On Feb 23, 2010, at 2:09 PM, Michel Jullian wrote:
2010/2/23, Horace Heffner <hheff...@mtaonline.net>:
...
Therefore ion motion in the electrolyte proper is mostly
due to random walk and concentration gradients.
...
The ion motion is due to a force, what kind of force do you think, the
"concentration gradient force"? It's of course an electric force,
There are three ways a flux of ions can come about (Bockris p. 288):
diffusion, migration (also called electromigration and conduction)
and hydrodynamic flow.
entirely due to an electric field. Ultimately, that's what it is. Same
thing for electrons in a metal.
Michel
Consider Frick's first law of steady state diffusion, which states
the flow vector J_i for species i is proportional to the
concentration vector (d c_i)/( d x) in typical cell conditions, i.e.,
one dimensionally speaking:
J_i = - D (d c_i)/( d x)
where D is called the diffusion coefficient. There are other
adjustments to be made, but this version of Frick's law is adequate
for this discussion. Notice it works for species independently, and
can work in a zero electrostatic field environment with respect to
subsets of species.
The electrostatic gradients measured in the center of very large
cells can not possibly account for the species flows required to
support the current.
Just because diffusion of charged species is happening doesn't
necessarily even mean there exists an electrostatic potential
gradient. In an electrolyte having more than two charged species
(e.g. Na+, H3O+, and OH-), just because a large concentration
imbalance develops between two species at the electrodes, (e.g. H3o+
at one end and OH- at the other) doesn't mean a correspondingly large
electrostatic gradient necessarily develops. A very small
simultaneous migration of *all* the charged species not involved in
the electrolytic reaction (e.g. Na+) happens very quickly to
neutralize any large potential gradient in steady state conditions,
with the very slow diffusion velocities involved having very little
effect due to the motion in unison. It then only remains for the
electrode created species to reach concentration gradients that
handle the individual specie currents.
This was a key point in my discussion of how the hot spots formed on
the back side of the cathode mesh in the SPAWAR experiments. Hydrogen
ion (H3O+) conduction (and OH- in the opposed direction) can occur
right through the mesh, right through a zero electrostatic field
surface, by diffusion alone. However, there is necessarily a line
somewhere on both sides of the periphery of each mesh wire where zero
potential exists, and thus no interface layer. This is an ideal
location for recombination to occur right on the mesh wire, because
the H3O+ and OH- species are migrating in opposed directions and can
attach directly to the metal surface there.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
