387 replies to this topic

[Old1953]

Tantalium sounds like the next best thing to platinium but its still very expensive...If i cant get a decent coating of lead dioxide on these graphite rods i have ill try tantalium wire...if you read the Patents it makes a very good substrate for lead dioxide coating..

Tantalum isn't nearly as expensive as platinum. Usually, its a good bit cheaper than silver, though the price runs up and down due to demand for capacitors. It is hard to find as metal though, because there really isn't much use for it outside the chemical/electronics industries.

Tantalum foil capacitors sound like a pretty good bet, though I'd be careful of any tantalum cap when breaking the seal. Some are filled with sulfuric acid. The oxidized anode type of tantalum cap often has a sintered anode made of essentially compressed powder. I doubt that has enough strength to be reworked. Solid tantalum is supposed to be very hard, but actually work like copper.

Ebay usually has samples of tantalum available, though the industrial surplus listings are a lot cheaper than the element collector listings. Sometimes the surplus listings are priced way under the actual metal value of what they are selling.

All that information, and I can't find anything about erosion of tantalum in a chlorate or perchlorate cell.

[Pretty green flames]

Report:

Well just tried the electrolytical method for making NaClO3......
Unsuprisingly the graphite anode fell apart after 11min 23sec of operation.
But it didn't fell apart in the electrolyte. It fell apart at the poing where that Anode is exposed to air. It just broke in half. But it did work.....for starters i could smell chlorine.

Technical Data:
Anode was taken from a pencil (The graphite core)
Cathode was an Iron Rod

I ran
25Volts
1.5Amperes through the elctrolyte.

Should i run more Amperes and Volts through the electrolyte

Edited by Pretty green flames, 17 February 2005 - 04:04 PM.

[alany]

Anode was taken a pencil (The graphite core)
Cathode was an Iron Rod

Pencil "lead" is actually very fine clay and graphite powder pressed into a solid and fired. The clay/graphite ratio changes with the grade of the lead. A normal HB pencil is largely clay, not graphite. The 9B pencil is the softest and contains the most graphite but still has a lot of clay in it. Commercial extruded graphite is similar but has much less binder.

Sometimes cheaper pencils are bound with waxy materials not fired clay, much like carbon electrodes from dry cells, they will fall apart very quickly from the high temperatures melting the binder. Mechanical pencil lead is often bound with a rubbery material not sure what it is, a troll through the patents might find quite a few methods of making pencil leads.

The resistance of a pencil lead is pretty high, that may have contributed to its failure so quickly and near the top of the solution due to ohmic heating were the current density was highest. Try to find gouging rods if you can, they are bigger and will erode less quickly, especially if you treat them. Can't say much about treating graphite, I haven't tried linseed oil, just wax and I wasn't impressed. At the current densities and temperatures I want to run carbon and graphite electrodes are useless so I've basically given up on them.

When I did experiments with carbon dry-cell anodes they lasted only a few hours, but the erosion pattern was always conical, sharpening the rod to a point as it went, until eventually there was no submerged electrode left. I believe this effect is used commercially to sharpen fish hooks and even make stylus for field-effect microscopes.

25Volts
1.5Amperes through the elctrolyte.

That's quite a high voltage to achieve 1.5 Amps. It is probably because of the small surface area of your electrodes and the high resistivity of the anode. The spacing will effect the voltage requirements too, closer spacings produce less resistance and are more efficient electrochemically.

The cell voltage of a chlorate cell is typically about 6 V.

[sasman]

Dont forget PGF graphite can only take a very low current..How many pencil leads did you use and what length & diameter were they ..

You need to know the surface area of the graphite to workout how much current to apply..If you want to make some usable amounts of CLO3 you need to at least use some Graphite electrodes out of a Battery...

Old1953 what do you need to know about corrosion of Ta in a chlorate/perc cell?

I have read lots of info on plating lead dioxide and worked out how much PbO2 to add to my plating tank but the only reference to adding CuCO3 is on wouters page

Although many articles state that the pH should be kept constant by addition of a mixture of CuCO3 and PbCO3 (1:1),

..

Surely this is wrong??? I have worked out i need to add about 35g of Pb02 and hour to maintain lead concentration...thats would mean adding a more CuNO# than i had started with?..Unless he means in a % ratio of the intial concentration?..

Eg Starting solution Pb(NO3)2 250g liter CuNO3 50g liter = 1:1 ratio PbO2 35g + CuCO3 7g?

Does that make any sense?...

[Pretty green flames]

The graphite in batteries is not the same as it used to be.
Now there's a graphite tube and in the middle there is some blue substance.

The diameter of the pencile core was 2mm and the lenght was 100mm

And i just used 1 for an experimental 50ml Cell.

[Phoenix]

I've found that some batteries have the graphite tube you describe too, but not all of them. Try a different brand. I've got a few graphite rods out of a 6v lantern battery (not got round to using them yet though). Welding rods are probably better for any serious production though.

EDIT Serious production of chlorate, that is. Obviously they're not good for perc...

Edited by Phoenix, 17 February 2005 - 06:49 PM.

[alany]

I've never seen hollow anodes in dry cells, interesting.

For the experiments I did I used lanten batteries too. Extracting and cleaning up the carbon rod isn't much fun, all kinds of gooey sealant, MnO2 and carbon black paste, yuck! Better to purchase graphite gouging rods I think, then you actually get good fine-grain graphite, not something cut with carbon black and $deity knows what else.

Better yet, just call around electroplating and corrosion control suppliers. They sell MMO/Ti, Pt/Ti and Pt/Nb by the sheet cut to size. I spoke to several today, some even understood my application and had lots of good advice.

[Old1953]

Sasman, I'm just trying to find out if tantalum really is good for the cathode in both chlorate and perchlorate.

I bit the bullet and bought a mess of surplus tantalum wire. Guess I'll find out when it comes in. If the erosion is light, say a gram per kilogram of perchlorate, I don't much mind it. That would be pretty minor contamination given the feed product.

As for the CU salt in your tank, I think its just there to buffer the remaining solution and help keep the PH steady.

Has anyone looked at the spec on commercial grade technical (99%) potassium perchlorate besides me?

http://globalchem.en...rchlorate_.html

[sasman]

Old1953 You dont use Tantalum for your cathode you would use it as a substrate for your anode..

I have not read any were about Ta being used as a cathode ..Your cathode should be Titanium or stainless steel much cheaper Ti is the best .....If you are using Tantalum for your cathode what are you using for your anode?..

The CuNO3 is in the solution to stop the lead from plating out , without the Cu the lead tends to plate on the cathode ..

[Old1953]

Old1953 You dont use Tantalum for your cathode you would use it as a substrate for your anode..

I have not read any were about Ta being used as a cathode ..Your cathode should be Titanium or stainless steel much cheaper Ti is the best .....If you are using Tantalum for your cathode what are you using for your anode?..

The CuNO3 is in the solution  to stop the lead from plating out , without the Cu the lead  tends to plate on the cathode ..

Tantalum isn't attacked by HCL or its relatives at temps under 150C. That's why I'm wondering if it is a suitable material for an anode (I meant anode in the above, that'll teach me to stay up late.).

The chemical and physical properties look right, I'm sure it would erode faster than platinum, but the question is "how fast"?

So I'll measure the wire with a micrometer, run a cell for a while, then measure again. That'll answer the question.

I must say, I'm having trouble visualizing that CuNO3 reaction at the cathode, have you got a link for that? It's certainly buffering the solution, but buffers are often chosen for more that one reason.

[sasman]

Hmmm ... i dont really understand what i have read but From my understanding Tantalum polarizes and becomes inactive in a chemical cell?..Thats why its a good substrate it still conductive but chemically inactive?..Have you read somehwhere that it can be used as a Anode?..i Have not found any reference for Ta being used as an anode in a CLO3/CLO4 cell.

The other question about CuNO3 i dont have a link but read it a few times on various articles on the web..The Cu is added so that preferential plating of Cu instead of lead at the Cathode..Also the addition of Cu reduces the hydrogen over-potenail of the cathode..god knows what that means?..the only part that concerns me is that if it plates out copper on the cathode at what rate do i need to add CuCO3 to make up for the loses?..

[Old1953]

Hmmm ... i dont really understand what i have read but From my understanding Tantalum polarizes and becomes inactive in a chemical cell?..Thats why its a good substrate it still conductive but chemically inactive?..Have you read somehwhere that it can be used as a Anode?..i Have not found any reference for Ta being used as an anode in a CLO3/CLO4 cell.

The other question about CuNO3 i dont have a link but read it a few times on various articles on the web..The Cu is added so that preferential plating of Cu instead of lead at the Cathode..Also the addition of Cu reduces the hydrogen over-potenail of the cathode..god knows what that means?..the only part that concerns me is that if it plates out copper on the cathode at what rate do i need to add CuCO3 to make up for the loses?..

Beats me, I can't find any information on tantalum for this purpose, so I'm trying the experiment to find out. If not, it will still make the ideal connecting wire, it won't react at all. It does oxidize, anf the oxide is an insulator, but I don't think this will be a problem in this instance - we'll see.

[Old1953]

Beats me, I can't find any information on tantalum for this purpose, so I'm trying the experiment to find out.  If not, it will still make the ideal connecting wire, it won't react at all.  It does oxidize, anf the oxide is an insulator, but I don't think this will be a problem in this instance - we'll see.

There are some interesting hints in the literature I can find.

http://www.ingentaco...000004/00450910

http://www.edpscienc.../jp4Pr4p69.html

I suspect tantalum erodes at a rate that makes it too expensive over time, due to more frequent replacement of solid electrodes, than platinum plated electrodes which only erode a thin film away and are then replated. IOW, in the long run the platinum is cheaper - if you have to buy the tantalum at actual cost.

[alany]

i dont really understand what i have read but From my understanding Tantalum polarizes and becomes inactive in a chemical cell?

Yep, I suspect it would behave like Titanium, Niobium, Zirconium, etc. It is directly under Niobium and Diagonal to Zirconium so its oxide formation and anodic breakdown potentials would be similar (much better than Titanium). If you push the voltage higher to punch through the oxide you will only grow it thicker, up to some physical limit where it will flake off due to internal stresses (the oxide is "bigger" than the original metal) and allow the process to repeat, slowly eating away the electrode.

[Old1953]

Yep, I suspect it would behave like Titanium, Niobium, Zirconium, etc.  It is directly under Niobium and Diagonal to Zirconium so its oxide formation and anodic breakdown potentials would be similar (much better than Titanium).  If you push the voltage higher to punch through the oxide you will only grow it thicker, up to some physical limit where it will flake off due to internal stresses (the oxide is "bigger" than the original metal) and allow the process to repeat, slowly eating away the electrode.

Yes, the exact behavior is what will tell as to whether or not it will be suitable for these minor cells and their applications - the rate of erosion is the critical number and I can't find it anywhere. Tantalums other properties would make it nearly ideal. So I'm going to try the experiment, and I do know a cheap trick that may work if the pure tantalum doesn't.