Re: Oxalate standard electrode potential

[Paul Sears <psears at nrn1_NRCan.gc.ca>]

> Subject: Oxalate standard electrode potential
> 
> High Paul,
> 
> >Does anyone have a number for the standard electrode potential
> >for oxalate/CO2?
> 
> I looked up in a few old chemistry books in a friend's house, and
> found the values of standard electrode potentials for CO2 and
> Oxalic acid:
> 
> 2CO2 + 2H+ + 2e- = H2C24  (electrode potential is -0.49V)
> 
> H2C2O4 + 2H2O = 2H2CO3 + 2H+ + 2e- (electrode is potential -0.386V)

	Thanks.  I suggest you keep the voltage pretty low in your
experiments, say less than 2 volts to start with.

> >        I really don't think that oxygen evolved at an anode is
> > going to erode the carbon, the claims of the manufacturers of certain
> > devices notwithstanding.
> 
> There was also the following reaction in the book:
> 
> C (graphite) + 2H2O = CO2 + 4H+ + 4e- (electrode potential +0.207)

	One can calculate the standard potential for lots of half-cell
reactions - it doesn't mean that they will actually occur.  An equivalent
statement is that the free energy of reaction for:

	C  +  2H2O  ->  CO2  +  2H2   is 79.9 kJ/mole, which is about right.

The point is that oxygen _is_ evolved, even though the voltage required
is higher than that theoretically needed for the production of CO2.

> There was also another reaction listed in the electrode reactions
> section, which I thought might also be used for cheap electrolytic CO2
> production: 
> 
> CH3OH + 2H2O = CO2 + 4H+ + 4e- (electrode potential +0.207V)

	There is something wrong here - this doesn't balance.  Once again,
how would you get the reaction to go?

> Do you think CO2 can be produced by electrolytic oxidation of
> methanol, perhaps in H2SO4 solution? Or, mabe sugar can be oxidized
> too, again with H2SO4 for electrolysis? This may become another
> sugar-based CO2 generator :), but much more controllable compared with
> yeast? 

	Don't forget that neutral species (alcohols, sugars) have no reason
to go to the electrode and take part in reaction.  Methanol _is_ used in
certain fuel cells, but that is a different story....

-- 
Paul Sears        Ottawa, Canada