Overpotential. Also known as Kinetic Polarization.
This is the big one for production of Oxygen. This phenomena
has to do with the rate at which you drive the reaction. If you
want to produce
large volumes of Oxygen gas, you will need to apply on the order
of 1.5 to 2 volts
in excess of the 1.229 Volts listed as a standard reduction
potential.
Overpotential is dependent upon the current density (amperes per
square meter)
and the metal that the electrode is composed of. Evolution of
Hydrogen and Oxygen
are particularly problematic as concerned with overpotentials.
The amount of excess
voltage required decreases with increasing temperature of the cell.
The best metal to use is platinized platinum, that is fine
deposits of platinum on a
conductor comprised of platinum.
Chloride ion in solution will become oxidized at a potential of 1.359
volts, thus if the overvoltage
for the electrode of choice is large, then the chloride ions will be
predominantly oxidized.
Now as we lower the concentration of chloride ions, there are less of them
to react at the surface,
permitting some of the water to be decomposed into oxygen gas.
Remember that overvoltage is not absolute.
Small amounts of oxygen are being produced, but no evolution of bubbles is
evident.
As far as the actual concentration necessary to generate oxygen gas, that
is going to depend
on the specifics of the cell. i.e. applied voltage, electrode material
and area, volume, stirring, temperature, etc., and I cannot give you a
definitive answer.
If you wish to generate oxygen at relatively large rates, or to
demonstrate the 2 to 1 ratio of
the composition of water, then you need to change electrolytes to
something that does not
oxidize at the anode, leaving only water to decompose into oxygen and
hydrogen gases.
Sulfuric acid fits that bill nicely. A few drops is all that is required,
but a higher concentration will make the Ohmic Potential drop
Any textbook on quantitative analysis would be a marvelous reference for
a description of electrochemical cells and techniques. Keywords to look
under are the terms described above, plus electrolysis and
electrogravimetry,
A good question!
Michael M. Gallagher Ph.D.
Senior Research Chemist
J.R. Simplot Co.