Under standard conditions, which set of conditions defines the standard electrode potential?

• A. 1 M, 0 atm, 25°C.
• B. 1 M, 1 atm, 0°C.
• C. 1 M, 1 atm, 25°C.
• D. 1 M, 2 atm, 25°C.

Answer: (C)

The main idea is that the standard electrode potential is defined at standard state conditions where all activities are taken as unity. That means solutions are at 1 M concentration, gases are at a partial pressure of 1 atm, and the temperature is 25°C. In the Nernst equation, E = E° − (RT/nF) ln Q, using those standard conditions makes the reaction quotient Q equal to 1, so E equals E°. If any condition changes—like a gas at 2 atm, a solution not at 1 M, or a different temperature—the potential would differ from E°. Therefore, the set that defines the standard electrode potential is 1 M, 1 atm, 25°C.