[hrwise89]

I'm working on a self-directed project for my general chemistry class at community college. I wanted to explore the process of purifying silver. My goal was to take sterling silver and refine it to 99% pure silver. I took a 9.45 gram sterling silver bracelet and dissolved it in about 35mL of concentrated nitric acid. I then diluted the solution to about 500mL. To this I added excess copper wire, totaling about 25 grams. I let the silver fall out of solution and recovered the solid silver by decanting and filtering the copper nitrate. I had excess copper wire that I then had to pick out of the solid silver once it was dry. Now I have powdered solid silver. I would like to take this powdered silver and refine it further, by plating it onto a silver cathode that is already of high purity.

There are a few finer points in this next step that I’m unsure of. I will lay out what I think I should do, and if you see any glaring issues, please let me know.

I plan to take this impure powdered silver, and melt it down into a bar. I’ve already melted some down into a little button just to be sure that it was possible for me to do that with the equipment I have. I will then take this impure silver and attach it to positive side of a power supply. I will take my pure silver and attach it to the negative side of a power supply. I will suspend my cathode in a beaker of 1M silver nitrate. I will also place my impure silver anode in the silver nitrate solution, but it will be placed in a filter so that any of the copper impurities that found their way into my anode will not fall into the beaker. The silver ions in solution will build up on my cathode, and the silver in my anode will dissolve to replace the silver ions in solution. I should get a build-up of at least 99% pure silver on my cathode.

Does this make sense? I’ve had a hard time finding verifiable sources for any of this, as industrial silver electrowinning relies on cyanide.

Now for some questions:
How do I know how much power to put through my cell? Is there some way I can calculate this on my own? I’ve seen other people (on youtube, so I’m not sure where their calculations from) use between 3.1 and 3.5 Volts, which flows at about 1.1 amps through the cell.

How important is the molarity of the silver nitrate solution? I assume it has some relevance to how quickly the silver ions will plate onto the cathode, but I don’t know the relation.

As for my level of knowledge, if it helps you help me, I am almost done with year one chemistry. I selected this project at the beginning of the term because I assumed I would know more about electrochemistry than I do by now. It seems most I’ve what I’ve learned has been more useful for determining how many volts a cell will produce, and if a reaction with proceed spontaneously etc. I have a pretty strong qualitative understanding of oxidation and reduction, and the mechanism by which this works. I don’t, however, have a quantitative understanding of this reaction. If you help me, or point me in the direction of a resource that I can use to teach myself, I’d be very grateful.

Just a quick edit here: The stated goal of my project is to demonstrate that you can purify silver through electrolysis.

[billnotgatez]

Just wondering how you are going to prove how pure the silver is now and will be in the future?

[Borek]

Why not try to remove the silver by electrolysis from the first solution?

Required potential is a sum of the one described by the Nernst equation, ohmic drop on the solution and kinetic component (if the reaction is slow, you need to apply higher potential to speed it up; unfortunately, it may mean other, interfering reactions kick in). The only sure way of selecting the right potential and the right current density (that is, combination that produces best quality silver) is through the experiment.

You will need Faraday's law of electrolysis to estimate what charge has to flow through the solution to reduce all silver you have.

[Enthalpy]

I wouldn't go the chemical route at all.

Copper and silver have very different vapour pressures, so just distill the alloy under low pressure. This takes less energy, is clean, etc - I see only advantages. By the way, several metals are produced that way, like magnesium is separated from silicon.