Sulphuric cell.

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Well-known member
Apr 6, 2014
Melbourne, Australia
Hello all.

I have a slowly growing pile of bluetooth antennas I would like to deplate. I posted something almost a year ago about my first attempts to put them through AP / copper(ii) chloride but soon discovered what a fool's pursuit it is, to try to dissolve so much base metal for such a thin coating of gold.

I've been researching sulfuric cells for a little while now. The forum is just loaded with a wealth of information on the topic and I've spend many hours of reading. Not only here, but other sites including wiki, Nurdrage and even Tom's Hardware. There are however a few things that are still a little unclear to me. So I would like to ask for some help before I make a complete balls-up of it, er, "start".

Please pardon me if I ramble a little; I'm trying to put my thoughts out there with my two questions.

I have not yet happened across copper mesh. But the antenna bodies, once the steel wire is removed, have a central hole. So I'm intending to simply suspend them on bare copper wire. Cheap. Easy. Disposable. Ok, onto the two questions.

1: Required sulphuric acid strength.

Forum posts generally indicate 98% is used. Easy if you can buy it at 98%. One post says 95%+. I'm having just a little trouble finding such concentration locally. Of course battery acid is always plentiful. My bike's top up bottle doesn't indicate its concentration but I will assume it to be in the order of 30%. So I can just concentrate it and I believe this is most easily done "by volume". But I don't necessarily trust my own evaporation accuracy to 98%, especicially if I don't know whether I'm starting with 28% or 35% in the first place.

From my studies I know diluted sulfuric will chew through copper like a hot knife through a leper but there must be a point where passivation gradually begins. You'd think this would be easy to find. But no. WHERE? Is 90% ok? 92%? 95%? 80%? My understanding behind the theory of passivation is "marginal" to say the least but I will presume that it will have a gradual curve rather than a sharp cutoff point. With that in mind, it would be good to start with a rough percentage. One thing in my favour is that I'm planning to use just a cheap wire for suspending the target (assuming it works) and that means all is not lost if it happens to dissolve, unlike some hard to find copper mesh.

2: Deplated metals

I'm finding it hard to determine what metals will deplate under electrolysis, while the current is turned on.
Gold, obviously. As I undertand the process, it's dissolved only under current so that when ions move away from the current source (or when current stopped) they are no longer held in solution and drop as black powder.
(2.1: why black? If it's gold, why not brown as when precipitated from auric chloride?)

Other than the steel wire which I yank out with pliers, I am not really sure what my antennae are even made of. Steel, brass, nickel, who knows?

What will NOT dissolve in sulfuric and under current?

3 (because I can't count to 2): Electrode Material: +Cu -Pb

So gold dissolves in the presence of current near the anode, for some kind of electrony reason I don't need to understand in order to accept. Copper and Stainless both work.
But if the ions move too far from the anode, or if the current stops, then gold precipitates as black pwder. But gold ions don't go even near the cathode. So why must the cathode specifically be lead?
Lead as the cathode is somewhat inert in conc. sulfuric.

Copper will dissolve to some degree if sulfuric is under 95% and/or too hot. Further conc. sulfuric gets far more dangerous beyond let's say 60-80°C, at its boiling point it will even generate SO3 smoke (never tried, don't want to!). Btw. remember NEVER to put water into conc. sulfuric.

At the anode formed persulfuric will dissolve the gold, but this compound is instable and gold will reduce to metallic state as finest black dust as soon as it leaves the anode. It is black because it is impure.

Yes, dilluted sulfuric can be heated until all water is vaporized. (never tried, never dared to)

Sulfuric cell works with base metals like copper, iron, nickel, cobalt and their alloys. I would not try other metals without further research or testing.
Jason I feel your pain as I've some of your questions too however I think I'm able to answer some of your doubts.

From an unknown concentration I reckon controlling the temperature could be the key. [stt]Temperature wont go over 100°C till all water is gone, so if it raise over that point should means the water is gone.[/stt] Also 1ml of 98% sulfuric acid weights 1.83gr, having a good scale it could be another tool to help you out. [stt]If I correctly remember, while evaporating, if you see white smoke coming out you are already at ~98%.[/stt]

At the anode, when current passes trough, sulfuric acid gains one atom of oxygen forming persulfuric acid and this is able to hold oxidized gold. Moving from the anode to the cathode the persulfuric acid chase to exist releasing the black gold powder.

Why black? No idea. But I guess it's just the way it reflect/absorb the light. Might be the just shape of the powder.

Why lead? Would be great to know! I always promised my self to learn more about anodes, cathodes and electrolytes, how, when and why.

Please accept my answers as not correct ones, sometimes when I feel to be able to answer I take the opportunity to also test my learning.

P.S.: just noted solar_plasma's reply but I'll post anyway, cheers.
I can't find the thread now, but I think it was GSP that suggested 85% concentration would work but keep the temperature down. The combination of excess moisture with heat will speed up copper dissolving more that just the heat or the moisture alone. I will keep looking and see if I can find the thread again.

If you can't find the mesh, and aren't doing tiny pins, you might find some window screen made of copper. I found a large roll at the local scrap yard and traded 5 pounds of aluminum scrap for it.

I too have been studying a bit on this subject and have a partially constructed cell - waiting for good weather so I can work outside.

On the copper mesh front, yes, it is hard to come by, and expensive. Some of the suggestions were to check a hobby store -I did and they used to carry it but no longer. I live in a small city with only one store so if you are in a bigger center you may find it there. I wound up finding it on eBay out of California and got a little less than 2 sq ft to my door for about $25 CND.

In regards to why the powder is black - I read that because the particles are so fine that they diffuses light in all directions, hence, appears black or having no color.

A source of 96% H2SO4 can be found in certain drain cleaners. I called up the MSDS for the stuff I have here and that confirms it. If you want the brand I can go dig it out of storage and advise. I believe it was GSP that said he doesn't like to use it because of the additives and agents in it but don't quote me that it was him. The stated problem is with foaming. That said, I am going to try it as I have 4 quarts of it and because I am in no hurry or am not a production outfit so I can slow things down to control that (hopefully). If it doesn't work out then I will go the concentrating battery acid route - there is good information here about doing that with a jeweller's scale (think LazerSteve has posts on it but again don't quote me).

And finally, I do not want to insult your intelligence, but I will reinforce - NEVER add water to conc H2SO4! Always H2SO4 to water. The way I remember that is that Acid comes before Water alphabetically, thus, A to W.

Regards, Jeff B
I just re-read the original patent, 2185858, and it uses sulfuric acid of specific gravity of 1.65. That computes to a solution of 68% sulfuric/32% water, by volume, assuming you're starting with 96%, by weight, sulfuric acid. I've never used it that weak (and probably never will) but, since it's a Western Electric patent, I assume it will work. The purpose of the patent is to strip faulty gold plating without damaging the substrate so the parts can be re-plated. The patent says to keep the solution temp under 100F. Doesn't mention current density

Would be interesting if someone tried this weaker solution. Might work faster. I know that sulfuric absorbs moisture from the air and becomes more dilute. Therefore, in production, I would probably start with a minimum of 85%, by volume. It seems that about any strength will work if it's strong enough to form a passive layer on the copper to prevent dissolution. I know that even battery acid (32%, +/-, by weight) will strip the gold but it also will eat copper.
MarcoP said:
From an unknown concentration I reckon controlling the temperature could be the key. Temperature wont go over 100°C till all water is gone, so if it raise over that point should means the water is gone. Also 1ml of 98% sulfuric acid weights 1.83gr, having a good scale it could be another tool to help you out. If I correctly remember, while evaporating, if you see white smoke coming out you are already at ~98%.
That is so wrong! The sulfuric acid is grabbing hold of the water and raises the boiling point of the acid. All the way up above 300 C / 570 F. Imagine what damage that hot acid would do to you in case of an accident.

The behaviour of sulfuric acid and water at different concentrations is shown here
And as shown, there is sulfuric acid going off as vapor even at lower concentration but becomes severe above 90%.

Concentrating sulfuric acid by evaporation is dangerous and should only be done with proper equipment and security procedures. Always plan for an accident.

GotTheBug said:
Is there any reason not to use stainless for an anode?
The only metal I've had problems with was nickel when I was stripping gold from inconel, which has a very high nickel content. The stripper tended to dissolve the nickel and, after only a couple of cycles, the solution became saturated with nickel and stopped stripping the gold. I would guess that cobalt (like in a Stellite alloy) would cause similar problems.

300 series stainless (non-magnetic) contains nickel and might cause problems. However, I have stripped gold from 410 stainless (magnetic - no nickel) hundreds of times with no problems.

A big disadvantage with any stainless steel is that it's not very electrically conductive. It's like a big resistor. Copper is 44 times more conductive than 316SS and 79 times more conductive than a 400 series hi-chromium alloy.

If you can keep the temperature low, I would think that copper would be the best choice. I once used plating racks to hold all-gold IC's for stripping, using spring contacts made from magnetic 400 series SS piano wire. The racks, themselves, were made from uncoated 1/4" square yellow brass stock (probably 65Cu/35Zn). Each rack spent at least 2 hours a day in the sulfuric (about 90%, by volume), as the anode. After 2 years, there was no visible attack on the brass or the spring contacts. I would guess the temp of the soln. didn't exceed 105F during that time.

A lead cathode was probably chosen because of its inertness in the sulfuric. I never have used lead. I always used mild carbon steel as the cathode(s), with nary a problem. In fact, the 50 gallon mild steel tank I used for the solution was made the cathode.
Thank you for correcting me! I should study more :oops:
While learning about it I never noticed any reference to vacuum distillation and thought about it as a relatively easy method ... I'll edit my post just in case.
Thank you all for your wonderfully helpful replies. I still have a few more rabbit holes to wander down, but I think I'm nearly ready to give this a go on a very small scale.

Göran, I couldn't access your link as the server is playing up. I'll try again later. You have me a little worried now about concentrating my own. I was aware it evaporated out to 300°C and I am used to working with large pots and waves of molten solder. I will be extremely careful and start testing with just 100ml.

I tried two major hardware chains last week plus some little local stores looking for sulphuric drain cleaner. All I can find is sodium hydroxide, and even that's now 98%. (Last time, I was able to get 100% so I will assume it's got a decaking agent.) It might be an "Australia" thing.

Based on GSP's advice, I think I shall try it at 85%, which means I have will to evaporate battery acid to about 230°C. Not wanting to put the cart before the horse, but if that works well I will try progressively lower concentrations, running as cool as possible and report back my findings. It sounds like some people might be interested.

I should have been more specific in Q3. I understand that it turns into persulfuric near the electrode. (How exactly that electrony stuff happens is a bit beyond my basic chemistry, but I don't need to know everything.) I can't find it now but remember reading perhaps a few months ago; Harold made a point about target items not even needing to touch the anode but just be within the zone, about half an inch or so (current dependent). So then, if at the cathode it's just plain sulphuric acid, and if the copper anode can survive sulfuric/persulfuric, perhaps a copper cathode would too. I have access to heaps of copper shim but no lead handy. On the other hand, my objective should not so much be preventing loss of cheap electrode but preventing loss of quality of acid.

Finally, Jeff, I am not the slightest insulted by you telling me something I should already know: "Do as you oughta, add acid to water". Safety before hurt feelings.

My grateful thanks to all.
Actually, I think the evaporation of weak sulfuric acid to make strong sulfuric should be on the list of very dangerous processes discussed recently on another thread.
Especially when looking on who would most likely want to try it: hobbyists with poor experience. Almost everyone else would rather be able to buy sulfuric in all concentrations.
For those of you looking for copper mesh, go to lasersteve's store at and he sells it for a very reasonable price.

Edit: A simple search using "copper mesh" as the search criteria would have found it very easily as well.
goldsilverpro said:
Actually, I think the evaporation of weak sulfuric acid to make strong sulfuric should be on the list of very dangerous processes discussed recently on another thread.
I would go along with that. Everything to do with it had me rather on edge. I will share what I did and if you think it inappropriate to share then please delete my post and I will not be offended.

As I intended, I started with a 100ml beaker. I only ultimately used 75ml because
  • that's the highest graduated marking on my 100ml beaker, and,
  • the first time you work with something that can scar you for life, you DON'T want to fill it to the top
  • As it evaporates through the 50ml mark, I am smart enough to NOT top up hot acid with another 25ml of cold 32% acid.
The initial 75ml weighed 93g, which is an SG of 1.24 or 32%. I figured there was therefore 24ml of acid in it.

I left it on the portable outdoor gas stove until it was down to 30ml, letting it continue to evaporate as it cooled. Final volume "about 27ml", or "about 88%", assuming the 24ml of acid was still in there. However, that ~27ml weighed 44.6g or SG 1.65, or around 78%. So some acid has fumed out too but I figured that was close enough to test concept. (I'm probably making weight/volume errors.)

I bend some copper wire into a small basket that would hang in a 50ml beaker, into which I could put just 3 or 4 antennas as a time. I also used copper wire bent several times for the cathode. It seemed to work quite well and I put a few cycles through with no degradation noticed. The temperature stayed below 27°C.

But then my power supply gave out. Instead of 6V limited to 0.5A, it slammed up to 40V unlimited current. My single strand wire basket broke in the centre even before I could kill the power. I don't have a photo of the little wire basket handy.

Nevertheless, I have proof of concept with 78+ % and I had made this: There's a nice layer of brown forming on the bottom. It's still not completely settled, but it is settling enough now to see there's a tinge of green in the remaining solution: probably indication of a little copper being chewed up.
Overall I'm pleased with the result. It would be better if it were stronger acid but I also prefer the idea of working with 85% than 96%: I expect it to be a little more forgiving around water, should I make any accidental slipup.

I'll have to repair my power supply before I can try anything else, or on a larger scale.
I have read a little about concentrating sulfuric on a german chemistry forum: One should be knowing, that concentrating sulfuric to about 85% implicates temperatures about 200°C and sulfuric acid vapors. A water containing liquid at this temperature is high stress for the glass ware and obviously it happened to more than one, that their (low quality) glass ware exploded on the thermal stress.

Thinking of drops of water condensing at the colder glass areas might run back into the acid would cause me nightmares. Not to think of, if someone would make this experiment in a closed rigg and forgets to prevent water to get sucked back into the acid when he is reducing the heat.

Again one should also be aware of, that 1l hot H2SO4 is exponentially harder to control than 10ml, - in case of something unexpected is going to happen!

While this might be safe for most of the seniors and chemists, this is an accident waiting to happen for everyone else, included myself - even though I have a large school lab to do a lot of not harmless experiments safely.
You know Björn, you are starting to talk me out of repeating a perfectly good first test run. :lol:
I am not certain either way about cool 80% being as susceptible to small water droplets as hot 96%, which is part of my reason for wanting lower % in the first place. But I can also see the wisdom in what you are saying.

Since the probability of horrible accident = individual chance of accident * times repeated, I think I will likely keep reusing what I already have as long as possible and not evaporate down any more, but instead wait until I can find a local source of supply.
solar_plasma said:
Thinking of drops of water condensing at the colder glass areas might run back into the acid would cause me nightmares.

This should not be a problem because the water droplets that condense should be close to the temp of the solution you are heating up.
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