stamasd
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Cathode deposit in chlorate cell
I am using a MMO anode with a titanium mesh cathode cell for chlorate production. Until recently I used it for potassium chlorate but for the past 2
weeks I've used it for sodium chlorate instead.
With the potassium salt, most of it falls to the bottom of the cell but minor amounts end up encrusting the anode and cathode. They are not too
adherent and can be easily scraped off.
Since I switched to sodium chlorate though, I get very thick and adherent, hard to remove crusts on the cathode. The buildup after a 4-day run is up
to 5mm on the upper part of the cathode, and 2-3mm at the bottom of the cathode. The anode is fine, almost no buildup of anything on it.
The deposit on the cathode is really hard and takes a lot of work to remove, scraping with a knife and then a metal scouring pad, and even after that
it still leaves a white layer behind that I can't remove. See the picture (cathode in foreground, MMO anode assembly behind it):
It's a porous but very hard deposit, almost like a ceramic. I tried a q-tip dipped in HCl on a part of the deposit, it didn't do anything.
Any ideas what that may be, and how I can clean it up? I'd rather not have to disassemble the electrodes from the lid as I did a pretty good job of
gluing them together.
(edit) The picture above is after the 2nd sodium chlorate run. The deposit seems to be getting worse with each run. There is a second cathode behind
the anodes that I haven't been using, it's there for redundancy - just in case something happens to the first cathode, like it seems to be now; but
since I plan to switch the cell back to potassium chlorate at some point, I'd rather have the deposit removed before I do that. Worst case scenario, I
may have to physically remove the first cathode.
[Edited on 25-5-2018 by stamasd]
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phlogiston
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I there is a small amount of calcium or magnesium in your electrolyte, it will cause buildup of the corresponding hydroxides on the cathode. This can
affect the electrical resistance enough to almost completely block any current from flowing.
Small amounts of Mg carbonate are often added to tablesalt as an anticaking agent. Ofcourse MgCO3 is insoluble in water, but if you acidify the cell
(typically by adding hydrochloric acid) to optimise the pH for chlorate formation, (some of) it could dissolve.
However, that does not fit with your observation that the deposit does not dissolve in hydrochloric acid.
[Edited on 25-5-2018 by phlogiston]
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stamasd
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I have thought of it being a magnesium salt, because the NaCl I use is not pure (it's sea salt); however I use pH control in my cell, I check it twice
a day and adjust the pH to 6.5-7 with HCl. That would keep magnesium oxide/hydroxide/carbonate from forming I think. Also I don't think it would be
magnesium chlorate because that one is soluble in water. Other impuroities in the salt I use? I don't know. I make my electrolyte with distilled
water. But I also reuse all the electrolyte from previous runs, so any impurities in the salt would accumulate - that may explain why it's getting
worse with each run.
I'll try to switch salt sources and see if it makes a difference. That still doesn't answer the question on how to remove what I've already got. Since
acid didn't work, maybe I'll try ammonia next. I don't expect that it would attack the MMO anodes it it gets on them accidentally, am I correct?
FWIW I haven't noticed the deposit to influence the electrical parameters of the cell at all. I run in constant current mode at 10A, and with an upper
limit on the voltage at 5V. The voltage during a run always stays between 4.35 and 4.5V regardless of the cathode buildup.
[Edited on 25-5-2018 by stamasd]
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phlogiston
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NaCl is so cheap and widely available, why use sea salt?
Are you using actual sea water? If so, to safe money or to prove a point? (eg. demonstrate making a powerful oxidiser from a common natural resource
that can not realistically be regulated)?
Otherwise, I don't see any reason to use sea salt, and it may well be the cause of your problem.
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stamasd
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No I don't use actual sea water. I use locally purchased sea salt from the grocery store. It was the only salt I found that didn't have additives
listed - but obviously it has trace other ions. I may end up sucking it and getting one of the other salts with anticaking agents and purify it
myself.
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fusso
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Probably CaSO4?
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stamasd
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There is no sulfate and no calcium in the system unless they come from the salt itself. And the initial solution was crystal clear, no precipitate. I
know CaSO4 is slightly soluble but I don't know why it would precipitate, and only on the cathode of all places.
Actually when running with the sea salt, the anodes at the end are much cleaner than when using KCl. With potassium, there's always some perchlorate
that sticks to them at the end and needs some gentle nudging off. But with NaCl they remain squeaky clean.
In fact I think I may be done making NaClO3 for a while. I have accumulated a few hundred grams of it and it will need serious cleaning with multiple
recrystallizations, that will keep me busy for a while. Once I get the cathode cleaned I'll probably set the cell back to KClO3 for the time being.
And concentrate on working on an idea I've had for making large platinized titanium electrodes on the (relatively) cheap at home. 
[Edited on 26-5-2018 by stamasd]
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fusso
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Maybe TiO2 [edited:] or Ca/Mg titanate?
Also did the product label of the sea salt mentioned the composition?
[Edited on 26/05/18 by fusso]
[Edited on 26/05/18 by fusso]
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stamasd
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"natural sea salt, no additives"
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