Physical chemistry

Electrodes and electrode potentials

An electrode turns redox tendency into a measurable potential. Standard electrode potentials are measured relative to the standard hydrogen electrode.

An isolated electrode has no directly measurable absolute potential; tabulated E° values are relative reduction potentials, with the standard hydrogen electrode defined as 0 V.

Model and quantities

Read the relation together with assumptions about state, experimental conditions, and sign conventions. Keep units consistent and check dimensions before interpreting a result.

E^\circ(2H^+ + 2e^- \rightarrow H_2)=0\ \mathrm{V}

Definition: Standard hydrogen electrode (SHE)

The SHE uses inert Pt, H₂ at standard pressure, and solution with standard H⁺ activity. Platinum conducts electrons and facilitates exchange without being consumed.

Quantities in the relation are defined for the reaction or system at hand. In particular, distinguish standard-state quantities from actual conditions and do not infer a mechanism from a general expression alone.

Example: Worked example

A half-cell paired with the SHE gives a measured emf of +0.34 V when the unknown electrode is positive. Infer its standard reduction potential.

Solution

Since E°SHE=0 and the unknown electrode is the cathode, E°red = E°cell = +0.34 V.

Concept summary
Concept	Description	Unit / note
Key relation	Use under stated conditions	Check units and sign convention
Measured quantity	Relates a state or process	Compare data with model
Scope	Model specific conditions	Check assumptions first

A more positive standard reduction potential generally indicates a stronger tendency for the oxidizing species to accept electrons under standard conditions. Compare the two half-reactions to predict the cell direction.

In the worked example, which result follows from the given data?

Which statement is consistent with this lesson?

References

Peter Atkins, Julio de Paula (2014). Physical Chemistry