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Basic Electrochemical Concepts

In the broadest sense, electrochemistry is the branch of chemistry that deals with electron transfer processes and their relationship to chemical changes.

A simple yet classic example of electrochemistry in action is the lemon battery. Here, a copper coin and a galvanised (Zn coated) nail are inserted into a juicy lemon. A voltmeter connected between the two electrodes will register a potential difference of around 900 mV, with the copper electrode being positive and the zinc electrode negative.

At the two electrodes, redox (oxidation/reduction) reactions are taking place. Zinc metal from the nail gets oxidised to zinc ions, while at the copper electrode, hydrogen ions from the acidic lemon juice are reduced, forming bubbles of hydrogen gas.

Overall, this system can be represented by the following two equations

Zn -> Zn2+ + 2e-
2H+ + 2e- -> H2(g)

The relative tendencies of two metals to give up electrons can be determined in an experiment using a galvanic cell. In the diagram shown opposite, a Zn electrode dips into a 1M zinc nitrate solution in a beaker and a Cu electrode dips into a 1M copper nitrate solution in a second beaker. A salt bridge containing a conducting solution connects the two beakers (known as "half-cells") as shown. If the leads of a voltmeter are connected + to Cu, and - to Zn, a potential difference will be registered - approximately 1.10V.

What is happening here ? In the left hand compartment, the zinc electrode is undergoing oxidation, forming zinc ions, while in the right hand compartment, copper ions from solution are plating onto the copper electrode. Accompanying these processes is a spontaneous flow of electrons in the external circuit.

Overall the process occurring here is
Zn + Cu2+ = Zn2+ + Cu

The Electrochemical Series

By conducting many experiments like the one above, electrochemists have worked out a scale of "standard reduction potentials". The more positive the reduction potential E0, the more favourable the reduction process involving the metal ion being reduced (gaining electrons) to the corresponding metal.

The standard reduction potential or E0 value for the Cu2+|Cu half-reaction is +0.34V, while that for Zn2+|Zn is -0.76V. In order to decide the overall outcome in a system of two half-cells, the rule is simple - reduction takes place in the compartment with the higher standard reduction potential (E0), and oxidation (or loss of electrons) in the compartment with the lower E0. Furthermore, the difference in the two E0 values gives the overall cell potential.
The potentiostat is an important instrument for electrochemical research. Potentiostats have many analytical applications, including the detection and measurement of extremely low concentrations of particular heavy metals (such as lead, Pb and cadmium, Cd) in water samples.

Electroanalytical experiments with a potentiostat are performed in an electrochemical cell containing three electrodes, as illustrated in the photo opposite. The electrodes within the cell are known as the working electrode (WE), the counter (or auxiliary) electrode (CE) and the reference electrode (RE).

A solution of interest is placed into the cell and the three electrodes are connected to the potentiostat, whose function is to precisely control the potential difference between the WE and the RE. It does this by injecting current at the counter electrode CE. While this potential control is being maintained the instrument measures the current at the working electrode. Different “waveforms” are then applied ie (VWE - VRE) varies vs. time depending on the user’s application.

Here we describe two potentiostats that we have developed and illustrate some of their applications.