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Author: Subject: More on PbO2 electrodes
dann2
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[*] posted on 7-4-2007 at 10:41


Quote:
[QUOTE]


number is GB850380. Sorry for the confusion.

-Alembic


Hello,

The patent was very hard to find from the espace site for some reason or other. I found it eventually.
It is also published under:

US2872405 (A1)
FR1168150 (A)
DE1094245 (B1)

The patent mentions nothng about precoating the valve metal with Sn or Sb semiconductor oxides. I guess you added that in yourself.


It also mentions using Graphite to make Perchlorate. It says that it erodes too quick for commercial use.
It also says that Magnetite is only useful at low current densities.
Also Magnetite will make Perchloarate at a current efficiency of 4 to 5% according to the Patent.


Cheers,

Dann2

[Edited on 7-4-2007 by dann2]
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Rosco Bodine
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[*] posted on 7-4-2007 at 18:15


Quote:
Originally posted by dann2
Quote:
Originally posted by Rosco Bodine
This could be useful information , and I don't recall seeing it posted before . Evidently the co-plating of a controlled amount of bismuth peroxide with lead peroxide forms a
harder and denser , much more erosion resistant PbO2
anode which also operates at better current efficiency .
This is only useful in a limited thickness because cracking problems develop with heavier layers . It might be possible to build up a thicker anode using cloth wrap or intermediate layer reenforcement as was mentioned earlier in this thread .

The patent also describes an interesting improvised flow through coaxial electrode which was used for one of the experiments , where the electrolyte is pumped through
the electrode assembly .

[Edited on 5-4-2007 by Rosco Bodine]


Hello,

It is interesting to note that they only mention in the patent (page 6 lines 10 to 15) very brifely that it may be needed to coat the substrate with Pt or some of its oxides. You would think they were trying to hide the fact................

Cheers,

Dann2


Yeah , another little detail I saw mentioned in one patent is
that fluoride in the electrolyte is especially corrosive to
even a titanium substrate if it permeates through the
PbO2 ...which it is pretty much guaranteed to do sooner or later . This is one area where a graphite substrate may actually fare better ....plus the fact that it doesn't form a blocking diode junction to current flow when electrolyte does permeate through to the substrate .

The electrical connection can be made to the naked graphite on the unplated top section instead of having to clamp directly onto the PbO2 which is brittle .

Another little note I made was concerning the integrity of
the PbO2 plating , that besides iron being a contaminant to be avoided is also any organic materials on the substrate . So I would conclude that linseed oil would only be useful on
an already plated carbon , like dipping the unplated section .
That information about needing absolutely clean and degreased graphite for the substrate pretty much rules out any pretreatment of the graphite to make it hold up better .
The dense and impermeable structure of a high quality PbO2
plating is the most important factor in the longevity of
the anode ....no matter what substrate is used .

I expect that some of that thin wickable silicone cement as
is used for sealing windshield leaks could be useful on the
section where the PbO2 transitions to unplated graphite ,
sort of as a primer for heavier silicone glue used to secure
the electrode into a PVC or poly pipe fitting ....if a coaxial electrode was being made like they describe .
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dann2
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[*] posted on 8-4-2007 at 04:57


Quote:
Originally posted by Rosco Bodine
Quote:
Originally posted by dann2


Hello,

It is interesting to note that they only mention in the patent (page 6 lines 10 to 15) very brifely that it may be needed to coat the substrate with Pt or some of its oxides. You would think they were trying to hide the fact................

Cheers,

Dann2


Yeah , another little detail I saw mentioned in one patent is
that fluoride in the electrolyte is especially corrosive to
even a titanium substrate if it permeates through the
PbO2 ...which it is pretty much guaranteed to do sooner or later . This is one area where a graphite substrate may actually fare better ....plus the fact that it doesn't form a blocking diode junction to current flow when electrolyte does permeate through to the substrate .

The electrical connection can be made to the naked graphite on the unplated top section instead of having to clamp directly onto the PbO2 which is brittle .

Another little note I made was concerning the integrity of
the PbO2 plating , that besides iron being a contaminant to be avoided is also any organic materials on the substrate . So I would conclude that linseed oil would only be useful on
an already plated carbon , like dipping the unplated section .
That information about needing absolutely clean and degreased graphite for the substrate pretty much rules out any pretreatment of the graphite to make it hold up better .
The dense and impermeable structure of a high quality PbO2
plating is the most important factor in the longevity of
the anode ....no matter what substrate is used.

quote]

Hello,

My understanding of the Ti Oxide build up between the Lead Dioxide and the Ti (or other valve metal) is that the oxide simply appears after some time even if no electrolite is getting through the PbO2 when the anode is being used in an electrolythic cell. This is why you need a Pt layer or some other sort of layer to stop the oxide from forming. The Pb02 simply does not 'agree' with the Ti (or other valve metal). Perhaps Alpha (plated from alkali PbO2 baths) Lead Dioxide is better at 'agreeing' with Ti (or other valve metal) that Beta Lead Dioxide (plated from Acidic Nitrate bath) as in many patents it is recommended that Alpha be plated first. Then again usually the patents recommending Alpha are coming from a point of view of releaving stress in the Lead Dioxide.

All commercial valve metal substrate Lead Dioxide anodes have a Pt coat (probably very thin) of some description on the valve metal first. I read this somewhere, can't remember where.

I totally agree with the fact that you need a good coating of Lead Dioxide to protect the substrates. The idea behind the plastic, ceramic, substrates is that 'any old coating' of lead dioxide would do. This is not the case. You still need a good coating to carry heavy current etc.



Cheers,

Dann2
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[*] posted on 8-4-2007 at 16:27


Quote:


Hello,

My understanding of the Ti Oxide build up between the Lead Dioxide and the Ti (or other valve metal) is that the oxide simply appears after some time even if no electrolite is getting through the PbO2 when the anode is being used in an electrolythic cell. This is why you need a Pt layer or some other sort of layer to stop the oxide from forming. The Pb02 simply does not 'agree' with the Ti (or other valve metal). Perhaps Alpha (plated from alkali PbO2 baths) Lead Dioxide is better at 'agreeing' with Ti (or other valve metal) that Beta Lead Dioxide (plated from Acidic Nitrate bath) as in many patents it is recommended that Alpha be plated first. Then again usually the patents recommending Alpha are coming from a point of view of releaving stress in the Lead Dioxide.

All commercial valve metal substrate Lead Dioxide anodes have a Pt coat (probably very thin) of some description on the valve metal first. I read this somewhere, can't remember where.

I totally agree with the fact that you need a good coating of Lead Dioxide to protect the substrates. The idea behind the plastic, ceramic, substrates is that 'any old coating' of lead dioxide would do. This is not the case. You still need a good coating to carry heavy current etc.



Cheers,

Dann2


You're right, about the PbO2 not agreeing with the Ti. The problem is quite simply the fact that PbO2 (whether Alpha or Beta) is a very strong oxidizing agent and Ti is actually pretty easily oxidized (at least until a sufficiently thick layer of TiO2 is formed). A layer of PbO2 plated directly onto Ti is inherently unstable, at least thermodynamically.

SnO2 is resistant to most acids and when appropriately doped electrically conductive. It is also not much of an oxidizing agent, which is why it works well when used as a buffer layer between PbO2 and Ti.

-Alembic
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[*] posted on 9-4-2007 at 06:53


Anyopne tried to use Tin as a subtrace or a contact?
Mybe as an electrode ... IIRC it is pretty resistant ... might be wrong
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[*] posted on 9-4-2007 at 11:06
little success making PbO2 electrode


Hi guys, I tried in the past few weeks making a good PbO2 electrode for perchlorate.
I think I succeeded making an usable electrode.
I used CD cut to squere as a base, wet it with MEK and powdered it with PbO2 powder.
After this I connected it to a graphite electrode and for 48H I coated it - at first with minimal current and in the last 4 hours with about 0.05A/cm^2.

This is the result:




I think I will try to convert NaChlorate to NaPerchlorate in the coming month.
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[*] posted on 9-4-2007 at 12:33


Nice man looks a lot like my first electrode(and thats not good news ;) )... try using scotch... i also tried to make a plastic subtrace.. my problem was that for some reason the electrode just cracked and fell off the plastic.
Also there is quite a problem with the connection, you do need silver paint. I ordered some so when ill recieve it i can give you some of it. If youll use just a metal youll get a lot of resistanse on the contact thus low potential on the anode and then no perchlorate .. thats what happned in my experiment.
Soon ill try to make a proper electrode with scotch and silver paint. :cool:
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dann2
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[*] posted on 9-4-2007 at 17:07


Quote:
Originally posted by hashashan
Nice man looks a lot like my first electrode(and thats not good news ;) )... try using scotch... i also tried to make a plastic subtrace.. my problem was that for some reason the electrode just cracked and fell off the plastic.
Also there is quite a problem with the connection, you do need silver paint. I ordered some so when ill recieve it i can give you some of it. If youll use just a metal youll get a lot of resistanse on the contact thus low potential on the anode and then no perchlorate .. thats what happned in my experiment.
Soon ill try to make a proper electrode with scotch and silver paint. :cool:


Hello,

The problem with the bad contact is that heat will be generated in the contact area which may damage the anode.
It will waste electical power too but we dont worry too much about power wastage.
The voltage dropped will NOT stop Perchlorate (or Chlorate) from forming so long as you have a sensible current density on the Anode.

Dann2

[Edited on 10-4-2007 by dann2]
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dann2
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[*] posted on 9-4-2007 at 17:15


Quote:
Originally posted by hashashan
Anyopne tried to use Tin as a subtrace or a contact?
Mybe as an electrode ... IIRC it is pretty resistant ... might be wrong


Never heard if being used anywhere. It is easily (IMHO) attacked by the (Per)Chlorate electrolite. You will need a perfect coating of Lead Dioxide. It will also be attacked by the Nitric Acid in the plating tank. You will have to plate it from a neutual (or close to neutral) plating tank or use a alkali plating tank (Lead Tartarate bath) You might as well use graphite, or Iron or Nickle.
The whole idea of the inert (plastic/cloth/ceramic or whatever) substrate is taht a poor coating of Lead Dioxide willl give a workable anode.
This has proved not to be the case (alas) as you still need a fearly good coat of Lead Dioxide for to carry heavy currents when anode goes into (Per)Chlorate cell.

Cheers,

Dann2
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[*] posted on 9-4-2007 at 17:29


Quote:
Originally posted by Alembic




You're right, about the PbO2 not agreeing with the Ti. The problem is quite simply the fact that PbO2 (whether Alpha or Beta) is a very strong oxidizing agent and Ti is actually pretty easily oxidized (at least until a sufficiently thick layer of TiO2 is formed). A layer of PbO2 plated directly onto Ti is inherently unstable, at least thermodynamically.

SnO2 is resistant to most acids and when appropriately doped electrically conductive. It is also not much of an oxidizing agent, which is why it works well when used as a buffer layer between PbO2 and Ti.

-Alembic


Hello Alembic,

That is the explanation I never had before. Was probably told before though!

Way way way back in the year 1999 :D I used to think that Ti
was too exotic as a substrate and was determined to stay away from it. In the mean time ebay etc has come along and Ti is now readily available. Think it's time I moved with the times.
Are there any other coatings between the valve metal and Lead Dioxide that may work that you are aware off.



Cheers,

Dann2
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[*] posted on 10-4-2007 at 01:32


dann, my problem with the cell was that I had a lot of resistance in the contact and thus very low potential at the anode. I didnt use NaF and so instead of the chlorate that was formed from the NaCl converted to tht perch it was converted to the NaOCl and even after very extended periods of time no perch or even sensible amounts of chlorate were formed.
Now i am running a chlorate cell and then ill use pure chlorate and a good anode with silver conection to convert it to perch
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Alembic
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[*] posted on 11-4-2007 at 11:35


Quote:
Originally posted by dann2

Hello Alembic,

That is the explanation I never had before. Was probably told before though!

Way way way back in the year 1999 :D I used to think that Ti
was too exotic as a substrate and was determined to stay away from it. In the mean time ebay etc has come along and Ti is now readily available. Think it's time I moved with the times.
Are there any other coatings between the valve metal and Lead Dioxide that may work that you are aware off. I will post the patent number as soon as I manage to find it again.

-Alembic



Cheers,

Dann2


I also used to feel that Ti was too exotic and expensive, till I just got so tired of my ceramic and graphite substrate anodes cracking or failing otherwise. The resulting amount of toxic waste was also getting a bit embarrassing:D

There are other interlayers that can be used such as Pt for TiO2/Ta2O5 just to mention two examples.

I chose the SnO2/Sb2O3 system because it is easy to apply and the raw materials are easy to come by and above all cheap. SnO2 also has a higher oxygen overvoltage than PbO2 which supposedly means that any exposed SnO2 will not interfere with the anodic formation of perchlorate.

-Alembic
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[*] posted on 15-4-2007 at 12:45


In chlorate cell using graphite anodes, when I use sodium dichromate and tried to ppt. dichromate as barium dichromate using 10% excess BaCl2 and tried to ppt. excess with NaCO3 but my solution still pale yellow!! how can I get colorless electrolyte "full procedure" ??
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[*] posted on 15-4-2007 at 14:29


Why do you need to precipitate it?

Clearly, barium (di)chromate is quite soluble! Did you check the ionic strength, by any chance? It doesn't take much chromate to look yellow, either.

My electrolyte is all green at the moment. More curious to me is, why is it even dissolved? Cr(OH)3 should be precipitated; I don't think the pH is so high (I'm not adjusting it any, mind you) that Cr(OH)4 is present.

Tim

P.S. To mods: split these two posts into the KClO3 thread. This has nothing to do with PbO2.




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dann2
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[*] posted on 15-4-2007 at 18:44


Quote:
Originally posted by SAM4CH
In chlorate cell using graphite anodes, when I use sodium dichromate and tried to ppt. dichromate as barium dichromate using 10% excess BaCl2 and tried to ppt. excess with NaCO3 but my solution still pale yellow!! how can I get colorless electrolyte "full procedure" ??


Hello,

I shall quote Garage Chemist from the thread 'Thoughts on Anodes'.

I destroy residual chlorate by acidifying the solution, adding sodium disulfite and boiling. The solution must give off SO2, otherwise more disulfite must be added. A SO2 smell from the solution is the sign that all chlorate has been reduced.
This also reduces the added dichromate to trivalent chromium, which is then precipitated as the hydroxide by adding NaOH solution and filtering.

The above was in relation to a Perchlorate cell btw.
Hope this helps.

As for the green colour in a Chlorate cell. Is there any Copper about?
Are you still using the bucket with the Carbon suspended on Copper pipes?

Cheers,
Dann2
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[*] posted on 15-4-2007 at 19:27


Nope, haven't had copper in it in some time. The only green thing, besides the possibility of chromium, is the mild steel plate bottom of the cell. I keep voltage on the cell pretty much at all times (including when boiling to decompose hypochlorite, but not when pouring out the liquor), so that's a low priority, and would most likely appear light purple (FeO4) anyway.

Green would be consistent with copper in a strong chloride solution, but not with the pH, which is soapy to my fingers after a minute of contact (maybe...pH 9?).

I am reusing the salt (which had since dried completely) from previous runs, so if the contaminant remained soluble, it could possibly be residual.

Tim




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[*] posted on 16-4-2007 at 13:44


I add NaOH for my solution to reach pH=8-9 and added barium chloride 10%excess "my dichromate conc. 2g/L" there was ppt and filtered but filtrated solution still has yellow color!!? what is the problems here



Sam
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[*] posted on 16-4-2007 at 14:52


What is the solubility of BaCrO4 anyway? Ksp = 2.2 x 10^-10, so with nothing else in solution, you'll have roughly 20uM of each. Maybe 10uM CrO4(2-) let's say. That's still in the ppm range, which is quite visible. Spectrophotometric limits on chromate go into the ppb range.

You might try lead instead (about 28 times less soluble). Mind the PbCl2 or PbSO4 precipitate when in excess.

Tim




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dann2
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[*] posted on 16-4-2007 at 15:28


Quote:
Originally posted by SAM4CH
I add NaOH for my solution to reach pH=8-9 and added barium chloride 10%excess "my dichromate conc. 2g/L" there was ppt and filtered but filtrated solution still has yellow color!!? what is the problems here


I once had a conversation with guys about a persistent yellow colour in their Chlorate solutions. Someone suggested it may be collidal Iron.

Dann2
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[*] posted on 17-4-2007 at 02:17


Ok.. not colloidal iron because I prepare fresh solution from 2g/L dichromate in 1L distilled water without electrolysis but the yellow color remains:(

I added before minutes very excess of BaCl2 "around 10-30 times of required in balanced equation" and I get very clear solution !!!???

[Edited on by SAM4CH]
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[*] posted on 22-4-2007 at 01:46


Great resultd.
Ive tested out my cloth reinforced anode with the silver paint and results are good :) made about 100rams of perch from chlorate.
Now a question:
I converted it to K-perch, how can i destroy the chlorate? the sulfite solution will still work?
how much sulfite should i add? is there a cheaper method? can i convert this perch to AP after i destroy the chlorate without the fear of all the thing going off, i dont have any indicators to check for chlorate traces.
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[*] posted on 22-4-2007 at 07:32


No, potassium perchlorate is much less soluble than ammonium perchlorate.

Use a reducing agent such as Fe(II), sulfite, etc., and acid if necessary to get it going. The KClO4 will remain undissolved (keep it cool to minimize loss).

Tim




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[*] posted on 22-4-2007 at 08:40


destroyed the chlorate with excess of HCl,
after the filtering and wash it doesnt react with conc. H2SO4 and doesnt color HCl in yellow is it a good enough indicator? i just dont have anythin better
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[*] posted on 22-4-2007 at 20:39


Quote:
Originally posted by dann2
Silver is the best by far connection to Lead Dioxide as this article describes.

http://www.geocities.com/CapeCanaveral/Campus/5361/chlorate/...



Silver is actually the best physical contact surface even more generally , particularly for any gently clamped connection , because when it oxidizes , the oxide itself has good conductivity . So even if you are clamping onto graphite for example , and even where some wicking or moisture corrosion is inevitable , silver should fare better than copper .

Thin silver sheet can be soldered onto a copper backing for economy and used to line the jaws of spring clamps or
used in fabricating connection terminals for anodes .
Silver sheet can be hard to find . Here is one source which
seems fairly reasonable in pricing , and supplies useful thicknesses .

Spot silver price on unfabricated forms is presently about $14.00 troy oz. just for reference .

http://www.silversupplies.com/catalog/metals/fine_silver/she...

[Edited on 22-4-2007 by Rosco Bodine]
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biggrin.gif posted on 26-4-2007 at 00:34


As promised, a photo of my cloth reinforced anode.

and a close up, looks like a volcano rock :D
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