How to clean lead and tin off silver the smart way?

[Mister_Better]

Greetings electro chemistry wizards! A quick speculation here... should be easy to sort out whether feasible.

I have nice old pocket watch made of sterling silver that some numpty tried to fix with soft solder - lead & tin i presume but god knows when repair was attempted so it could be newer non toxic alloy. Anyhow i want to effect a repair but the small amount of structural silver that is left is riddled with a horrible mess of lead and tin. Apparently hydrochloric acid is one way to proceed (any info on why would be cool btw) but I wondered if the galvanic series could be used in some way to electrically liberate the silver / copper alloy of the leaded mess... Like a sacraficial zinc anode, or electrically deannodizing the silver (as in lead annodizing on ali).

Lead and tin are both less noble than silver and copper. Is it possible to rig up an electrolytic cell to do this? I guess the pocket watch case would need to be one end of the cell (cathode I am thinking) and Id hope to wind up with all the lead and tin of the solder on the other end.

I don't want to oxidize anything particularly, only to gently persuade the nasty mess that is standing in the way of my repair, to go elsewhere!

If theoretically possible what electrolyte etch should be used? This is one shot by the way if I stuff it up it's just going to be a pile of scrap silver and a rather nice 19th century timepiece mechanism!

 

[TeethWhitener]

This book:
https://books.google.co.in/books?id...IRTAJ#v=onepage&q=silver + aqua regia&f=false
along with several websites and common (old folks’) knowledge claims that aqua regia reacts only very slowly/poorly with silver, thanks to the formation of a silver chloride passivating layer. I’ve never tried it, so it’s at your own risk. I know, however, that aqua regia will dissolve lead and tin (possibly vigorously). At any rate, aqua regia is quite a powerful reagent, so you’d need to take the proper precautions before using it.

Electrochemistry might also not be a bad idea. You might try an electrolyte that contains chloride ions for the same reason as the aqua regia discussion above: the passivating insoluble silver chloride. Hydrochloride acid works on the same principle (metal is oxidized, HCl is reduced to hydrogen gas), but I’m not sure HCl alone will react with lead (it definitely reacts with tin).

 

[Mister_Better]

Hi thanks for info. It seems ar doesn't dissolve silver and is definitely used in refining gold and platinum but would it attack the copper in the sterling?

I think hcl has same effect as aqua regia here... does the nitric add anything for this case? Also any idea why tin and lead get eaten by hcl or aqua regia but silver produces protective chloride layer? Tin chloride is quite soluable so makes sense for the tin to get dissolved on formation of salt but lead and silver chloride both have low soluablility. It seems the lead should also wind up with a protective layer also!

A few, probably I'll informed, thoughts on this...

If the contaminated silver is set as the anode of electrolytic cell filled with brine as electrolyte and an electrical power source is connected there would be a formation of clorine around the piece... this would bond with the tin and lead to form salt rather than the silver or what not. Agitation using a submerged nozzle might help to dislodge any salts formed and this would enable a more controlled cleaning of the piece though i guess this is basically what happens in the application of hcl to the soft solder anyway so not sure if there's any advantage to this method.

The same arrangement with water and maybe sodium hydroxide or similar as electrolyte would oxidise the lead and tin at the anode through the hydrolysis of water, again I'm thinking the contaminants should oxidize first and that it will work with either pure water (at suitable voltage or at poor efficiency) or using a base as electrolyte such as sodium hydroxide.

Oxidizing the contaminents at the cathode to associate with ions in the electrolyte, such as sulphuric acid or similar might also work (isn' this basically a lead acid battery?). I think this is just reverse electroplating.

Clearly I'm no chemist so any suggestions would be great! Will these methods actually work in this application and would there be any advantages/drawbacks to any of them?

 

[Borek]

Hi thanks for info. It seems ar doesn't dissolve silver and is definitely used in refining gold and platinum but would it attack the copper in the sterling?

As far as I know - yes, copper will be quickly eaten by the aqua regia.

I think hcl has same effect as aqua regia here... does the nitric add anything for this case?

Nitric acid is a game changer, are it is a strong oxidizer. Due to additional reactions in AR it gets even more oxidizing strength.

Also any idea why tin and lead get eaten by hcl or aqua regia but silver produces protective chloride layer? Tin chloride is quite soluable so makes sense for the tin to get dissolved on formation of salt but lead and silver chloride both have low soluablility. It seems the lead should also wind up with a protective layer also!

Passivation depends on the interactions between the metal and the compound that is produced. Some stick to the surface, some don't. Just because something is weakly soluble doesn't mean it can't flake off.

 

[Mister_Better]

"As far as I know - yes, copper will be quickly eaten by the aqua regia."

Thanks, pure HCl is likely to be a preferable acid for this purpose then as i don't think it is particularly reactive with copper but should react with the lead and tin.

These clarifications are much appreciated but the main point is trying to work out whether there is an advantage to using an electrochemical cell for this application, and if so what setup would be best... i realize chemistry is not particularly active on this forum but I am kind of surprised this can't be cleared up really quickly!

Anyhow my feeling is that a weak H2SO4 solution with work piece as cathode seems best approach (if feasible) as I'm hoping the setup will not cause any reaction with the piece until a current is applied, and when it is, the reaction should only occur in order of nobility of the metals on the electrode which will allow a controlled and thorough cleaning of the piece.

 

[TeethWhitener]

would it attack the copper in the sterling?

Maybe. Alloys act differently than pure metals. It would depend on the surface chemistry of the alloy.

does the nitric add anything for this case

As @Borek said, nitric acid is an oxidizer. HCl by itself is not. The reason HCl dissolves tin is because tin is a particularly good reducing agent. I'm not so sure about lead.

lead and silver chloride both have low soluablility.

Their K_sp differ by 5 orders of magnitude. Lead chloride is not particularly soluble, but silver chloride is really really insoluble.

i realize chemistry is not particularly active on this forum but I am kind of surprised this can't be cleared up really quickly!

Anyhow my feeling is that a weak H2SO4 solution with work piece as cathode seems best approach (if feasible) as I'm hoping the setup will not cause any reaction with the piece until a current is applied, and when it is, the reaction should only occur in order of nobility of the metals on the electrode which will allow a controlled and thorough cleaning of the piece.

It's your watch; you're free to take our suggestions or not. Why can't you take a small piece of silver wire or something, put a blob of solder on it, and test out your theory? Silver wire is a few dollars a foot max.

 

[Mister_Better]

"It's your watch; you're free to take our suggestions or not. Why can't you take a small piece of silver wire or something, put a blob of solder on it, and test out your theory? Silver wire is a few dollars a foot max."

Thanks and i appreciate the suggestions relating to how acid attacks metal but the thread seems to be missing the central thrust of the original post which is examining the theoretical and practical aspects of the idea to use an electrochemical cell arrangement to clean soft solder from precious metals.

I could just blindly experiment with limited knowledge, or learn the theoretical aspects myself and spend a lot of time accruing experience in chemistry, but arent these forums for pooling and sharing knowledge and ideas in open discussion to realize greater group efficiencies? Personally i'd have thought this would be a reasonably interesting, if not too trivial, problem for a chemist to consider.

Can anyone offer some insight or suggestions relating to cell arrangement, electrolytes, and reactions?

The other problem with just experimenting is getting some clean and well diluted sulphuric acid in the uk. It used to be possible to buy 30% odd suphuric battery acid and drain cleaners here but these are now restricted unless you have a license and registered business. Technically it should be legal to acquire sulphuric below 15% concentration but i'll be blown if i can find anyone selling it so any suggestions on alternatives may be really useful.

 

[Mister_Better]

BTW piece of lead/tin solder and modern non toxic solder are currently sitting in 32% HCl solution... Quite a bit of activity on the lead one and a little bubbling on the non toxic but extremely slow progress. Actually the lead piece reaction seems to be accelerating after a while in the acid... presumably this was due to an oxide layer preventing the acid reacting with the lead. Anyhow its very slow and will take a very long time to dissolve by the looks of it. I want to use electrons!

 

[TeethWhitener]

the central thrust of the original post which is examining the theoretical and practical aspects of the idea to use an electrochemical cell arrangement to clean soft solder from precious metals.

Can anyone offer some insight or suggestions relating to cell arrangement, electrolytes, and reactions?

I want to use electrons!

I offered a suggestion in the second half of post 2. You decided you wanted to use sulfuric acid as the electrolyte instead of a chloride, without giving a reason, theoretical or practical. In fact, you give a practical reason not to use sulfuric acid (you can't get hold of a bottle), and furthermore, a practical reason to use chloride (you currently have a 32% solution of it). I'm not sure what else you want.

This isn't a problem which lends itself particularly well to theory. It's a case where we could debate what's going to happen for 2 weeks, or you could go out and do the experiment and be done with it in 30 minutes.

 

[Mister_Better]

Thanks. I am not sure I missed something but I thought post two related to just pickling the piece in hcl which I know is an option. This may well be the best way to do it but the reason I was proposing to use an electrochemical cell, aside from a genuine interest in learning, is to achieve a gradual stripping of the contaminants from the piece without reacting with the precious metal alloy.

I guess the ideas about putting the piece on the anode and inducing the production of chloride or oxygen has no real advantage to pickling in acid, however it seems that if the piece is placed on the cathode and an electrolyte with available ions is used then application of current should, according to my basic understanding of chemistry, cause the cathodic metals in contact with the electrolyte to turn to a sulphate or whatever in order of the galvanic series.. is this basic supposition correct?

It just seems so basic I'd have thought chemists would be able to either confirm whether my proposal is viable, and if so what the electrolytes might be worth considering.

There must be a whole raft of things that could be used as an electrolyte to facilitate this "deplating" process under current it's just with a basic knowledge of lead acid batteries and electroplating h2so4 is the only possibility I'm aware of.

The discussion and analysis I was hoping to enter into on this forum doesn't seem a waste of time because others may be interested in it and because it might attract input from people with sound theoretical understanding and gems of experience. This seems much more effective and valuable than undertaking I'll informed experiments in private but, that said, I do have a bottle of drain cleaner on order so will have a shot at playing around at home none the less. Of course id be happy to post the results in case anyone else is considering it.

 

[Borek]

It just seems so basic I'd have thought chemists would be able to either confirm whether my proposal is viable, and if so what the electrolytes might be worth considering.

To someone dealing with these thing on a daily basis - probably yes. To everyone else - well, we know what to expect in general. Trick is, part of this knowledge tells us that behavior of specific systems is controlled by several competing things. Quite often we have no idea what will be the winner. Thermodynamics tells us what is the final stable state, but sometimes kinetics makes the system move in a different direction and it settles down in some local minimum. This is hardly predictable without experiments or a ton of a specific experience.

You basic idea - of using electrolysis to dissolve only unwanted metals - is sound. Unfortunately, choosing the right conditions can be really hard.

 

[Mister_Better]

Thanks, probably with a basic understanding one expects a simple solution to a problem but no doubt the expert will know that no simple physical process exists!

So if the basic principle is sound, excluding the multifarious potential practical caveats, are there any other good candidate reactions / electrolytes worth considering?

Criteria for good electrolyte would be to have available ions that would easily reduce/oxidise (need to read up on all this!) on application of current to bond with the low nobility metals, that preferably would not attack the piece without current being applied, and that preferably would result in a soluble or non passivating layer forming on the piece.

Incidentally would attaching piece to cathode in hcl solution speed up the process compared to just pickling in hcl?

I'm assuming that NaCl solution with piece on cathode would not work because Na is more reactive than lead/tin... right?

What about ammonia? This might yield nitrates/nitrites. Aren't these dangerous?

 

[Mister_Better]

... just discovered lead acetate... would acetic acid work?

Might be good as lead acetate/nitrates are soluble.

 

[TeethWhitener]

I thought post two related to just pickling the piece in hcl which I know is an option.

From post 2:

Electrochemistry might also not be a bad idea. You might try an electrolyte that contains chloride ions for the same reason as the aqua regia discussion above: the passivating insoluble silver chloride.

I thought it was pretty clear that I was referring directly to an electrolyte in an electrochemical cell.

There must be a whole raft of things that could be used as an electrolyte

Yes, pretty much any ionic solution.

Criteria for good electrolyte would be to have available ions that would easily reduce/oxidise (need to read up on all this!) on application of current to bond with the low nobility metals,

Not necessarily. For instance, with lead in a chloride solution, you already have Cl^-. The potential applied to the lead will take electrons from it to give you Pb²⁺, which can bind with the chloride to give lead chloride.

Incidentally would attaching piece to cathode in hcl solution speed up the process compared to just pickling in hcl?

Possibly. At some point you'll electrolyze the water, which will slow everything down.

I'm assuming that NaCl solution with piece on cathode would not work because Na is more reactive than lead/tin... right?

No, the sodium is already oxidized in NaCl. NaCl will be a decent solution as long as you're under the potential required to electrolyze water (this is a complicated issue).

What about ammonia? This might yield nitrates/nitrites. Aren't these dangerous?

Ammonia is also dangerous, or at least requires good ventilation.

... whay about acetic acid? This will produce lead acetate I believe.

Might be good as lead acetate/nitrates are soluble.

Solubility might be an enemy. If the salt you make crashes out of solution, it will drive the equilibrium to make more of the salt. Google Le Chatelier's principle.

 

[Mister_Better]

Hey thanks for so many answers in one post!
Well so far it seems that sulfuric acid is probably the best option. Will have a go with this. Does 15% strength with at least the estimated weight of contaminants in acid sound sensible? Anyhow will keep try with gradually increasing voltage and keep an eye on current as it progresses.

Any tips/advise on safe disposal... can small quantities of lead/tin sulphate go down drain or is this very bad idea?

 

[TeethWhitener]

Well so far it seems that sulfuric acid is probably the best option.

Why? You've still given no reason for using sulfuric acid other than "they use it in batteries."

Does 15% strength with at least the estimated weight of contaminants in acid sound sensible? Anyhow will keep try with gradually increasing voltage and keep an eye on current as it progresses.

I think if you're really serious about keeping your watch intact, you'll do a test run on silver wire with lead solder.

can small quantities of lead/tin sulphate go down drain or is this very bad idea?

This is a very bad idea and probably illegal. You'll have to dispose of it as hazardous waste. Contact your local municipal authorities for the proper procedure.

 

[jim mcnamara]

PF is meant as a service to our members. Some members avail themselves of it usefully. Others may not. The choice is up to the member. I choose to lock this thread as it it appears to be thrashing uselessly.