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Author: Subject: Passivation of MMO anode?
Simoski
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[*] posted on 16-2-2019 at 22:59




Does this buildup of chlorate crystals on the anode make you think the electrolyte needs resaturation?

its definitely causing passivation




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[*] posted on 17-2-2019 at 13:02


It has become saturated with chlorate.

Harvest the chlorate by cooling the electrolyte and removing any chlorate that crystallises.
The re-saturate with chloride and continue.

If the chlorate that formed on the anode does not redissolve, add a little water and/or heat the cell. The current flowing through the cell will also heat it, but if the crust of crystals prevents any significant current flow you may have to use an external heat source until a significant flow of current begins. A little insulation may also help, aim for about 70 deg C.




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[*] posted on 17-4-2019 at 20:57


Quote: Originally posted by woelen  
A friend of mine just purchased an MMO anode with titanium substrate off eBay and used it to make KClO3. He used diet salt (a mix of 1/3 part NaCl and 2/3 part KCl by weight). He had a run from a 5V power supply and let this cell run until current had dropped strongly and as a consequence the cell cooled down and solid KClO3 crystallized. He used no additives in the solution and used a stainless steel cathode. After a few days of electrolysis he had a lot of solid KClO3 which easily could be separated.

He disassembled the cell, cleaned the electrodes and worked up the KClO3. Now he has purchased pure KCl and wants another run, but to our horror, the anode does not work anymore! When a concentrated solution of KCl is used, then only 0.2 A is running through the cell. Different cathodes were used, but always the same result, just around 0.2 A of current. He tested the power suppy and the cathode and these are just fine. It really has to to with the MMO anode.

It seems as if the anode has passivated. The anode was thoroughly cleaned and kept in clean water for some time, hoping that it would remove some invisible layer, but this also does not work.

What is causing this problem and what can be done about it? I do not have real knowledge about this subject and could not find good info on internet. Does anyone else have a similar experience or know of this?


You should not have used pure KCl as your electrolyte because if the chloride concentration becomes low then passivation of the anode will occur.

It is always better to use diet salt or a 50/50 mix of KCl and NaCl by weight and then gradually increase the concentration of NaCl until all KClO3 precipitates out and NaClO3 starts precipitating then your yield will be at the 99 percent.

then if you want you can add KCl to the remain solution to Salt out more KClO3 and recrystalize it.
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[*] posted on 17-4-2019 at 22:55


Quote: Originally posted by mysteriusbhoice  


You should not have used pure KCl as your electrolyte because if the chloride concentration becomes low then passivation of the anode will occur.

It is always better to use diet salt or a 50/50 mix of KCl and NaCl by weight and then gradually increase the concentration of NaCl until all KClO3 precipitates out and NaClO3 starts precipitating then your yield will be at the 99 percent.

then if you want you can add KCl to the remain solution to Salt out more KClO3 and recrystalize it.


I've always used pure KCl for the purpose, as sodium impurities are tenacious and require a lot of extra work and time (and also losses) to get out the product afterwards. No problem with MMO anode passivation so far and I have used the same one for almost a decade now. Seems indestructible to me, but perhaps it is just a suitable grade. The only precaution I take is to not aim for full conversion of the chloride contained in the cell and always top up with concentrated KCl as the liquid level falls off due to evaporation and discharge of water at the electrodes. The precipitated KClO3 is harvested when a sizeable amount has dropped out, the liquour is replenished with chloride and the process is repeated. In essence you have full conversion of chloride, just some always remains in the cell liquour.




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[*] posted on 18-4-2019 at 00:19


No need to use NaCl. Using KCl is fine, but it helps if you put your cell in a not too deep layer of water. This water provides cooling at the bottom and at that coldest place you get crystals of KClO3. In that way you will have less deposit on the anode. Deposit on the anode reduces current and removal of the crust may lead to mechanical damage.

I myself like using KCl only. With a single recrystallization you get quite pure KClO3 already, with two recrystalizations you get really pure KClO3, white like snow and free of sodium ions, and free of dichromate.

With NaCl you get NaClO3, but this is much harder to separate and cleanup, due to its much higher solubility, also in cold water, and its somewhat hygroscopic properties.




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