# Does silver oxide react with hydrogen sulfide?

Electronic connectors are often silver plated. However, silver tarnishes fairly quickly and heavily. There exists a widespread misconception that the tarnishing of silver contacts is harmless, because silver oxide has about the same conductivity as silver itself. The problem however is that silver does not oxidize under normal conditions. The tarnish on the contacts is not silver oxide, but silver sulfide that develops due to the presence of some hydrogen sulfide in the air. Unlike silver oxide, silver sulfide is not a conductor, but a semiconductor with various potential adverse effects for the connection.

I have come across a reference that suggests oxidizing silver contacts before using them in order to prevent the development of the silver sulfide layer. This implies that silver oxide does not react with hydrogen sulfide in the air under normal conditions. Is this claim correct?

EDITS

I have corrected the typo by changing "sulfur dioxide" to "hydrogen sulfide" in the title and body of the question. Thanks for pointing this out in the answer!

Silver sulfide is formed by Ag and hydrogen sulfide not sulfur dioxide. You need reducing conditions where the latter can be reduced to

$$\ce{SO2 +reduction -> H2S}$$

Actually, sulfur dioxide chemisorbs on ultraclean silver surface, however heating can remove it. So this is reversible sorption, as suggested by Lassiter [1].

Just note that Auger electron spectroscopy is done under extremely clean environment. There is no trace of water, oxygen or any other component! Real atmosphere is far more complicated and tons of photochemical reactions occur in the atmosphere. A typical indoor air has plenty of undesirable components. How does $$\ce{SO2}$$ react with Ag must be another story because we cannot avoid or control other factors.

Coming to the second part of the query: If we have surface layer of silver oxide, will it prevent sulfide formation. In principle, possibly yes, because $$\ce{Ag2O}$$ is decent oxidizing agent. The moment traces of $$\ce{H2S}$$ come in contact with the oxide, it will reduce the oxide to elemental silver. This is my personal speculation.

As per Franey et al. [2]:

Polycrystalline silver has been exposed to the atmospheric gases $$\ce{H2S}$$, $$\ce{OCS}$$, $$\ce{CS2}$$ and $$\ce{SO2}$$ in humidified air under carefully controlled laboratory conditions. $$\ce{OCS}$$ is shown to be an active corrodant while $$\ce{CS2}$$ is quite inactive. At room temperature, the rates of sulfidation by $$\ce{H2S}$$ and $$\ce{OCS}$$ are comparable, and are more than an order of magnitude greater than those of $$\ce{CS2}$$ and $$\ce{SO2}$$. It appears that $$\ce{OCS}$$ is the principal cause of atmospheric sulfidation of silver except near sources of $$\ce{H2S}$$ where high concentrations may render the latter gas important. At constant absolute humidity, the sulfidation rate of silver by both H2S and OCS decreases from 20 to 40 °C and then increases to 40 to 80 °C.

So may hydrogen sulfide may not be a major culprit!

### References

1. Lassiter, W. S. Interaction of Sulfur Dioxide and Carbon Dioxide with Clean Silver in Ultrahigh Vacuum. J. Phys. Chem. 1972, 76 (9), 1289–1292. https://doi.org/10.1021/j100653a011.
2. Franey, J. P.; Kammlott, G. W.; Graedel, T. E. The Corrosion of Silver by Atmospheric Sulfurous Gases. Corrosion Science 1985, 25 (2), 133–143. https://doi.org/10.1016/0010-938X(85)90104-0.
• Thanks for the correction, yes of course, hydrogen sulfide is exactly what I meant :) Please forget about sulfur dioxide, it was just a silly typo, sorry. I will edit the question momentarily. So, does silver oxide react with hydrogen sulfide? I am asking for a very practical purpose. – safesphere Apr 20 at 20:56
• Yes it does, almost instantaneously, especially if traces of water are present. Add a new paragraph in your question with a heading EDITS. – M. Farooq Apr 20 at 20:58
• Added, thanks! So the claim is wrong. Oxidizing silver contacts does not protect from silver sulfide developing on top (or in place) of silver oxide. Did I get this right? Thank you! – safesphere Apr 20 at 21:07
• Interesting, does OCS have a name? Also, this is a bit unclear to me: "*The moment traces of $\ce{H2S}$ come in contact with the oxide, it will reduce the oxide to elemental silver.*\$ - By oxidizing what, hydrogen or sulfur or both? I mean, would this not produce water and silver sulfide? (I've no idea, just asking.) so, even in the best case silver oxide would be only a temporary protection until it's all consumed. – safesphere Apr 20 at 22:34
• According to Wikipedia carbonyl sulfide is the most abundant carbon sulfur compound in the atmosphere (ppb levels). Typically when hydrogen sulfide is oxidized, the product is elemental sulfur. You will have to read the ref 2 get more details. Silver oxide is an oxidizing agent, hydrogen sulfide is a reducing agent. Again this is speculation. What happens in reality esp. in the presence of light or moisture! – M. Farooq Apr 21 at 1:28

When you mate connectors there is mechanical abrasion. Thus electronic connectors with silver contacts would not be resistant to silver sulfide formation because of this mechanical abrasion which would remove the silver oxide coating. That is why gold plating is often used on higher quality connectors. Gold is resistant to tarnishing of any sort and the gold layer is tick enough to resist mechanical abrasion for a significant number of matings.

• Thanks for your answer. I did not want to overcomplicate the question, but a related issue is tarnishing of a bare silver wire introducing undesirable skin effects of semiconductive silver sulfide. – safesphere Apr 20 at 22:19