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Author: Subject: What about silicon as an anode in a chlorate cell?
jpsmith123
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[*] posted on 28-6-2005 at 02:39
What about silicon as an anode in a chlorate cell?


I see several discussions here related to chlorate and perchlorate production, so I apologize in advance if I should've posted this elsewhere.

In any case, I'm interested in building a chlorate cell myself, and, after seeing this archive:
http://yarchive.net/explosives/chlorate_mk.html

I'm wondering, has anyone here experimented with silicon as an anode material for chlorate production?
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Marvin
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[*] posted on 28-6-2005 at 12:25


When silicon oxidises the product is quartz, so non conductive.

The same thing happens to aluminium. The surface oxidises and conduction then only happens by breakdown of the oxide at much higher voltages. On the bright side the same effect gets us electrolytic capacitors.
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Lambda
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[*] posted on 28-6-2005 at 14:12
Production of Chlorates


Jpsmith123, here are a few threads that may be of interest to you:

http://www.wfvisser.dds.nl/EN/chlorate_EN.html
http://www.geocities.com/CapeCanaveral/Campus/5361/chlorate/...

The information on these sites is excellent.

[Edited on 28-6-2005 by Lambda]
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jpsmith123
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[*] posted on 28-6-2005 at 16:49


Hello Marvin,

Yes my first thought was that you'd get SiO2 and that would stop it, but, according to one of the participants in the discussion I cited, he supposedly tried it, and, surprisingly, it appeared to work.

I then looked into it a little bit more and found out that you can buy silicon coated stainless steel tubing (www.mcmaster.com)

Here's the product description:

"A thin layer of silicon permanently bonded to the ID and OD of this tubing provides an extremely smooth surface that prevents deposits from forming resulting in excellent corrosion resistance. Great for use with hydrochloric, sulfuric, and nitric acids plus sulfides, alcohols, and chlorinated compounds. Not for use with caustic liquids".

Based on this, I thought it might make for an interesting experiment.

********************************
Hello Lambda,

Thanks for the links. I'm studying it right now. I'd like to prepare sodium perchlorate. I was going to buy some, but it seems difficult to find and relatively expensive.
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Lambda
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[*] posted on 28-6-2005 at 20:12


jpsmith123, buy Chlorax (sodium hypochlorite solution) or start from scratch with kitchen salt (NaCl). Sodium perchlorate is hygroscopic, it would be better to by "Sodium free Salt". Wel, actually it often contains about 60 % KCl and 40 % NaCl. Potasium perchlorate is however easilly seperated due to it's lower solubility.

Then this link will also iterest you (Perchlorates):

http://www.wfvisser.dds.nl/EN/perchlorate_EN.html

[Edited on 29-6-2005 by Lambda]
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[*] posted on 29-6-2005 at 04:51
Silicon


jpsmith123, part of your silicon product description gives me pause -
"Not for use with caustic liquids" - hydroxides are created during the process.




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[*] posted on 29-6-2005 at 13:02


Yeah, but that's why you don't use it for cathode.

Tim




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[*] posted on 29-6-2005 at 15:29


But at the anode, chlorine gas is produced, which could react with the Si forming SiCl4, which would immediately hydrolyse to form silica gel and the acid.
This might not be the case in solution however. Dissolution of the anode is however possible.

Don't forget btw, Pt-coated Ti meshes are available at ebay, and Pt wire (hammer flat) at the jewellers. Pt is superior in its catalytic efficiency. Hmm. REally I should merge this thread with an existing one.




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Lambda
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[*] posted on 29-6-2005 at 15:43
Cathode material


Like in chlorate cells, stainless steel is a suitable cathode material. Mild steel may also be used. While copper and brass will also work, they may cause problems with copper contamination when they erode.

Graphite electrode that can be used in non oxidising (Oxygen evoluting) compartments:

I have obtained hughe graphite electrodes, used as the current pickup grid of a train or tram. They are used as "slip" electrodes in combination with the copper (actually an alloy) current carrying above line. The trams here opperate at I think (not shoure) 600 volts and the trains at 1500 volts. The pickup grid electrodes look similar in construction and composition. Some kind of baked graphite composition material is used.
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jpsmith123
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[*] posted on 29-6-2005 at 16:34


I was thinking that products like SiCl4 might not form in an aqueous environment.

In any case, I think I am going to just go with platinum wire, first, in order to minimize the number of things that can go wrong, and just try to make some sodium chlorate (perchlorate after I have some experience).

Then in the future I may experiment with other anode materials.

BTW, some other possible sources of platinum wire are:

http://www.elementalscientific.net/downloadcat.html

http://www.hooverandstrong.com/
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Lambda
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[*] posted on 29-6-2005 at 22:22
How to grow a beard


The surface area is the determinating factor for your current. Platinum wire has a very small surface area, unless you are going for foil, and a wallet-buster strategy. If not, then buy some good shaving gear, you are going to need it. Unless you want to look like Noach, by the time you have some Sodium chlorate. It would then be easier to start from Sodium hypochlorite (5% chlorox), for this will give you a "head" start.
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[*] posted on 5-7-2005 at 19:16


I have enough platinum that I could run at a few amps, conservatively, but the more I look into it, I'm beginning to think that PbO2 is really the most practical way to go.

I'm interested in the idea of a plastic substrate electrode as described here:
http://www.geocities.com/CapeCanaveral/Campus/5361/chlorate/...

I wonder what would hold up better, the plastic substrate, or the ceramic one?

Quote:
Originally posted by Lambda
The surface area is the determinating factor for your current. Platinum wire has a very small surface area, unless you are going for foil, and a wallet-buster strategy. If not, then buy some good shaving gear, you are going to need it. Unless you want to look like Noach, by the time you have some Sodium chlorate. It would then be easier to start from Sodium hypochlorite (5% chlorox), for this will give you a "head" start.
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[*] posted on 5-7-2005 at 22:31
Electrode material


Jpsmith123, I will quote what stands in the text:

1.- Metal connections will not give as many problems as with the porous ceramic substrates when the anode goes into service as there will be no conduction of salts up the plastic substrate.

2.- The advantages of the plastic substrate it that it is easy to obtain and is non porous and therefor will not suffer to a great extent from the problem of the salts from the Chlorate/Perchlorate cell getting to the current connection by cappillary action.

3.- Plastics that are suitable for the task are polyester, polypropylene, polyethylene, teflon, ployvinylchloride (PVC) and polystyrene (...and counting).

What you can allso do, is to go all the way with Graphite. You can find hugh graphite electrodes at places where they repair an sevice trains or trams. This Graphite composite material is used as the contact-electrode from the train/tram to the above powerline. When they wear out, then they are replaced, and here you come in. Bring the train/tram mechanics a few beers in a box, they will very much appreciate this.

You can allso find Carbon electrodes of about 1 cm diameter and 30 cm long in certain weld-supply shops. They have a 0.1-0.2 mm copper foil wrapt around them, that can easilly be torn off to the point were you connect the electrode with the powerlines.

Certain new unfilled (~30% H2SO4) lead acid car batteries, allso have Lead dioxide plates in them. The active mass on the plates (lead with traces of Antimony, and lead oxide with additives called a paste) have a spongy structure with a certain porosity. A high porosity structure has more surface which enables high currents (greater reactive surface for starting engines). I have not tried this, so I can't tell you in how far this material will hold up, or pollute you Chlorate synthesis. These plates may have to be oxidized further.

Maybe just sticking to Graphite or Carbon electrodes may be the best solution. Plastic with a Lead dioxide layer may allso be the solution, if you don't run the Chlorate cell excessively warm. A water cooling bath may then be required.

[Edited on 6-7-2005 by Lambda]
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jpsmith123
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[*] posted on 6-7-2005 at 00:54
What about Pb as a substrate for PbO2?


I wonder if a piece of pure lead could be somehow anodically oxidized and/or plated with PbO2 by the lead/copper nitrate plating process?

As I understand it, the PbO2 coating on graphite or titanium for example has to be very high quality, i.e., no pinholes, otherwise the substrate will be attacked and the electrode destroyed.

Maybe an imperfect PbO2 coating on Pb will not result in early failure? IOW, a lead substrate might be self-healing to a certain extent?

Moreover, unlike nonconductive substrates, it may be easier with lead to get a robust electrical connection.
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[*] posted on 6-7-2005 at 02:50


Idunno, I've electrolyzed lead in a salt solution before; as long as it's not hot (such that PbCl2 is mobile and makes Pb(OH2) as I'd like), it passivates, then slowly the coating transforms to Pb(OH)2 and then PbO2, which sloughs off. Problem is, current is really low once it passivates, so it takes forever to get much PbO2 formation.

A coated electrode might work (suppose I'll go anodize Pb in H2SO4 and try it), but it may also slough off at pinholes and passivate anyway.

Tim




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jpsmith123
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[*] posted on 6-7-2005 at 04:37
Method for anodic oxidation of lead.


Hello 12AX7,

I found the following by way of a Google search. I don't know if it works or not, but it sounds interesting. (Unfortunately it gives no idea what the current density should be, and I think the usage of the worde "chromate" is an error).

"METHOD FOR PEROXIDING LEAD ANODES
This process will provide a protective coating of lead chromate
to lead anodes, and yet allow current to pass freely for the purpose
of electroplating. If properly performed, the lead anodes will cover
with a hard, dark brown coat of lead peroxide...as the name implies.
This coating will protect the anode from attack of the chromic acid,
which on "clean lead " would erode and fall into the tank. If at
anytime you should see a yellowish powder look on the anode, that is
an indicator at which it should be put thru this process again to
restore the coating . Now..for the process:
Step 1. The peroxidised anode is connected to the ANODE buss bar of
the plating tank.
Step 2. Another anode of similar size is connected to the CATHODE side
Step 3. The anodes are then placed in a solution of water with 5%
sulfuric acid
Step 4. A current of 5 amps is now applied for 15 minutes
Step 5. Reverse the current by swapping the electical connections
and apply current once again for 15 minutes
Step 7. After completion of step 5, reverse the electrical
connections back to the same as in step 4.
Step 8. Apply current as used in step 4 for about 20 minutes, at
which time you will see the dark brown coating.
Step 9. After removal from the solution, rinse in clean water and
allow to dry
You now have your peroxidised anode, and as with any anode when not
in use, rinse after using and keep out of plating solutions until you
are ready to plate."



Quote:
Originally posted by 12AX7
Idunno, I've electrolyzed lead in a salt solution before; as long as it's not hot (such that PbCl2 is mobile and makes Pb(OH2) as I'd like), it passivates, then slowly the coating transforms to Pb(OH)2 and then PbO2, which sloughs off. Problem is, current is really low once it passivates, so it takes forever to get much PbO2 formation.

A coated electrode might work (suppose I'll go anodize Pb in H2SO4 and try it), but it may also slough off at pinholes and passivate anyway.

Tim
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[*] posted on 6-7-2005 at 04:52


in reply to the first volley of posts.

Am I the only one to think that Si is non conductive... regardless of the oxide layer?

In order for the Si to be conductive, it must be doped with another element that is either conductive on its own, electron deficient (P type) or have abundance of electrons (N type).

IF the Si is doped with a small w/% of these materials, it still behaves like Si, but is conductive, regardless of the tiny oxide layer.

I use Si wafers as anodes once or twice a week.
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jpsmith123
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[*] posted on 6-7-2005 at 07:07


I believe that pure silicon has a high resistivity, so I was under the impression that you would need to use either cheap relatively impure silicon, or doped silicon.

In any case, you use silicon to make chlorates only, or have you also tried perchlorates?

Quote:
Originally posted by uber luminal
in reply to the first volley of posts.

Am I the only one to think that Si is non conductive... regardless of the oxide layer?

In order for the Si to be conductive, it must be doped with another element that is either conductive on its own, electron deficient (P type) or have abundance of electrons (N type).

IF the Si is doped with a small w/% of these materials, it still behaves like Si, but is conductive, regardless of the tiny oxide layer.

I use Si wafers as anodes once or twice a week.
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[*] posted on 6-7-2005 at 09:42


There’s no need to quote a post right above yours, especially not the whole thing.
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[*] posted on 6-7-2005 at 15:34


I use the Si butted against Al (both have oxide layer that can not be cleaned conventionaly) in the cell. But I do it for something other than what your doing.
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[*] posted on 6-7-2005 at 16:12


~ quote of Lambda
1.- Metal connections will not give as many problems as with the porous ceramic substrates when the anode goes into service as there will be no conduction of salts up the plastic substrate.

2.- The advantages of the plastic substrate it that it is easy to obtain and is non porous and therefor will not suffer to a great extent from the problem of the salts from the Chlorate/Perchlorate cell getting to the current connection by cappillary action.

3.- Plastics that are suitable for the task are polyester, polypropylene, polyethylene, teflon, ployvinylchloride (PVC) and polystyrene (...and counting).
~

Yeah, but someone has to stand up for ceramics around here!

1 In response to the first problem... yes, salts coming up the anode will be a problem, unless you plate the anode tip in silver, have a smooth lead dioxide plate, or otherwise prevent the salts from reaching the top. I made some thin ceramic substrates (not fired yet, ~0.4 cm thick x 20 cm x 6 cm) which may be thin enough to soak one part of it in a plastic dissolved by some solvent, and then dry. The lead dioxide will still conduct, but perhaps the plastic layer will seal out water and salts. I made plenty of ceramic pieces, so I can do plenty of tests.

2 Ceramics are easy to obtain too (flowerpot) and you can easily build a kiln/furnace. If you make them yourselves, they are extremely easy to shape into the exact size and configuration you want. (easier than plastic, IMO)

3- about any clay will do (IMO) as long as it's fired well and is made of alumina and silica mostly.

There are several advantages that ceramics have over plastics;

1- they are ceramic, and therefore cool
2- they are very stiff, unlike plastics, and so will not bend/warp and cause the lead dioxide coating to flake and crack.
3 -they are completely inert to the solution; imperfect plating or whatever is not a problem. There will be no contamination (as far as I know).
4- they can take much greater current densities. Axehandle and Madhatter have both had problems with plastic substrates being oxidized by the lead dioxide and burning up. Clay will not react, melt (in this situation) or burn. Ever.

That's why I'm trying the ceramic route. But if anyone succeeds with the plastic substrates, tell us all about it. I'm not trying to critique Lambda or anything, I just want to make sure that people know that ceramics could be a good choice too.




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jpsmith123
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[*] posted on 6-7-2005 at 18:42


Hmmm. I didn't know that several people had already unsuccessfully attempted to use plastic substrates.

I followed one thread in which Axehandle made some PbO2, but that thread apparently ended without any discussion of electrolysis results using the material.

Is there another thread somewhere that I've missed regarding these plastic substrates?

Quote:
Originally posted by Cyrus

"...Axehandle and Madhatter have both had problems with plastic substrates being oxidized by the lead dioxide and burning up...But if anyone succeeds with the plastic substrates, tell us all about it"...
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[*] posted on 7-7-2005 at 06:56


Anodized lead in salt solution; graphite cathode. PbO2 appears to be slowly flaking off and reforming, but with bad connection to the underlying metal. Bubbles are seen at both electrodes, suggesting oxygen production in addition to chlorine (it has a chlorinated odor, presumably OCl- and such are forming in quantity given the low operating temperature). Yellow to green to black corrosion products are seen around the copper wire anode connection (the steel bolt supporting it isn't too happy either).
Solution is mostly clear. Mild lead plating is seen on the cathode.

Tim

PbO2atedPb_Cell.jpg - 53kB




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jpsmith123
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[*] posted on 7-7-2005 at 16:47


Looks really messy. Rats.

I'm wondering, did you anodize the lead anode using the method I posted? Did you plate any PbO2 (from lead salt) on top of the anodized PbO2?

Unfortunately I won't be able to try it myself for a few weeks yet.

(BTW, I emailed a company that makes PbO2 anodes, and asked the price of a 200 cm^2 anode. They want $3,000.00 USD per anode, with a minimum buy of 2 anodes).

[Edited on 8-7-2005 by jpsmith123]
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[*] posted on 8-7-2005 at 00:33


Uh, sorta... :P I just stuck it in 50% H2SO4 and anodized for an hour or so. Rinsed and dropped in 15-20% NaCl solution where it's been since. Nothing special like mentioned in the method you posted.

Y'know, I bet the metal-PbO2 contact resistance is so high that it's only producing oxygen, not having the potential at the surface to oxidize chlorine. That would explain both the low current and bubbling.

Some current is gained by heating the solution, but I don't have a hotplate so I can't tell what it'll do long term. Probably just oxidize more lead than chlorine, wasting current.

How do you plate out of a lead salt solution? Last time I tried I got nice dendrites of lead metal on the cathode, as...happens to make sense. :rolleyes:
*Edit: PbAc2.

Tim

[Edited on 7-8-2005 by 12AX7]




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