What constitutes the standard state for a solute in electrochemical problems?

• A. 1 M concentration in solution, 1 atm for gases, 25°C
• B. 0.1 M concentration in solution
• C. 10 M concentration in solution
• D. 1 mM concentration in solution

Answer: (A)

In electrochemistry, the standard state is the reference point where the activities of species are taken as unity. For a solute in solution, this means using a standard state of unit activity, which corresponds to a 1 M concentration in dilute solutions. This choice defines the standard electrode potentials, E°, under those conditions, so comparisons between reactions are consistent. The broader standard-state definition also includes 1 atm for gases and 25°C for temperature, but when focusing on a solute, the essential point is that 1 M in solution serves as the reference. Concentrations like 0.1 M, 10 M, or 1 mM do not serve as the standard state because they change the activity away from unity and would shift the calculated potentials.