Sciencemadness Discussion Board - Passivation of MMO anode?

Passivation of MMO anode?

woelen - 9-1-2011 at 01:15

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?

Jurre10 - 9-1-2011 at 01:37

Hi,

I'm the one Woelen is talking about.
He exactly writed how it went.

So we think there is an (invisable) layer on the anode,
but we both don't know how to remove it.

Hopefully, somebody who know this problem can give us a good answer.

Jurian

[Edited on 9-1-2011 by Jurre10]

hissingnoise - 9-1-2011 at 03:24

I haven't used MMO anodes so I'm just guessing . . .
I assume you've checked and double checked the connections.
Does the Losalt possibly contain additives that could be deposited onto the anode as a polymeric insulating material which could be removed by a solvent other than water?
A good microscope might reveal something . . .

Jurre10 - 9-1-2011 at 03:52

Hi,

Thanks for your tips, but LoSalt is just 66%KCl and 33%NaCl
So i think it can't be something from, the LoSalt.

Maybe it's a bit of the glue i used to glue the anode/cathode in place?
I don't think so because i used Epoxy glue that easely holds at 100°C.

The pont is that i don't have a microscope.

There is a little bit of corrosion on the lug, but i think that won't make the connection so bad that there is just 0.23A.

Jurian

hissingnoise - 9-1-2011 at 04:51

Hmmm . . .
LoSalt - from wiki; "An iodised version is also available in the Nederlands and Middle Eastern markets".

Jurre10 - 9-1-2011 at 05:13

hi,

I know, but thats a different color package and there's a little text on it hat says : iodised
So i don't have the iodised version, just naturel.

I have threats of this on 3 different forums but nobody knows whats happening?
So i'm afraid i have tobuy a new one

Jurian

hissingnoise - 9-1-2011 at 05:29

Quote:

So i'm afraid i have tobuy a new one

Or save some pennies for Pt foil . . .
But don't discard your old one - keep it for further tests.
Solving the puzzle might be advantageous at some point!

Jurre10 - 9-1-2011 at 05:44

Quote: Originally posted by hissingnoise

Quote:

So i'm afraid i have tobuy a new one

Or save some pennies for Pt foil . . .
But don't discard your old one - keep it for further tests.
Solving the puzzle might be advantageous at some point!

hi,

I know, hopefully i will find it out.
but when i buy Pt, im also able to make KClO4

i just hope my mmo anode will come back to life

Because it waas not that cheap

Jurian

woelen - 9-1-2011 at 06:13

Testing the connection can be done easily. Just put a clamp on the titanium strip of the anode and put another clamp at an other position of the strip and connect these through a decent 5 V lamp (e.g. lamp from a bicycle, which takes at least 500 mA). But I do not believe this is the issue, I really think this is due to formation of some ultrathin layer, which passivates the anode. And no, having an optical microscope at hand will not help here. Usually such layers are just nm sized and they can only be observed using extremely powerful (and expensive) microscopes, such as electron microscopes.

Quote:

I have threats of this on 3 different forums but nobody knows whats happening?
So i'm afraid i have tobuy a new one

Just wait for more input from other members, this thread is just a few hours old.

hissingnoise - 9-1-2011 at 06:49

Yeah, people like dann2, or Swede, although I haven't seen posts from them for a while.
A bit of experiment seems in order though - like reversing polarity for a few minutes, or increasing voltage, current density, or both.
Dipping the anode in acid or basic solutions (H2SO4/HNO3/NH4OH) for short periods.
Carefully buffing a small area of the anode with a soft wheel and then checking the treated area for increased gas evolution.
And if all else fails, high quality graphite rods give very acceptable chlorate production rates with NaCl . . .

metalresearcher - 9-1-2011 at 06:57

Recently I tested a Ti anode which passivated shortly. By scouring with fine sandpaper I removed the passivation layer.
But I don't use it anymore for my chlorate cell. Last week I also ordered an MMO anode via ebay (probably the same as @woelen) as graphite anodes corrode and I purchased probably the same LoSalt as @woelen from a large supermarket chain called Albert Heijn..
I don't know whether the scouring of the MMO anode is a good option as it might destroy the MMO layer.
I know that LoSalt contains very small amounts of MgCO3 and KI (iodated) but that should not disturb the cell.
@woelen: did you try a carbon rod just for test whether now it really draws 5 A ?
Now I am warned what to do when I receive my MMO anode as I have the cell ready.

Jurre10 - 9-1-2011 at 07:25

Hi,

@hissingnoise : @Woelen told me Swede is verry ill

@metalresearcher so you just removed the layer with sandpaper

sounds good!
But isn't it dangerous? i think you may damage de MMO layer?

I don't have graphite rods right now so i can't try them.

Jurian

b.t.w The LoSalt on your photo is the iodised version....

hissingnoise - 9-1-2011 at 07:33

I'm sorry to hear about Swede, woelen; I hope it's not too serious and that he recovers fully soon.
I had missed his very informative posts for a while now . . .

Jurre10 - 9-1-2011 at 07:57

I'm sorry to tell you
I just heard it from Woelen.

I gonna ty things like higher voltage and reversing polarity for yust few secs.

Jurian

hissingnoise - 9-1-2011 at 09:19

Quote: Originally posted by metalresearcher

Now I am warned what to do when I receive my MMO anode as I have the cell ready.

I'd use NaCl rather than KCl for chlorate cells even with MMOs - NaClO3's high solubility make it less convenient but it seems somewhat less corrosive than KClO3 (possibly because of Na's smaller atomic radius).

- Jurian, I'm wondering if having two chlorides with widely differing solubilities in the cell could have had some kind of effect on the surface oxides.
But, perhaps dann2 will chime in with a different perspective . . .

woelen - 9-1-2011 at 09:38

Yes, Swede seems to have health issues (at least that is what I know from my last contact several weeks ago). He also is very busy with his work. Whatever is the situation at the moment, let's hope he will be back soon in good health with his invaluable input and knowledge.

Treating the anode with sandpaper is really a last resort action only to be applied when everything else fails. The MMO layer is very thin and very little abrasive action is needed to remove that layer completely. If this is done, then a very gentle abrasive action should be used (e.g. ultrafine CeO2 or SnO2 may be used, applied by gently rubbing with a wet piece of fabric which has some of the powder on it).

The other suggestions can be used, such as increasing the voltage. But only do that for a brief amount of time. High voltage may actually increase passivation and more and more voltage may be needed for operating the cell.

metalresearcher - 9-1-2011 at 09:43

What about using nonpolar solvents like white spirit or acetone ?

Jurre10 - 9-1-2011 at 09:55

oke guys,

Something really strange happened!
I first applied 5v and there was a little 0.15A trough the cell.

I had two things i wanted to test :
- 12V instead of 5V
- Reverse Polairity

I wanted to start, by applying 12V for 5 sec, so i did but what happened?!
The stainless steel cathode begon bubbling verry hard! Also the anode wass bubbling, but the bubbles where much smaller.
I looked to my multimeter en gues what! : 7,53A!!!

Then i stopped the cell picked up some 5V wires... just 0.13A.
So is it the anode?I don't think so!

Jurian

b.t.w i did test the power supply, it gives the right voltage,but not amperage?!

[Edited on 9-1-2011 by Jurre10]

woelen - 9-1-2011 at 10:16

It is the anode, but its overpotential has increased. Your power supply is perfectly fine, also its 5 V part.

If you look at the operation of an electrolysis cell, then you can understand what happens. The redox reactions occurring in a cell needs some voltage (around 2 volts total for both the anode and cathode reactions). The product of current and this redox voltage is the amount of energy, which is converted to chemical energy (new more energetic compounds).

So, in theory, there would be a reaction if the applied voltage is just above the redox potential. In a real cell, however, there is more than that. Each of the electrodes needs takes some more voltage. This voltage is the so-called overpotential needed for the reaction to occur. It depends on the material of the electrodes, it depends on how the bubbles of gas are formed and on the concentration of the electrolyte. Finally, there is someohmic resistance of the cell. This mainly depends on the size of the ions in solution and on the geometry of the cell.

Now let's have the following simple example:

2 V for redox potential
1 V for overpotential at anode
1 V for overpotential at cathode
ohmic resistance is 0.5 ohms

When 5 volts is applied to such a cell, then 5 - 2 - 1 - 1 = 1 volt remains for the ohmic resistance. With 0.5 ohms, a current of 2 A will flow through the cell.

Now assume that due to some layer on the anode, the anode overpotential rises to 1.9 volts. In that case, only 5 - 2 - 1.9 - 1 = 0.1 volts remain. With a 0.5 ohm resistance only 200 mA can run through the cell. If now you apply 12 volts, then 12 - 2 - 1.9 - 1 = 7.1 volts remain. With that current you have 14 A through the cell.

So, what I assume here is that indeed the overpotential on the anode has increased.

Jurre10 - 9-1-2011 at 10:48

Hi,

So we are back to where we begun there's a layer on the anode.
But i dont know what kind of layer and don't know how to remove it

I gonna try to reverse polarity for short times and else..:

Quote:

Dipping the anode in acid or basic solutions (H2SO4/HNO3/NH4OH) for short periods.

Jurian

[Edited on 9-1-2011 by Jurre10]

metalresearcher - 9-1-2011 at 11:03

This layer is MMO, deliberately deposited on the anode before you purchased it I think.
I think @woelen is right. Applying more voltage will help. Moreover to get more chlorate ions ClO3- a higher bath temperature is better. So the 'waste' energy as heat is not actually wasted, but increases ClO3- formation.

And running the first hour on 5 volts changes the composition of the electrolyte (e.g. forming ClO3-) so continuing would require a highte voltage.

These are just my thoughts, I do not (yet) have esxperience with MMO anodes only with carbon rods in achlorate cell.

And did you add some K2Cr2O7 to improve efficiency ? That might also have some (adverse?) influence on the current through the cell.

Jurre10 - 9-1-2011 at 11:18

Hi,

So i can run my cell on 12V? or not?
I do have some Potassium Dichromate, that Woelen give me.

But i didn't use it yet.

Jurian

woelen - 9-1-2011 at 11:33

No, don't run it at 12 V. You will have really excessive production of heat and strong wear of the anode. First try reversing polarity for a brief amount of time and then use the cell in normal operation at 5 V. That might reverse the effect of formation of a thin layer. If that does not give satisfactory results, then you can use a higher voltage, say 6 to 7 volts. This can be achieved by using a series resistor between the 12 V power supply and the cell. A good value to start with might be 2 Ohm's (use a parallel connection of five 10 Ohm resistors, each resistor being a 10 W model). With that value you'll have 3 A through your cell.

Applying 12 V directly to the anode simply puts too much wear on the anode and your cell will become very very hot.

Jurre10 - 9-1-2011 at 11:41

Hi,

Ok, then i gonna try to reverse polarity this evening.
Hopefully, the layer will remove ...

Jurian

Jurre10 - 9-1-2011 at 12:40

hi,

Started with normal polarity : 0.15A
Reversed polarity : 4.54A

again : normal :0.20A?!
Reversed : 4.73A

Again normal 0.30A
Reversed:5A

Every time of reversed was 10 seconds.
So it looks like after each time its going better!

Stopped after the 3 times.
Can i go further with this or wil i damage it?

Jurian

woelen - 9-1-2011 at 13:12

Keep doing this, each time for one minute and then measure normal current also for at least 1 minute.

Xenoid - 9-1-2011 at 13:22

Hi Jurre10

Seems to me like you have been sold an anode made from used or partly worn out MMO which may have come from old, larger factory anodes.

Does the MMO coating appear fairly thick and slightly rough or black and smooth. Worn out MMO leaves a black smooth surface.

The oxide passivation surface will breakdown if the voltage exceeds about 10 volts and the titanium will be rapidly converted to TiO2.

To see what happens, have a look at my home made MnO2 anode when the coating wore off and the voltage was allowed to rise!

http://www.sciencemadness.org/talk/viewthread.php?tid=9572&a...

The photo is at the botom of page 11 of the "Cobalt oxide anode" thread.

EDIT:
About half way down the "Thoughts on Anodes" thread is a post by dann2 where he is trying to make perchlorate using LaserReds MMO from ebay. The electrode passivated in short time possibly due to the low chloride level. Later on, down the page, dann2 thinks the ebay MMO is second hand or poor quality!

http://www.sciencemadness.org/talk/viewthread.php?tid=5050&a...

[Edited on 10-1-2011 by Xenoid]

[Edited on 10-1-2011 by Xenoid]

I have had a similar experience

markgollum - 9-1-2011 at 14:05

I have had two MMO anodes passivate/wreck on me. This was a good 3 years back so my memory is slightly fuzzy. I had run one cell for a few weeks and had made 1-2 Kg each of NaClO3 and KCLO3 when the first anode suddenly passivated (Over the course of about a day).
The first anode had gradually built up a white, very hard, abrasive, non-uniform deposit that I could not remove or affect with hydrochloric acid, sodium hydroxide as I recall helped but not much. I remember having a feeling that the deposit was silica. The initially shiny titanium cathode had darkened with what I felt was a small amount of electrophoretically deposited MMO particles. However, no amount of scrubbing with plain soap and water would clean up the cathode. The second anode was wreaked after a 3-4 days of operation in the same electrolyte. It had only a very slight amount of the white deposit (a few bits here and there, perhaps 1% coverage) BUT the second titanium cathode was covered in the black film. The initially slightly glossy uniformly black anode was significantly lighter in color, comparing it side by side with a new anode; it's obviously lost most of the MMO coating.

If I were to use MMO anodes to produce chlorate again (I won't I would rather go back to graphite) I would not run KCl as an electrolyte since KClO3 builds up on the electrodes. I would also recrystalize the table salt to remove the iodide and the silicoaluminate anti-caking agent. I think some other brands contain some kind of fluorosilicate (also bad). I was using a silicone containing TiO2 to seal the electrodes to the cell lid, which was a ceramic tile I had carefully cut two slots into with a dremel and a diamond bit. A lexan plate with matching slots was siliconed and epoxied to the top of the tile.

I also think that silica could enter the electrolyte by the wet chlorine oxidation of the silicone.

I have attached
"Effects of Electrolyte Impurities in
Chlorate Cells"
Presented By: Richard A. Kus
Senior Research Chemist
ELTECH Systems Corporation
16th Annual Chlorine/Chlorate Seminar
Renaissance Cleveland Hotel
Cleveland, Ohio

Attachment: Effects_of_Electrolyte_Impurities.pdf (712kB)
This file has been downloaded 1982 times

[Edited on 10-1-2011 by markgollum]

markgollum - 9-1-2011 at 14:10

markgollum - 9-1-2011 at 14:14

Sorry for the multiple post

Xenoid - 9-1-2011 at 20:14

There is a post here, about halfway down page 30 of the "Thoughts on Anodes" thread, by Bikemaster. He is complaining about an MMO anode that has lost it's coating.

http://www.sciencemadness.org/talk/viewthread.php?tid=5050&a...

He did get 8 months running out of it, in a KClO3 cell, but eventually let the chloride level drop to below 100g/litre. Again, this may have caused the rapid decline of the MMO coating and passivation of the TiO2. Doesn't look like there is any coating whatsoever left on that anode.

Jurre10 - 9-1-2011 at 21:37

Quote: Originally posted by Xenoid

There is a post here, about halfway down page 30 of the "Thoughts on Anodes" thread, by Bikemaster. He is complaining about an MMO anode that has lost it's coating.

http://www.sciencemadness.org/talk/viewthread.php?tid=5050&a...

He did get 8 months running out of it, in a KClO3 cell, but eventually let the chloride level drop to below 100g/litre. Again, this may have caused the rapid decline of the MMO coating and passivation of the TiO2. Doesn't look like there is any coating whatsoever left on that anode.

Hi,

He sais he got the same anode as me.
It's from ebay from north-star pyro, and size is 2"x6"

I think mine didn't lost he's coating, but there migth be an coating over the anode like what Swede replies on Bikemaster

Today i gonna look how the anode is, which color,rough or soft... Maybe i can upload some pictures

Jurian

Xenoid - 9-1-2011 at 22:52

The reason I mentioned the nature of the MMO coating is that my MMO experience is wholly with swimming pool chlorinator electrodes. I get them in used condition from a local contractor. Some look perfectly serviceable as chlorate anodes, and are very black, but when you try to use them they are actually passivated. These would have been in use for several years.

On the other hand, some of the electrodes have totally lost the black MMO coating and are a bronze-silvery colour.

I assume your anode still looks black. Can you see any slight texture in the black coating or is it perfectly smooth. MMO has a slightly rough texture.

Did you run the anode with low chloride concentration?

hissingnoise - 10-1-2011 at 04:47

Quote: Originally posted by markgollum

If I were to use MMO anodes to produce chlorate again (I won't I would rather go back to graphite) I would not run KCl as an electrolyte since KClO3 builds up on the electrodes. I would also recrystalize the table salt to remove the iodide and the silicoaluminate anti-caking agent.

Yes, graphite (bought from a company specialising in contact brushes) gave me good results!
Cheaper rods disintegrated rapidly but they were made of compressed carbon powder with some kind of polymer binder which couldn't be filtered off.
I used to recrystalise salt by chilling a saturated solution to -20*C!
A fairly slow business but purity more or less guaranteed . . .

woelen - 10-1-2011 at 05:51

The remark about using NaCl instead of KCl is one which I do not agree with. I already did around 10 runs, both with KCl and with CsCl and in all runs no crust of material settles at the anode. The huge advantage of using KCl is that you directly get crystals of KClO3 and workup is much easier. You also get a product which certainly is free from Na-ions, allowing good colors in pyro-compositions.

I did have formation of crusts on the anode when KBr is used for making KBrO3, but with KCl no such problems exist.

Apparantly the clue to preventing problems is to assure that concentration of chloride does not decrease too much. I always stopped electrolysis when the current dropped to approximately half the value it has on its maximum. Apparently low chloride concentration leads to passivation of the anode.

Jurre10 - 10-1-2011 at 07:06

Hi,

Thanks for all answers.
My MMO anode looks as follows : black and really soft, it also feels soft, not rough.

Jurian

Jurre10 - 10-1-2011 at 12:31

hi,

Just reversed polarity few times for 1min each time.
...0.90A,1.0A,1.1A so i think this is the way to make it better.

Jurian

dann2 - 10-1-2011 at 12:38

Hello Folks,

It would be an understatement to say that I hope Swede gets back on his feet soon.
Sometimes the only way to get some extra time, if needed, when involved in the 'Chlorate and Perchlorate' business is to shut everything down and STAY COMPLETELY AWAY from the scene!

Great read that paper on Chlorate DSA impurity problems. It covers alot of ground that I have not seen before anyways.
Just wondering if you were to view the DSA under ultraviolet light would it perhaps show up some problems clearer.
I do not have alot of experience with MMO. I ran a piece of corrosion control MMO (Ti wire covered in a black coating of MMO which was Ta Oxide + Ir Oxide especially made for working as a corrosion control Anode) in a Perchlorate cell for approx. 25 days and it did not passivate. Some of the coating wore off in places though.
It's a wonder that DSA works at all in an Amateur environment after reading all the possible problems that crop up with impurities!
I would guess that Jurre10's DSA has passivated or partially passivated, some TiO2 has managed to form between the MMO's and the Ti substrate. Just reading your last post that the MMO is soft?? Perhaps you mean smooth? There are many types and textures and colours for MMO. Perhaps the pool chlorinations have some Ir Oxide in them.
That stuff is black. The Ir Oxide (perhaps together with Tantalum Oxide as opposed to Ti Oxide is better for
use where very low contentration of salt in encountered. (Swimming pools, corrosion controll applications). They are used at low current density though I think (when used in the swimming pools etc.) Just wondering Xenoid do
you know what current density the pool Anode are worked at in a pool?
A wire Anode for corrosion control that I had (Ir and Ta Oxide) was 100% smooth when new and black in colour.

What was the ebay source? (Answered above it's new) If the stuff is not new then obviously it's quality is completely unknown like the stuff 'Laserred' was selling (though he (to be fair) was really selling the stuff as Ti mesh that happened to have an MMO coat on it).
A piece of 'Laserred' MMO that I was using to see how %CE was in a Chlorate cell as Chloride concentration went lower and lower, passivated after approx. 9 days of operation.
It definitely was passivation (the MMO layer was still there, the TiO2 managed to appear on the Ti between the two) as no amount of 'cleaning' would make any difference.
Cleaning MMO with steel wool/sandpaper/ or whatever is hopeless IMO (if it's a layer that is on there) because you are only going to get rid of the layer is places. It would have to be a chemical for the cleaning to be successful.

(Looking at Marks system) the one Anode and one Cathode is a bad system IMO. Better to have two (small) Cathodes for good current distribution on the Anode.

Contact brushes ('Graphite')sometimes contain Copper so watch out for that!

There is another file about brine treatment from here:
www.chlorimax.com/pdfs/Brine_Treatment.pdf

The history bit is a bit hilarious.

____________________________________________
HISTORY
The Chinese were drilling wells for salt water in 2000 B.C.. The Chinese emperor Yu made the
royal decree that his subjects would supply his court with salt. (Hence the term "rubbing salt in a
wound" was coined referring to an early version of the IRS.) Roman soldiers were paid partly in salt
- or salaria. Our word salary and the expression, "not worth his salt," can be traced to the Roman
word salaria.
History is filled with references to salt. The Bible contains 32 references, the most familiar
probably being the story of Lot's wife who was turned into a pillar of salt when she disobeyed the
angels and looked back at the wicked city of Sodom. The widespread superstition that spilling salt
brings bad luck is believed to have originated with the overturned saltcellar in front of Judas Iscariot
at the Last Supper, an incident immortalized in Leonardo Da Vinci's famous painting.

In 1553 an English traveler visited Venice and noted the following legislation. "Neither may any
man in Venice buy more salt then he spendith in the city for if he be known to carry it out of the city
and be accused, therefore he loseth an ear." Salt is still used today as a means of exchange.
In West Africa, a man can buy a wife for 48 pounds of salt.
In Ethiopia, hunters exchange skins for salt. In 1584, Jean D. Marcounille said, "... the sacredness
and dignity of salt. This mineral is like unto
the four elements: earth, air, fire, and water. So universal, so necessary to life, it is the fifth
element."2
Our bodies contain 2% salt. It acts as a policeman patrolling our blood streams and keeps our fluids
in chemical balance. If all the salt was removed from our bodies, we would not live 48 hours.
Today, the uses of salt are manifold. Of the 25 million tons produced annually in the United States,
about 71% or 18 million tons are consumed by the chemicals industry in producing chlorine,
caustic soda, hydrochloric acid, sodium, and some 30 other basic chemicals. These basic
chemicals are used in the preparation of 14,000 other chemicals. In addition, large amounts of salt
are consumed by meat and fish packers, in water softening, road stabilization, ice and snow removal,
and recovery of bromine from seawater.

___________________________________________

I will not be posting for quite some time. I am going off to West Africa with a Matilda (I will Waltz the Matilda all of the way) packed with 48 pounds of salt to obtain myself a Wife!!!!!!

Dann2

Google Waltzing Matilda

[Edited on 10-1-2011 by dann2]

Jurre10 - 10-1-2011 at 13:38

Hi,

Woohoo! a reply from THE dann2 everyboddy is talking about!

I'm sorry but i don't have an ultravioletlamp or something like that.

You'r right i mean black and smooth. not soft.
I buyed 2 anodes that aren't used (hopefully its true)

Is it a good idea to use the two anodes, two cathodes methode? or do you prefer one anode, two cathodes?

So it might be something like: its not a layer but some componements of the anode have changed?
But wy can i get a lower resistance by reversing polarity for a short time? because anode bubbles wrong componements off?

Greetz,
Jurian

dann2 - 10-1-2011 at 20:04

Hello Jurian

Each side of the Anode should have a view of a Cathode. When you just put one Cathode into the cell the side of the Anode that is beside the Cathode is going to have a greater current density that the side that is not beside the Cathode. This will give uneven wear of the Anode. You can see the effect clearly when using Graphite as the Graphite is inclined to wear more on the side beside the single Cathode. Perhaps I am too particular.
Instead of installing one Cathode of size x, simply cut the Cathode and install two Cathods of size x/2, one each side of the Anode.

Ti (and Ta, Nb, Zr) are 'Valve' metals. When used as a Cathode in an electrolyte the current will pass from the electrolyte into the Ti. It seems to go 'through' the Ti Oxide layer. When you reverse the current (the Ti is now the Anode) the current stops flowing. The current refuses to go from the Ti, through the Oxide layer, and into the electrolyte.
(taking conventional direction of current going from + to -)
I must google up an explanation of how/why this actually works in detail.
Ti looks like a diode (a valve* if you are old enough) when put into an electrolyte and you start passing current in each direction. It won't rectify 50Hz ac as the 'diode' has very very slow recovery

So when you use Ti as a Cathode it always works, Oxide layer or no Oxide layer. When you use Ti as an Anode no current will flow as the Oxide layer stops the current from flowing. If a piece of (useless) passivated (layer of Ti Oxide between the MMO and the Ti substrate) DSA is used as a Cathode it will work OK. When the pasivated DSA is used as an Anode the current just 'sees' the Oxide layer and does not flow.
If you put the Voltage high enough the Oxide layer will be destroyed and current will start to flow and Ti metal will get eaten away. You need approx. 16 Volts for Ti (semi guess). Ta Oxide is able to withstand about 50 Volts.

Try using the DSA Anode as a Cathode for half an hour or so to see if there is a layer that will go away (this layer has to be on the very external surface of the MMO).
If Ti Oxide has managed to form between the MMO layer and the Ti substrate then you DSA Anode is not an Anode anymore but just a piece of Ti. No amount of using the DSA as a Cathode will get rid of the Ti Oxide (if it is there).

A bit of a rambling post.

Dann2

[Edited on 11-1-2011 by dann2]

markgollum - 10-1-2011 at 20:42

@ dann2

Yes, I would agree that 2 cathodes are better than one.

@woelen
I would also agree with woelen that its possible to set the cell up so that no KClO3 crystals form on the electrodes. However, I think it would be necessary to design the cell so that it has good circulation so that the crystals easily form on the cooler surfaces. Of course, the circulation/mixing of the electrolyte is important for other reasons as well. Poor mixing was a fault of my cell as it was a tall cylinder, it now seems likely that my cell could have been running at a localized low chloride concentration. I had read all of the stuff on Wouter Visser's page, whatever dann2 had up at the time, everything on the E&W forum, as well as Sciencemadness so I really should have known better

.

What I was trying to accomplish at the time was to focus on trying to build a clean, low maintenance cell. I was concentrating on solving the salt creep and sealant (epoxy, silicone, I tried so many brands and types) problems that had plagued my previous attempts.

I also got my anodes from northstar pyro on Ebay. I had bought 3 MMO anodes, one tubular Ti substrate Pt coated anode, and a Niobium mesh Pt coated anode. I have never used my Pt anodes.

I still have one more MMO anode, but I am saving it so that perhaps one day I can plate it with lots of PbO2.

woelen - 10-1-2011 at 23:25

Indeed, you have to assure that KClO3 crystallizes on colder places. For that reason I put my cell in a layer of water, which nicely cools down the cell (otherwise it becomes too hot), but the cell is not completely immersed in water. Approximately 75% is under water. The coldest part of the cell then will be the bottom and that is where KClO3 crystals are formed. No crystals at all are formed at the anode and cathode in such a setup.

metalresearcher - 11-1-2011 at 09:26

Another option is putting the cell into the fridge (or in the freezer). Of course after running the electrolysis.

[Edited on 2011-1-11 by metalresearcher]

metalresearcher - 11-1-2011 at 10:36

Here a test run still with graphite rod (MMO anode not received yet).

It is a 300ml "Calvé Pindakaas" peanut butter jar very suitable for this test.
The lid is sealed with sanitary caulking.
Voltage: 5.2V, current 10 A (meter has a 80/1 scale).

Contents look very dark now after less than an hour while is was yellow at the start (due to the K2Cr2O7).

mr.crow - 11-1-2011 at 11:13

I also bought one of those eBay MMO on titanium anodes a while ago from the States. It was total crap, didn't conduct electricity when I put it in the solution. I swapped it out for a regular piece of metal and conducted just fine, but was quickly corroded. Maybe the MMO needs to be activated?

dann2 - 11-1-2011 at 12:47

Hello,

@ Mr Crow
Who was it from? I would thinK North Star Pyro sell perfectly OK new Anodes.

Going back to the 'Valve metal thing'
The paper below explains why Valve metals (Ti and Nb anyways) conduct from Cathode (-) to electrolyte
and not from Anode(+) to electrolyte. Hydrogen doping of the Oxide film seems to be the reason. When used as
a Cathode, Hydrogen diffuses into the non-conducting TiO2 making it condutive. When used as an Anode
there is no Hydrogen so the TiO2 remains an insulator. Google will get the artical.

Electrocomponent Science and Technology
1974, Vol. 1, pp. 121-127

ELECTROLYTIC RECTIFICATION AND CATHODIC
CHARGE REVERSIBILITY OF SOME VALVE METALS

Going back to using MMO as a Cathode, somewhere in the 'Thoughts on Anodes' thread I think I posted info. from a pool
chlorination site on the time frame that was used to reverse the polarity of bipolar pool chlorinators for to clean them.
I don't think using MMO as a Cathode (to clean it) will do it any harm.

Xenoid - 11-1-2011 at 14:07

Quote: Originally posted by dann2

Going back to using MMO as a Cathode, somewhere in the 'Thoughts on Anodes' thread I think I posted info. from a pool
chlorination site on the time frame that was used to reverse the polarity of bipolar pool chlorinators for to clean them.
I don't think using MMO as a Cathode (to clean it) will do it any harm.

Dann2, perhaps your mind is wandering, are you referring to this post of mine from a few weeks ago!

Quote: Originally posted by Xenoid

All the pool chlorinator electrode assemblies from around here are now wholly manufactured from MMO. This is because they are all self cleaning now, so that the electrode polarities are switched back and forth regularly to stop build-up on the cathode. This also simplifies construction and inventory for the chlorinator companies as they don't need to worry about plain Ti electrodes.

I am currently running a 5 litre chlorate cell with an all MMO 10 element pool chlorinator electrode. I switch polarity two or three times per run at which time any whitish "crud" that has built-up on the cathode immediately flakes off when it becomes the anode.

Incidentally, I now note most companies are switching over to solid plate electrodes rather than mesh, they last longer I guess.

MMO is now so cheap and common

Perhaps to summarise, the use of MMO by amateurs is not without its pitfalls :

1. MMO being sold on ebay which may be "seconds" or low quality, or perhaps "used" to more or less extent!

2. Don't use MMO in low chloride environment, keep adding chloride to your cell.

3. Remember MMO of various compositions is not stable in a high pH environment (ie. an uncontrolled amateur cell). Industrial users can get 10 year lifetimes only by tightly controlling the pH just below 7. There are reports of MMO anodes losing 25% of their coating in just a few hours when industrial pH control has gone haywire

metalresearcher - 12-1-2011 at 02:45

Quote: Originally posted by Xenoid

Industrial users can get 10 year lifetimes only by tightly controlling the pH just below 7. There are reports of MMO anodes losing 25% of their coating in just a few hours when industrial pH control has gone haywire

How can I achieve this ? In a chlorate cell LOTS of OH- appears which will turn up the pH to fay beyond 10.
Adding HCl ?

Or, better buy a more expensive Pt foil anode (current price $2000/tr oz == $60/g).

[Edited on 2011-1-12 by metalresearcher]

woelen - 12-1-2011 at 05:55

Add some K2Cr2O7 and occasionally add a drop of HCl. This will keep the pH lower. Dichromate has some buffering action, HCl of course brings pH down. But adding of HCl should be done cautiously. You don't want the pH going down too much, as that will lead to desctruction of chlorate and formation of free chlorine.

My experience is that adding some K2Cr2O7 keeps the pH already much better under control.

metalresearcher - 12-1-2011 at 07:01

@woelen:

Thanks. I already have a knifetip of K2Cr2O7 (in 300ml) in the solution so the color is piss-yellow.
I'll check the pH tonight.

My girlfriend saw this and said: "You have to piss into the loo not into a jar ;-)"

Xenoid - 12-1-2011 at 09:49

Quote: Originally posted by metalresearcher

How can I achieve this ? In a chlorate cell LOTS of OH- appears which will turn up the pH to fay beyond 10.
Adding HCl ?

Yes, it's not easy for the amateur. There are long and detailed discussions about pH control by dann2 and Swede in (I think) the "Thoughts on Anodes" thread. It is usually achieved using auto syringes or simple dropper systems. As woelen says be careful adding HCL, use dilute acid. One drop of concentrated acid can drop the solution by several pH units.
I have never bothered with pH control, but then, all my MMO has come free!
Probably more trouble than it's worth.

dann2 - 12-1-2011 at 19:24

Hello,

I found the post. It is in 'Thoughts on Anodes' and posted 20-12-2009
Its just info. from a patent giving cycle times for cleaning MMO pool chlorinators.
"US Pat. 6391176 is about a pool Chlorinator. The cycle times are shown in the picture of some text below and are of the order of 10's of seconds."
(see the patent or post).

It is quite easy to contoll pH. There is no need for a closed loop controller with a pH probe and an acid doser. Just add acid at the rates suggested here:

http://www.oxidizing.110mb.com/chlorate/reaction.html

Approx. 0.146ml 12% HCl per amp per hour is what you need in my (and Swede's) experience.
A simple timer and valve is probably the best way to go.
I have used a second hand syringe dosing machine (ebay) it's great for small cells but costs approx. 60 dollars. Acid is worth the bother IMO so long as you can get acid easily.
Chlorine escapeing from the cell is what causes pH to rise.

Dann2

Jurre10 - 15-1-2011 at 05:34

hi,

got my new anodes.
They are verry thick and rough!

My passivated anode is not thick and verry smooth.
I can't remember my other anode ever wass that thick!

Can somebody tell me if there is a way to protect my new andes?

Jurian

Jurre10 - 15-1-2011 at 05:38

sorry, double.
chek my last post

[Edited on 15-1-2011 by Jurre10]

hissingnoise - 15-1-2011 at 08:32

Quote:

Can somebody tell me if there is a way to protect my new andes?

You could set up your cell in such a way that the electrode assembly can be lowered into and lifted out of the cell while the PSU is delivering current so that the electrodes are never inactive in contact with electrolyte.
Of course, that means foregoing cleaning or washing of the anode.

metalresearcher - 15-1-2011 at 11:32

I can't wait for the delivery of the MMO anode so I started another attempt of making KClO3 which appears to be succesfull even with carbon rod anodes, as the erosion was very low.

www.metallab.net/KClO3.php#exp201101

I made this webpage ten years ago when I made about 300g of KClO3 successfully. Recently I refurbished it and improved the layout and added my results of last week to it.

[Edited on 2011-1-15 by metalresearcher]

UncleJoe1985 - 22-2-2015 at 01:49

Good to see I'm not the only one having anode deterioration problems. I've been using an MMO anode I purchased from NorthStar Pyro in a chlorate cell for 3 weeks. For the first 2 weeks, it was running fine at ~11 amps. During the 3rd week, the current gradually decreased to 5 amps.
I kept adding more NaCl and water throughout to keep the solution saturated and to discourage O₂ production.

At first I thought the problem was because I had exhausted most of the Cl ions in the electrolyte. But when I tried the anode in a saturated NaCl solution, the current only reached ~6A. So, the Cl- concentration isn't the problem. Then I noticed that when the anode dried, it wasn't black anymore, but covered by a white deposit (see pictures).

I've tried cleaning it with HCl and NaOH, but the desposit remains. What could it be?
I read the Effects of Electrolyte Impurities in Chlorate Cells article markgollum found earlier in this thread. It says calcium and fluoride ions are both bad. I read the local water report, and it says fluoride concentration is ~1 ppm, which should be safe. However, for ground water, the calcium carbonate % is 310ppm, well past what the arcticle considers acceptable.

So, what could the deposit be? The fact that it doesn't get dissolved and washed away by water suggests it is CaCO₃. But I also soaked it in 15% HCl for 4 hours and the deposit still remains, which doesn't make sense.

Also, I noticed 2 weeks into the electrolysis that even when I disconnect the electrodes so that there's no circuit between anode and cathode, there is still vigorous bubbling on the anode. Could this be the RuO2 getting reduced (Wikipedia says it's "highly reactive with reducing agents") instead of getting clogged?

[Edited on 22-2-2015 by UncleJoe1985]

deltaH - 22-2-2015 at 02:21

Ruthenium dioxide coatings slowly leach as ruthenates during operation (I recall reading about this somewhere but don't have a reference on hand), ultimately this leave just TiO2 behind which might explain the light colour. Perhaps such 'spent' anodes can be reimpregnated by painting on a ruthenium salt solution and then heating to decompose to ruthenium dioxide. Ruthenium's spot price is extremely low of late ($56/ounce), but purchasing a suitable salt might still be expensive and/or not easy to find?

[Edited on 22-2-2015 by deltaH]

papaya - 22-2-2015 at 10:26

Unclejoe, can it be some kind of organic precipitation that blocks the current also? Did you try to use some solvents to get rid of that? But what deltaH says is more true I think..

UncleJoe1985 - 22-2-2015 at 14:36

Quote:

can it be some kind of organic precipitation?

I also tried cleaning with acetone, but to no effect.

Quote:

Ruthenium dioxide coatings slowly leach as ruthenates

OK, but how can it happen in only 3 weeks? I saw the specs of a pool chlorinator anode and it said the life time was 8000 hours.

So far, the mechanisms I heard about that destroy the MMO coating are:

1. Oxygen evolution, causing the RuO₂ to turn into RuO₄, which is a liquid.

http://www.waterstarinc.com/files/Resources/White_paper.pdf

2. "caustic accelerated wear from cathode scales" (from Effects of Electrolyte Impurities in Chlorate Cells). It said in one case, 90% of the MMO was stripped due to hard water scale bridging between anode and cathode.

3. Reducing agents (since RuO₂ is an oxidizer)

I still don't know if the white layer is an extra deposit over the MMO or if it's bare titanium. Guess I'll have to sacrifice this anode by scraping it off and see what's under it

[Edited on 22-2-2015 by UncleJoe1985]

UncleJoe1985 - 23-2-2015 at 21:02

I just put the anode under a microscope. I'm pretty sure now the white layer is a deposit, instead of a place where the MMO was stripped away.

I also tried 800 grid sand paper and a screw driver to clean it, but it's not possible without severely chipping off the MMO.

Can anyone identify what the mystery substance that resists all the cleaners I've thrown at it (HCl, NaOH, acetone)?

cleaner, wide angle

cleaner close up 1

cleaner close up 2

dirty, wide angle

dirty close up 1

dirty close up 2

[Edited on 24-2-2015 by UncleJoe1985]

papaya - 24-2-2015 at 11:27

TiO2 can be fused with molten NaHSO4 or even NH4HSO4 (it worked for me), however I'm afraid that it'll also dissolve that precious metal oxides by the way, you could try it on a small spot.

jock88 - 24-2-2015 at 15:13

Try the piece of MMO as a cathode and see if it will work as a cathode. If it will work as a cathode and will not work as an anode (different directions of current) then you can be sure that it is a layer of Ti oxide that has built up between the Ti and MMO layer that is stopping the current flowing (as an anode) and not a simple an insulating layer of something else on top of MMO layer (stopping current in both directions).

morganbw - 24-2-2015 at 16:24

I worked in industry which used Iridium oxide coated plates. They would eventually pacify and the current would not pass and the voltage would increase ( big time ). Big voltage, low current equals DSA's end of life.

UncleJoe1985 - 3-6-2015 at 03:00

Victory! My anodes have been resurrected!

It was quite simple - just soaking the anodes in an initial 5% solution of NaOH at 80C for 2 hours. Although tried NaOH before, I missed the importance of high temperature and time.

current measurements (5V, saturated NaCl solution)
new: ~10 A
dirty: ~2 A
cleaned: 4.5 A

I only did a crude current test and probably didn't have good electrical contact and had a larger distance between electrodes than in the actual cell. So the current might be bigger than 5 A. However, I'm expecting a large drop from 10A due to my earlier abusive cleaning efforts.

That anode is moot since I'm using a much bigger one now:

It only took 1 month for this anode to passivate (go from 18 amps to 5 amps)! I cleaned it the same way and was able to get 9 amps (again, it's probably higher due to the crude testing).

Here are some photos of the smaller, cleaned anode.
You should see a big difference compared to my earlier photos

after cleaning

after cleaning, 4x zoom

after_NaOH_cleaning_wide_angle.JPG - 981kB

after cleaning, 54x zoom

My conclusion?

It's the wrath of calcium silicate!

The Morton salt I'm using has calcium silicates to prevent caking. I don't know much about silicates but it seems they're glass like and insoluble. But I suppose the basic pH of most amateur cells could dissolve trace amounts of it. And since molten NaOH will utterly dissolve glass, it totally makes sense that a hot solution of NaOH will dissolve the deposits.

I received tremendous help from the "Effects of Electrolyte Impurities in Chlorate Cells" by Richard A. Kus referenced earlier in this thread. It mentioned silica in several places. Appreciate it.

It would be great if someone else can try this and report how much it helps.

[Edited on 3-6-2015 by UncleJoe1985]

papaya - 13-6-2015 at 15:47

Congratulate me! I run exactly into the same situation as UncleJoe1985, look at his pictures of the anode before cleaning - looks same as mine, terrible.
Not sure if it changed voltage/amperage very much (didn't measure carefully before and after), I ran it for 5 days at 4Amps and 4.4volts (surface area both sides: 100cm^2 of mesh) and got what I got - coating of white stuff on MMO. But I have a clue what this might be in my case - SnO2! Because I wanted to test cathode material from 60/40 Pb/Sn solder and it actually holds up the hot brine very well (even when not under cathodic protection), however it seems some Sn leached out into alkaline solution and deposited on the anode! It also deposited on the walls of the glass container and I can rub it off with my finger, but on anode is different - it's very rough, still possible to clean mechanically. I had to use Pb cathode instead of Pb/Sn solder, might do a good cathode as well (corrosion resistant, what do you think?), any suggestions how do I clean the surface from what is supposedly SnO2?

[Edited on 13-6-2015 by papaya]

UncleJoe1985 - 19-6-2015 at 15:24

I don't think having SnO₂ on your anode will cause passivation since it's quite conductive with a resistivity of 2 * 10^-5 ohm * m (ref. 1) or 1.1 * 10^-7 to 7.2 * 10^-6 (ref. 2)

1. from Handbook of Transparent Conductors, p. 157
2. Effects of Substrate Temp. on Structural Properties of Tin Oxide Films produced by Plasma oxidation after thermal evaporation

After all, tin compounds should have many similar characteristics as their lead counterparts and PbO₂ is very conductive.

I think a general rule is that acids will turn metal oxides into soluble salts.
CaO + 2 HCl -> CaCl2 + H₂O

So HCl might work (but can it also destroy the precious RuO₂ and IrO₂ too?). The Wiki for SnO₂ also says strong alkali solutions will dissolve it.

Why would you ever want to use tin or lead for the cathode considering how flimsy it is? For chlorate cells, titanium is the best:

http://www.oocities.org/capecanaveral/campus/5361/chlorate/c...

[Edited on 19-6-2015 by UncleJoe1985]

papaya - 19-6-2015 at 15:55

Conductivity of metal oxides depends on many things, such as how they are prepared. Most of them are not prepared by "wet" methods (except PbO2 maybe), but with some kind of pyrolysis. Also what deposited on my anode given under what conditions it happened is more likely to be stannic acid or SnO2.xH2O, which is not conductive. I tried 5% HCl but that acid changed color very quickly to some yellow-greenish and I stopped at that point since I thought this somehow may be related to leaching of RuO2. I tried nitric acid, but it had no noticable effect. Then I tried boiling NaOH and it finally did the job (in few hours), however after that when rubbed with a finger a black stain could be noticed, so some % of coating is lost. Anyway I tested that anode and compared with a similar new one - there's no difference in currents (at fixed voltage), and there's no difference in "overpotential" as long as I can tell (from the minimum voltage vallues at which electrolysis only starts at few milliamps).
Why use Pb for the cathode? First, it's very resistant to corrosion (even when the cell is off), second you can take a copper foil, cut whatever shape you want and then apply solder (or Pb) and that thin layer will last! However after this accident I'll not try Sn/Pb solder in the future, maybe pure Pb can still do the job. Also I want to mention that I got very low current efficiency for my chlorate - about 25% percent only (not using dichromate and Sn/Pb solder as a cathode), which may be related to higer hydroden evolution overpotential for this metal (Pb is almost the same as Hg in this manner), so yes, all I did is purely experimental and not to any use yet. Now I'm trying with some type of stainless steel, which turned out to be very robust in this electrolyte, next maybe will try cupronickel or melchiour if it's tight enough.

[Edited on 19-6-2015 by papaya]

UncleJoe1985 - 19-6-2015 at 18:10

Congratulations on restoring your anodes. Feels very relieving.

I don't want to reduce your curiosity, but that site (made by this site's dann2 I think) clearly says titanium has been the cathode material of choice for (per)chlorate cells because

Quote:

Using Titanium metal stops the reduction of Chlorate unlike Iron which reduces Chlorate

Quote:

may be related to higer hydroden evolution overpotential for this metal

I was thinking the same thing. I found that to be a problem too when making lead acetate electrolytically

papaya - 20-6-2015 at 02:59

Titanium is very good of course, but it's hard to me to work mechanically with it without suitable tools - it's very tough and not that easy to cut, then welding is impossible without inert gas, other problems. And chlorate reduction is not the major problem as far as I remember correctly, worse problem is hypochlorite reduction which takes place on all types of cathodes (unless dichromate additive is in there). Stainless steel shows unexpected stability btw - it doesn't rust even at the places above liquid layer, only minor blackening effect is observed. But the steel also is very hard to work with.

jock88 - 20-6-2015 at 04:40

Grade one, two, three and four Ti are easy to work. They are soft. About the same is mild steel.
It can be spot welded easily.
I think that is would be fair to assum that Ti reduces hypo. less than Iron. By how much is not known.
Stainless steel is hard. Someone suggested that there may be a layer of 'Chromate something' on stainless steel that may reduce or eliminate reduction.

There are many types of ss as far as corrosion is concerned.

Little_Ghost_again - 6-10-2015 at 14:52

I reverse my anodes in sodium hydroxide at 3.3V for 12 hours and aim for a current <300mA, the two small bits I originally cut from larger ones are still going strong, but I dont take mine above around 3.5A as i dont see the point of very high currents. I also swap the anode and cathode every other run but I dont do many and only small runs

Simoski - 14-2-2019 at 23:55

Hi

My postassium chlorate cell has been running great for months now, but all of a sudden chlorate is crystalising all over the anode to the point that its passivated.

Do you think I should add water or more KCL?

I get the feeling that its just super saturated with chlorate and at night when it cools the chlorate crystalises on the anode...

any ideas?

markx - 15-2-2019 at 07:44

Quote: Originally posted by Simoski

Hi

My postassium chlorate cell has been running great for months now, but all of a sudden chlorate is crystalising all over the anode to the point that its passivated.

Do you think I should add water or more KCL?

I get the feeling that its just super saturated with chlorate and at night when it cools the chlorate crystalises on the anode...

any ideas?

Harvest the crop and resaturate with chloride.....no point in trying to run it dry. You risk damaging your anode (MMO or platinum plated) by running at a very low chloride concentration.

Simoski - 16-2-2019 at 07:47

ok will do Markx, thank you

[Edited on 16-2-2019 by Simoski]

Simoski - 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

phlogiston - 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.

mysteriusbhoice - 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.

markx - 17-4-2019 at 22:55

Quote: Originally posted by mysteriusbhoice

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.

woelen - 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.