Student name:_________________________________________


CHEM 1079  Analytical Science

Practical: Voltammetry Simulation

Due date:


The program Polar41c has been downloaded onto the computers in the  Department's computer lab.  You can download a copy yourself (as a zip file) from, unzip  and install it on your own computer.  Note that the program is too large to fit on a floppy disc.


In this exercise you will perform a number of experiments on a voltammetry simulator.  You may not have covered this material in lectures and should refer to your CHEM 1079 lecture notes and textbook (Harris, Chapter 18).





DC Voltammetry

In voltammetry, an inert electrode (eg., Pt, C, Hg) is placed in a solution containing electroactive species.  (Polarography is a subset of voltammetry that uses Hg electrodes.)  The potential of the electrode is changed as a function of time:

If an electrochemical reaction occurs, electrons are transferred and a current flows.   The current is measured as a function of the potential applied to the electrode.  A voltammogram is a plot of the current, i, on the vertical axis versus the potential, E, on the horizontal axis.  Cathodic (reduction) currents are shown in the positive y-direction and negative potentials are shown in the positive x-direction.


Explain the purposes of the three electrodes used in voltammetry.








In this series of experiments we are looking at the two-electron reduction of Cu2+ to Cu.  The Eo of the reduction is +0.34 V relative to the standard hydrogen electrode, or +0.10 V relative to the saturated calomel electrode, SCE, (which is more commonly used as a reference in voltammetry).


If we apply a potential greater (more positive) than 0.10 V vs. SCE, the reaction

Cu2+ + 2e  ®  Cu

will not occur.  In voltammetry of metal ions, we start with an applied potential well in excess (more positive) of the Eo of the metal ion present and scan the potential in the negative direction as a linear function of time.  As the potential approaches 0.10 V, the reaction above begins to occur and a current flows.  As the potential is brought closer to 0.1 V, this current increases until, a little way past 0.1 V, it flattens out.  See for yourself:


Load the program from Windows:

·        Start/ Programs/ Polar/ Polar

·        Maximise

Now run the simulation:

·        Input/ Techniques ® DC voltammetry

·        Input/ Mechanism ® A + 2e®B

·        Run/ Simulate


Sketch the polarogram (i versus E):














Note the gradual increase of current with voltage around the Eo.  Current (coulombs per second) is a measure of the rate of the electrochemical reaction.  The reaction starts slowly at first but is driven faster as the voltage is increased.  Note also the levelling off of the current to form a plateau.


What causes this plateau?  As Cu2+ ions near the surface of the electrode react to form Cu (that is deposited on to the electrode), the region close to the electrode is


depleted of Cu2+ ions.  We have

to wait for more Cu2+ ions to diffuse from the bulk solution (where they are in high concentration) to the electrode surface (where they are in low concentration) before the reaction can continue. The rate of reaction, and hence the current, becomes limited by the rate of diffusion of ions from the bulk of the solution to the electrode surface, and the current plateaus.

There are a couple of parameters that can be obtained from the voltammogram that are of interest to us. The diffusion current , id, is the (diffusion) limited current at the top of the voltammogram.  The half-wave potential, E1/2, is the potential at id/2.


Find the values of id and E1/2:

·        Analysis/ Find Halfwave E

You can put gridlines in if you want:

·        Display/ Option ® Grid


Show id and E1/2, on the voltammogram you sketched on the previous page.


Let’s now look at the effect of changing the concentration of the Cu2+ ions will have on the voltammogram.


Run the simulation:

·        Display/ Option ® Overlap (this enables the two voltammograms to be displayed on the same plot)

·        Input/ Chemicals ® Canal(A) = 2e-3

·        Run/ Simulate

What has happened to id and E1/2?  Find out:

·        Analysis/ Find Halfwave E


Sketch the voltammograms for the 1 x 10-3 M  and 2 x 10-3 M solutions on the same axes.












You may well have expected to see a change in E1/2 with concentration since this is what is seen in potentiometry (as described by the Nernst equation).  In voltammetry, E1/2 is unchanged by concentration, and it is id that is concentration dependent.


Try another couple of concentrations, tabulating the data (to 3 sig. figs. only) below:

[Cu2+] / M

id / A










Now plot id versus concentration using the data you collected in the simulations in the space below.  What do you notice?