Thanks for that idea. I didn't realize it might be so simple to separate out the silver. I have a couple of basic questions about the plating process...
Originally Posted by Ian Grant
I presume any direct current source with several volts will work same as the battery?
After plating out the available silver, what do you do with whatever liquids and sludges that remain?
How disposable and safe is that remainder?
How do you know when you've got the plating job done?
How often does one do the plating job?
Are there any other basics one should know?
Before trying this method I suggest you to talk to someone who knows his ropes. Otherwise you might blow up your home with oxyhydrogen gas, produce some nasty chemicals or something else like that. AFAIK for seperating silver electrolytically you will have to thoroughly control the currence and voltage.
Originally Posted by CBG
Originally Posted by panastasia
in these days I was doing some electrolysis experiment (no photography related) and then I thought of some use for recovering silver... I'm no chemist, that's why I asked on APUG first: if there was a reliable system - I thought - some other must already be using it.
Running an electrolysis process costs time and money. I don't know if the quantity of silver one can recover from a fixing bath is worth the effort.
If making silver nitrate wouldn't be such a deadly task, I would be tempted to claim the metallic silver and make my own silver nitrate which I use extensively in my photography and printmaking. But that is definitively NOT a thing I will ever attempt to do.
Is there a similar way to restore the fixing bath? Ok, the fixer doesn't cost as much as silver does, but I wouldn't totally disregard the idea of having my fixer life extended a little bit. I'm into wet plate photography now and I'm consuming a lot of hypo. It's not expensive, but you need a lot of it and dies quickly.
The FT-1 hypo check uses potassium iodide to
Originally Posted by Fulvio
precipitate silver iodide. In excess it will clear a
hypo of nearly every last trace of silver. Same
for sodium sulfide. Both salts form extremely
insoluble salts with silver; the sulfide being
Fixer loads up with chloride, bromide, and iodide;
the remains of the silver salts which make up a
silver gelatin emulsion. Also, old fixer is likely
quite oxidized. Addition of sodium sulfite
after electrolytic extraction of the silver
may extend it's life some.
BTW, silver nitrate is safe; the component in the
FT-2 test and used in emulsion making. Dan
Electrolytic recovery is normal in much of the industry, but AFAIK it destroys the fix. I have forgotten over all the years since I worked on this. Ulrich is right though, it must be carefully controlled to prevent evolution of unwanted gases.
Saw Dan's post after this went up. He is totally correct. KI added to the fix will precipitate out most all silver salts and complexes as Silver Iodide.
Originally Posted by Photo Engineer
Sounds like it isn't so simple after all. Oh well. I had assumed the fix would be dead after silver recovery. Didn't plan on hearing about evolving oxygen and hydrogen.
Electrolytic recovery is simple and effective, in practice under some circumstances the fixer can be re-used and it isn't harmed at all by the process, but the build up of iodide from film emulsions limits the reuse. If the plating is done at too high an ampage then yes sulphurisation takes place, commercial electrolytic recovery units have circuitry to control the voltage & ampage to prevent this.
Some re-circulating units recover the silver electrolytically from Fixer or Bleach Fix on continuous process machines, allowing slightly more efficient use if the chemistry, however the sophistication of the control circuitry is far higher than for standard plating units, because as Ron (PE) says fixers and particularly bleach fix can be damaged, these units control the voltage and ampage to give a slower rate of recovery.
On an industrial scale spent fixer or bleach-fix is desilvered electrolytically and often used again and again to recover silver from old X-ray & Graphic Arts films. So often when fix solutions are sent for recycling they get used again before final treatment & disposal.
To have a any problem with Hydrogen & Oxygen liberation you would need a very large commercial rectifier to generate the high ampage required, commercial units are designed to cut out well before that point can be reached, as by then the plating process would have broken down anyway.
Small scale recovery from a litre or two of fix using a battery system would be very safe and easy, I would only advocate using a purpose built mains unit. The secret to good plating is to keep the ampage low and allow the silver to plate out slowly, there's plenty written about plating on other websites.
The electrochemical potential for the reduction of silver ion to silver is around 0.8V. So if you were to use a battery, or wall-wart style DC source, it would be best to arrange a series of cells in series to regulate the voltage in each. If you cannot get hold of silver wire, or sheet, to form the anode, you could use stainless steel (ie. any old unused stainless steel knife blades) in its place. If I remember correctly, silver plated on to stainless steel does not form a very strong bond, and can be removed by flexing the blade, or scraping off.
The gasses evolved using too high a voltage or amperage will contain dissociation products of the thiosulphate anion, which will be hydrogen sulphide - smelly, poisonous, and dangerous to unexposed photographic emulsions. Indeed this will probably take place in preference to the dissociation of water to form hydrogen and oxygen (though I would have to drag out the Rubber Handbook, and brush up on my electrochemistry to prove that one way or the other). :P
It has been too long. Now you are refreshing my memory. You have to control the voltage precisely to get silver without harming the fix, otherwise overvoltage causes the hypo to decompose. Also, with acidic fixes, the fix is probably going bad anyhow from oxidation by the time it is so well used that there is enough silver to warrant reclamation.
This is why I seem to remember that it is suggested that the fix be discarded after the silver is reclaimed.
Could anyone specify what levels of voltage are safe for not gassing out bad things. I do not seek to reuse fix. My interests are, first to make the spent fix safely disposable, and second, to not waste silver.