I recently gave a public lecture on my campus in which (among other things) I discussed the chemistry of metallic copper, silver, and gold. During the talk I conducted a chemistry demonstration very similar to the Journal of Chemical Education’s Classroom Activity #25: Silver to Black – and Back.1 Through the process of conducting this experiment several times, I learned quite a bit about the chemistry of silver. The things I learned touch on several topics in chemistry that might be of interest to your students. If you’d like to skip my musings on this subject and simply watch videos of the demonstration, scroll down to the videos at the bottom of this post.
The main focus of JCE Classroom Activity #25 is the demonstration of the formation and removal of tarnish from items made of silver. Silver tarnish results from the formation of silver sulfide from the reaction between silver, oxygen, and hydrogen sulfide:2,3
2 Ag(s) + ½ O2(g) + H2S(g) --> Ag2S(s) + H2O(l).
Hydrogen sulfide is released into the atmosphere from sewers, factories, and farms. Hydrogen sulfide can also form from the reaction between carbonyl sulfide and water:
OCS(g) + H2O(l) --> H2S(g) + CO2(g)
Carbonyl sulfide is expelled into the atmosphere from volcanoes and the ocean. Both hydrogen sulfide and carbonyl sulfide are considered to be particularly effective in forming tarnish on silver.2,3
Fortunately, it is relatively easy to remove tarnish from silver, because several metals are more reactive than silver. It is easy to see the metals that are reactive than silver by inspecting the metal activity series (Figure 1). Any metal higher than silver on the series is more reactive, is more easily oxidized, and is more likely to react with sulfide ions than silver.
Figure 1: Metal activity series. Because magnesium and aluminum are higher in the series than silver, both of these metals are more likely to bind with sulfide ions than silver.
Thus, by treating tarnished silver with aluminum foil, the chemical transfer of sulfide ions from the tarnished silver to the aluminum foil takes place:
3 Ag2S(s) + 2 Al(s) --> Al2S3(s) + 6 Ag(s)
Because I wanted to see this reaction take place for myself, I bought several tarnished silver items on eBay.4 As I experimented with the removal of tarnish using aluminum, I noted that some tarnish was particularly resistant to removal with aluminum. Seeing that magnesium is higher than aluminum on the activity series, I decided to try to use magnesium as a tarnish remover:
Ag2S(s) + Mg(s) --> MgS(s) + 2 Ag(s)
Sure enough, because magnesium is more easily oxidized than aluminum, I found it to be particularly good at removing the tarnish from silver. I’m not the first one to have this idea. Apparently a product called “Maggie Pan”, which is no longer sold, used magnesium as a silver cleaner.5
This experiment – the removal of tarnish with aluminum foil or magnesium – is quite easy to accomplish. While it does take a bit of time, the results are impressive and I think your students will enjoy seeing the process take place. Check it out in the video below:
Once you have chemically removed the tarnish from your silver item, you might want to conduct this experiment again for another group of students. However, the formation of tarnish on silver from gases in the atmosphere usually takes several years. In JCE Classroom Activity #25, it is suggested that egg yolk, mayonnaise, powdered sulfur, and mustard applied to silver will cause it to tarnish in about 24 hours. While testing out these suggestions, I quite by accident discovered a way to form tarnish on a silver item in less than one hour.6 You can see this process of discovery unfold in the video below:
I find it interesting to note that in JCE Classroom Activity #25, it is claimed that the yolk of an egg in contact with a silver item will cause the silver to tarnish. Therefore, I was surprised to find that a hot egg white did the trick in less than an hour.7 I wish I had monitored the experiment more closely to try to observe how quickly the tarnish formed. Because I set the experiment aside and didn’t check on it until an hour later, I don’t know how long it took for the hot, hard-boiled egg white to tarnish the silver plate. Did it take less than 30 minutes? What about less than 10 minutes? I don’t know the answer to this question! If you get the chance, maybe you or your students could repeat this experiment and report back how quickly tarnish forms on a silver item through treatment with a hot, hard-boiled egg white.
4. Surprisingly, I purchased all silver items for less than $10 each. It appears that people are quite willing to sell tarnished silver items for substantially less than untarnished silver.
5. Google “Maggie Pan silver cleaner” and you might find this vintage product on sale on Ebay or elsewhere online!
6. Do not attempt to add tarnish to your fine silver for these experiments. In some experiments I conducted I was not able to remove purposely added tarnish – even when using magnesium. This mostly occured when I used powdered sulfur to tarnish the silver items.
7. It makes sense to me that the egg white would form tarnish. Egg white is primarily made of protein, and the sulfur containing amino acids cysteine and methionine are found ubiquitously in proteins.