What is the difference between formal potential and standard potential?

• A. E° is defined under standard states; formal potential E°' accounts for nonstandard conditions by incorporating actual activities and complexation, often shifting from E°.
• B. They are identical in all circumstances.
• C. Formal potential is only defined for noble gases.
• D. Standard potential includes temperature effects.

Answer: (A)

The key idea is that the potential you measure depends on the state of the solution, not just the intrinsic chemistry of the redox couple. The standard potential is defined for standard states—activities of all species equal to 1 (usually 1 M for solutes, 1 atm for gases) at a chosen temperature. In real solutions, actual activities are not 1 and species often form complexes or interact with the solvent, so their effective activities differ from concentration alone.

Formal potential, E°′, captures these nonstandard conditions. It represents the potential you would obtain when you account for the actual activities (and any complexation that changes how much free species is available) rather than the ideal standard state. In practice, the Nernst equation uses these activities, so the measured potential can shift away from E°, often noticeably if complexation or high ionic strength is present: E = E°′ − (RT/nF) ln(Q′), where Q′ involves activities rather than simple concentrations.

That contrast makes sense: standard potential is fixed under standard states, while formal potential adjusts for how the real solution behaves chemically, including binding and nonidealities.