url: http://escribe.com/health/thesilverlist/m63427.html
Re: CS>Re: Measuring ppms
From: Malcolm Stebbins
Date: Sun, 12 Oct 2003 03:21:54
[... snip nice discussion on Bronsted Lowry Proton Transfer theory]
>> Electrons cannot propagate in an electrolyte. Current flow is
>> through the movement of ions only.
> BTW, Mike, this is not strictly true, but it may be so for aqueous
> electrolytes - dunno.
I think it's true for any liquid electrolyte, but I'm not sure about
solid polymers such as film batteries. Do you know of any examples
of electron conduction in a liquid electrolyte?
> Hey, if water swaps protons around in the form of hydronium ions,
> does that mean it's a semiconductor and 'holes' propagate??
This is amusing. The same thought occurred to me. I found some web
sites that discussed the movement of protons via water molecules.
This apparently occurs very quickly - on the order of a few
femtoseconds. So movement of the hydroxyl ion could also be
explained by proton transfer in the opposite direction.
Ivan's data indicated the hydroxyl ion moves 6 times faster than the
silver ion under an applied field. This should mean the diffusion
velocities whould also be different by the same ratio. So I set up
an experiment to visualize the ions using red cabbage juice and
managed to get just the right amount to see both species.
The ions met in the middle between the electrodes! This was amazing.
It contradicted everything I had learned up to that point. This is
described in experiment #2 at
http://escribe.com/health/thesilverlist/m61491.html
I was too astonished to take accurate notes of the time, and the
hydroxyl ion turned the solution a deep color that made it
impossible to see anything past the halfway point. So I repeated the
experiment using salt, which did not show the hydroxyl ion. This
gave the approximate velocity of the silver ion, and showed it is
much slower than even very gentle convection currents:
http://escribe.com/health/thesilverlist/m61527.html
[...]
>> The ones further away may know an electric field is present, but
>> their drift velocity due to the electric field is perhaps orders
>> of magnitude less than their thermal velocity, and much less than
>> the diffusion velocity.
> Diffusion velocity?? If I'm going 100 mph back and forth randomly,
> and at the same time I'm propagating steadily toward San Francisco
> at 10 mph, I'm gonna get there; the traffic opposing me cannot be
> any thicker than the traffic going my way.
Yes, it is very similar to the Fermi velocity of electrons in
metals, typically one-tenth the speed of light. Even though they are
moving very fast, the drift velocity is about the same as a person
walking.
But that's not the problem.
Current is the movement of charge carriers. This is electrons in
metals, electrons and holes in semiconductors, and positive and
negative ions in electrolytes.
Consider a long tube with silver electrodes at each end, filled with
dw. Apply a constant current.
From the above experiments, there will be no silver or hydroxyl ions
in the middle of the tube at the beginning when current is applied.
Yet as more ions enter the solution, the conductivity increases. So
my question is how do the ions know there are more of them in the
solution so the conductivity can increase?
Take a Hall current probe that can measure DC current.
Measure the current in the wires. It will give the same value
anywhere along the wires.
Now measure the current anywhere along the tube. It will give the
same value anywhere along the tube. It has to. The current in a
series circuit is the same everywhere.
But there are no silver or hydroxyl ions in the middle of the tube
yet. So where are the charge carriers coming from that generate the
magnetic field detected by the Hall probe?
My conclusion is they must be the ions from the trace contaminants
in the dw. But they have a much lower concentration than the ions we
are now making at the electrodes.
The second conclusion is they must move faster to keep the magnetic
field the same value. Therefore there will be a non-uniform voltage
distribution along the tube. This should be easy to measure.
But these ions must exist. There must be a continuous path through
the water from one electrode to the other, containing ions that can
move. That's why my freezing experiment was doomed to failure.
Hmm.. This was posted on Saturday, Sept. 27. But it doesn't show up
in the archives, so I can't give you a link. I'll try to post it
later when I have some time. The experiment failed, but there was a
very interesting foonote.
Thanks for your interesting analysis, Malcom. It's refreshing to
find someone who knows the theory of chemistry.
Best Regards,
Mike Monett
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