12AX7
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Chlorate cell/persistent color mechanism?
Just had a thought about why my chlorate cell liquor is still green.
Initially, some chromate/dichromate is added. Having a negative charge, this is attracted to the anode, where it passivates it (reducing graphite
wear), according to theory. Or something like that. At the cathode, the basic solution gives CrO4(2-) ions, which are perhaps reduced by nascent
hydrogen to Cr(3+).
In a cell running without pH control, the average pH goes somewhat alkaline. On average, Cr(OH)3 and Cr(OH)4(-) species are most common in solution,
then. Due to the stirring action of the hydrogen bubbles, the Cr(OH)3 remains suspended (or just remains dissolved). At the cathode, Cr(OH)4(-) is
repelled due to its negative charge (though some may be reduced further). At the anode, the acidic solution nearby shifts the chrome to Cr(OH)2(+)
and Cr(OH)(2+) species, which are repelled by the anode (some may be oxidized).
The only solid-state sinks are precipitation of Cr(OH)3, which may not occur significantly at this pH, and reduction at the cathode, which also
doesn't appear to go very far.
The high concentration of Cl- will also tend to encourage [CrCln] species, which may tend to be more soluble.
...And then there's the yellowish color I'm getting now. This happened after boiling some liquor in tin cans, so it's most likely an iron impurity.
This solution is next in queue so we'll see if it changes...
Tim
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alancj
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In electrolysis of NaCl to ClO3 I got green, amber, deep (almost red) amber, and yellow colors at various times in processing of the electrolyte. This
is using titanium cathode and graphite gouging rods for anodes in my chlorate cell. For a while I used stainless steel for my cathode. I never used
any additives, just plain NaCl and water for the electrolyte. The gouging rods apparently have iron specs in them because I got sparks when sanding it
on a disk sander. The salt was water softener grade.
I got green and amber/red after filtering the mother liquor, then it all turned to a yellow color after boiling. So whatever it was that was making
the darker color(s) gets destroyed by heat. Maybe iron hypochlorite? After boiling I suppose that would turn into iron chloride.
I used glass for the cell container and boiled in a glass beaker, so no other impurities were introduced but it still turned yellow like yours. So I
think it was iron to begin with.
Of course, I may have also hade some chromium ion present too... from the stainless steel that was used for awhile (a few days). But it shouldn't be
in near the concentration that was in your liquor, where it was an additive, yet we both seem to have had the same things happen.
Do you think you have iron coming from your graphite?
-Alan
Edit: I also used one of those graphite rods as an anode in a solution of NaOH to expose a clean, rough surface, before attempting to plate PbO2 onto
it. Using a copper cathode it turned the solution an amber color (after filtering out the graphite specs). That same rod did the same thing to my PbO2
plating bath, giving it a light amber color and producing a terrible coating. I don’t recall ever boiling either solutions to see if it turned
yellow.
[Edited on 21-4-2007 by alancj]
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12AX7
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Hmm, dunno. I'm using bulk graphite bar, no idea what kind of impurities might be in it.
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Per
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I used a grpahite rod and a Pb wire and no chrome at all and after a while the solution turned into a bright green color. There wasn´t any iron at
all, Í think that chlorine water is responsible for the green color.
But it doesn´t disturbed my NaClO4 production at all.
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