Chemistry » Introducing Electrochemical Reactions » Standard Electrode Potentials

The Standard Hydrogen Electrode

The standard hydrogen electrode

It is the potential difference (recorded as a voltage) between the two electrodes that causes electrons to flow from the \(\color{blue}{\textbf{anode}}\) to the \(\color{red}{\textbf{cathode}}\) through the external circuit of a galvanic cell (remember, conventional current goes in the opposite direction).

It is possible to measure the potential of an electrode and electrolyte. It is not a simple process however, and the value obtained will depend on the concentration of the electrolyte solution, the temperature and the pressure.

A way to remove these inconsistencies is to compare all electrode potentials to a standard reference electrode. These comparisons are all done with the same concentrations, temperature and pressure. This means that these values can be used to calculate the potential difference between two electrodes. It also means that electrode potentials can be compared without the need to construct the specific cell being studied.

This reference electrode can be used to calculate the relative electrode potential for a substance. The reference electrode that is used is the standard hydrogen electrode (see figure below).

Definition: Standard hydrogen electrode

The standard hydrogen electrode is a redox electrode which forms the basis of the scale of oxidation-reduction potentials.

The Standard Hydrogen Electrode

A simplified version of the standard hydrogen electrode.

The standard hydrogen electrode consists of a platinum electrode in a solution containing \(\text{H}^{+}\) ions. The solution (e.g. \(\text{H}_{2}\text{SO}_{4}\)) has a concentration of \(\text{1}\) \(\text{mol.dm$^{-3}$}\). As the hydrogen gas bubbles over the platinum electrode, the reaction is as follows:

\(2\text{H}^{+}(\text{aq}) + 2\text{e}^{-}\) \(\rightleftharpoons\) \(\text{H}_{2}(\text{g})\)

Fact:

The standard hydrogen electrode used now is actually the potential of a platinum electrode in a theoretical acidic solution.

The electrode potential of the hydrogen electrode at \(\text{25}\) \(\text{℃}\) is estimated to be \(\text{4.4}\) \(\text{V}\). However, in order to use this as a reference electrode this value is set to zero at all temperatures so that it can be compared with other electrodes.

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