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]

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.

Am I right or wrong ?

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

I realize 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.

 

[fergus1]

you would have to mix 2x 2mdm-3 to get one solution with concentration 1mdm-3 for each ion.

 

[Borek]

you would have to mix 2x 2mdm-3 to get one solution with concentration 1mdm-3 for each ion.

Getting 1M solution is not a problem, problem is, 1M solution won't work as you expect it to do.

 

[fergus1]

what is the best approximation?

 

[Borek]

I am afraid there is no easy way of dealing with the problem.

The best way is probably to prepare series of solutions containing diluted ions, measure the potential in each one, calculate ionic strength of these solutions and activity coefficients, and to extrapolate to solutions with activities equal 1. Unfortunately we don't have a good theory allowing calculations of activity coefficients from the first principles in solutions with ionic strength above 0.1 (which is quite diluted).