Re: CS>basic dumb question
From: Ode Coyote
Date: Sun, 27 Mar 2005 06:41:17
http://escribe.com/health/thesilverlist/m78970.html
> I've seen the silver smearing off either electrode [should be just
> one, right?]
If you are talking about misting, it can be both. Usually one
electrode will have some feature that makes it different from the
other. Such as length, a cut or burr on the side, different amount
of etching and wear from being used as the anode, etc.
The neat thing is if you can get the reaction to start at a specific
electrode, the ions normally produced at that electrode go into
making oxides instead of diffusing to the other electrode. So the
other electrode doesn't see as many ions and doesn't make as much
oxide.
I use that principle in my high-uS generator. I split the cell into
two rectangular chambers separated by a 4 inch polyethelyne pipe
with 1/2 inch inside diameter. Each chamber holds 500ml of dw.
(Here's the diagram. Please use a monospaced font)
#A Electrode 1/2" pipe #B electrode
| / |
+-|-------------+ / +-------------|-+
| | -------------- | |
| | -------------- | |
| | | | | |
| | Compartment | | Compartment | |
| | | | | |
| | "1" | | "2" | |
| | | | | |
| | | | | |
| | | | | |
| | | |
+---------------+ +---------------+
The theory is the ions in each chamber will tend to stay there, and
only a few will go through the pipe to the opposite compartment.
This requires a very high voltage to supply the current, since there
are few ions in the pipe to contribute to current flow. In fact, the
initial resistance is over 3 megohms. So I use a 330 volt constant
current source that supplies variable current. Right now I am using
755uA. It takes a while to come out of saturation.
The electrodes are flat 0.999 silver plates 1 inch wide. The wetted
length is 4 inches, so each electrode is about 4 square inches. I do
not count the reverse face since there seems to be relatively little
current there.
The magic part is if I get things working right, the silver ions go
through the pipe and enter the opposite chamber. Since the current
is so low, many of them reach the cathode and accept an electron and
plate out on the cathode.
But this has extremely beneficial results! It means one less
hydroxyl ion is created, so the population of hydroxyls is reduced.
It takes a very slight asymmetry in electrode length and placement
to achieve this. One electrode is slightly longer than the other. I
use this for the cathode. The chambers are also slightly different.
Since there are fewer hydroxyls, the chance of them combining with a
silver ion is reduced, so there are more silver ions to reach the
cathode and take an electron. This process avalanches, and a point
is reached where few hydroxyl ions are generated, and few are
available to travel to the anode compartment.
By now the ion concentration in the anode compartment is mostly
silver, with a deficiency of hydroxyl ions.
But the Hanna is calibrated for equal numbers of silver and hydroxyl
ions. So in theory, if there were zero hydroxyls, the Hanna reading
should be doubled. This means a reading of 40uS actually means a
silver ion concentration of 80ppm!
Of course, things never get this good. But the salt test is
considerable stronger than would be indicated by the Hanna reading.
Without more sophisticated (and expensive) equipment, I have no way
to tell what the actual ion concentration is, so I just go by the
PWT reading in uS.
Now, my problem is the cell voltage would stalls during the brew,
indicating everything was going to making oxides or plating out on
the cathode. I found by removing and cleaning the electrodes, the
process would continue and produce very concentrated cs.
> Anyhow, you can buy Silver Hydroxide Reagent? and it's described
> as a white powder, virtually insoluable in water.
I think there's some mistake. Silver hydroxide is black or dark
brown. Here's how it is used to make a mirrored flask. The
instructions say:
--------------------------------------------------------------------
"4. While stirring, add concentrated ammonium hydroxide dropwise
to the silver nitrate solution in the beaker until the gray-black
silver hydroxide, AgOH, precipitate forms."
"Continue adding concentrated ammonium hydroxide dropwise with
swirling until the silver diammine ionic complex, Ag(NH3)2+ (aq),
forms and the solution JUST becomes clear and colorless."
http://www.flinnsci.com/Documents/demoPDFs/Chemistry/CF10145.pdf
--------------------------------------------------------------------
So they use the black color of silver hydroxide to tell how much
stuff to add to make it colorless.
> There is a white powdery deposit that appears now and then under
> certain conditions.
Yes, I have seen that also. I don't know what it is. But much more
often the deposit that forms is black or dark brown.
> If the electrodes are very close to the bottom and no stirring is
> used, you get that white deposit on one side, a black spot on the
> other side under the electrode that turns black and shiny metallic
> plateout in between.
> Ode
I don't know what the white spot is, but I'm pretty sure the black
one is silver hydroxide.
I haven't put the electrodes that close to the bottom, but will try
it next time I play. I have seen the shiny metallic coat on the
sides of the glass when using high current density. This always
formed on my first cs generator long ago. A description is on my
Shingles page (caution graphic photos)
http://www.geocities.com/mrmonett/shingles/0shin.htm
Silver hydroxide is fairly stable but decomposes around 100C. It
also decomposes under slight pressure of wiping, which is how it got
on my glass. H2O2 will remove it, but slowly. H2O2 is much more
vigorous on the black oxide - it bubbles and fizzes.
Best Regards,
Mike Monett
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