Long John Silver

I wanted to explore oxidation, reduction, and electrochemistry a bit more. Luckily, I stumbled across an old tarnished silver ring that my wife needed cleaning. Silver tarnish is a coating of the mineral acanthite, a grey-black compound of Ag2S. It forms on silver over time by the following equation:

    4Ag + 2H2S + O2 -> 2Ag2S + 2H2O 

As a minor note of interest, there is more H2S in the environment (such as your home) than you might think. Eggs are a source, which is why you don't use silverware to eat eggs. In any case, this ring is over 80 years old and was quite tarnished. I didn't quite realize how tarnished it was, but more on that later. Here is a picture:

Anyway, we need to reduce the silver. Ag2S contains two silver atoms that have lost one electron each (oxidized). The silver therefore needs to be reduced, adding the electrons back so that it will let go other the annoying sulfur ions. 

    Ag2S + 2e- -> 2Ag + S2-

It just so happens that the standard reduction potential of this half-reaction is 0.69V. So we will use electrochemistry with a reduction potential above this voltage to reduce the silver. 

It just so happens that zinc has a reduction potential of 0.77V. So, in theory, I can melt enough of a penny to expose its zinc core and put that in a solution with various ions. We then place the ring in conductive contact with the zinc so the electrons can flow into the acanthite. For style points, I attached a copper wire to the half-melted penny and placed the ring onto the wire. The copper is not strong enough (0.34V) to reduce the acanthite but will conduct the electrons from the zinc to the acanthite. It would have been simpler to just melt the zinc core out and place the ring on that, but I wanted to prove the conduction theory.

The electrolyte solution I used was 100ml of water with 10g of salt (NaCl) and 5g of baking soda (NaHCO3). It really doesn't matter which ions are used in the solution. In this case, the cations are all Na+. The anions are a mix of Cl- and HCO3-. This "recipe" is common on the internet but I can't see what difference it makes personally. It should work with just one or the other salts in theory.

Here is what the setup looked like:     

After a few minutes, not much appeared to be happening. I used a voltmeter to make sure electrons were flowing into the ring. They were. After a while, I decided it was working - just really slowly. I removed the ring at this point:

Aluminum has a bit more reduction potential, 1.66V. So I put some scrap aluminum foil in the bottom of the solution instead of the zinc/copper and laid the ring onto the foil:

Again it seemed to be working but not quite fast enough for my desires. 

So I decided to just pump a massive number of electrons into the ring with an actual battery. I took a 9V and attached the anode to the aluminum foil. For the cathode, I attached it to some steel tweezers. That was an unfortunate choice of cathode, but more later on that. I just needed to conduct electricity into the solution.

As you can see in this video, the reaction is vigorous but still takes some time. The sulfur ions released kept clouding my solution with yellowish material bubbling to the top. Perhaps this was sodium sulfide, as both cations in the solution were Na+. In any case, I had to keep replacing the solution. Perhaps I didn't *have* to, but I wanted to see the progress.

In the end, I decided not to completely clean the ring. It was looking pretty good and had a nice antique-looking finish with tarnish mostly in crevices. Plus I had already proven the chemistry and would need even more solution and time and to continue. Here it is:

In retrospect, that ring was very tarnished and not suitable for a simple bath solution without external current applied. Unfortunately I didn't have anything lightly tarnished that I was willing to test. Some antiques and coins can lose value if you clean them.

I mentioned a mistake of using tweezers in the solution. While they were a cheap pair, I more or less ruined them. The sulfur ions adhered themselves to the tweezers as it gave off electrons from the iron. I ended up with what appears to be a yellow-green iron (iii) sulfide (Fe2S3) coating on my tweezers. This material was very hard and I had trouble removing it with a steel file. I consider the tweezers mostly ruined for chemical use and/or aesthetics now. I should have just used a bit of copper wire and then throw that away. 

Thanks for reading,

Paul