Explain why standard electrode potentials must be measured under standard conditions, and state what those standard conditions are.

Standard electrode potentials are measured under standard conditions so that the potentials of different half‑cells can be compared directly. The potential of a half‑cell depends on the activities of the ions involved; if the concentrations, temperature or pressure were different the measured potential would change. By fixing the conditions the values become reproducible and comparable.

The standard conditions are:
1. Ion concentration of 1 mol dm⁻³ for all aqueous solutions.
2. Temperature of 298 K (25 °C).
3. Pressure of 100 kPa (1 atm, 101 kPa accepted).