Standard Electrode Potential of Fe2+/Fe3+

elemis

If I try to determine the standard electrode potential of

Fe²⁺ -------> Fe³⁺ + e^-

What would be the concentrations of Fe ions ?

Would I mix 1 mol/dm3 of Fe2+ and 1 mol/dm3 of Fe3+ ions ?

Borek

More or less.

Standard means all substances involved are in standard condition, which further means their activities equal 1. Unfortunately, it doesn't mean their concentrations equal 1 M, although that's a reasonable first approximation.

elemis

Am I right or wrong ?

You seem to be hinting I am wrong so what would the concentrations need to be ?

I realise we are aiming for all concentrations to be 1 mol/dm3....

Borek

This is not easy.

Using 1 M solutions you will not get the correct result. Actual definition of the standard solution is "hypothetical 1 M solution exhibiting infinitely dilute solution behavior" (see http://goldbook.iupac.org/S05925.html). Hypothetical, as such solution doesn't exist - when the concentrations is 1 M, it is rather obvious substance can't behave as if it was infinitely dilute.

As such thing as standard solution doesn't exist, standard electrode potentials are not measured using a standard solution, but extrapolated from known real solutions.