodes:

a working electrode, an auxiliary electrode (sometimes

called the counter electrode), and a reference electrode.

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Page 15: Anodic Stripping Voltammetry

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.

Page 16: Anodic Stripping Voltammetry

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.

Page 17: Anodic Stripping Voltammetry

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.

Page 18: Anodic Stripping Voltammetry

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.

Page 19: Anodic Stripping Voltammetry

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.

Page 20: Anodic Stripping Voltammetry

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.

Page 21: Anodic Stripping Voltammetry

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.

Page 22: Anodic Stripping Voltammetry

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)

Page 23: Anodic Stripping Voltammetry

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.

Page 24: Anodic Stripping Voltammetry

Anodic Stripping Voltammetry The concentration of the reduced metal(s) in the

mercury will be given by

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

Hg

dlHg nFV

tiC

Page 25: Anodic Stripping Voltammetry
Page 26: Anodic Stripping Voltammetry

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 the 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.

Page 27: Anodic Stripping Voltammetry

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.

Page 28: Anodic Stripping Voltammetry

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,

  A: area of the film l: length of the film v: potential scan rate

2.7RT

AlCvFni Hg

1/222

p

Page 29: Anodic Stripping Voltammetry

Differential Pulse ASV- Several metals in scan- Extremely sensitive- Hydrogen wave- Mercury oxidation wave

Page 30: Anodic Stripping Voltammetry
Page 31: Anodic Stripping Voltammetry

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).

Page 32: Anodic Stripping Voltammetry

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