How does foil, baking soda and hot water clean silver?

[WarmNPrickly]

@Hari_Seldon

Basically, silver is less reactive than aluminum. I am not sure of the valences involved, but the primary reaction is that silver sulfide + aluminum —> silver + aluminum sulfide. I see no reason why it wouldn’t also reduce silver oxide. I would assume (but I might be wrong about this) that the main function of the bicarbonate is to increase electrical conductivity, but I guess there is some reaction with it as there is some hydrogen sulfide being given off.

Yes, it does produce a matte finish. But you don’t lose the silver. For solid silver, this might not matter too much, but if you keep polishing silver plate you will eventually wear it down to the base metal.

The sodium bicarbonate solubilizes the oxide layer on the aluminum. Otherwise, the aluminum remains passive. You could try it with sodium chloride, but I suspect it wont work.

I actually think that most of the sulfur goes to hydrogen sulfide. The aluminum will react with the water very quickly once it’s exposed and there just isn’t all that much sulfur there.

I suspect that the matte finish comes fom the fact that the silver sulfide had a diffeent crystal structure from silver metal. This means that the atoms had to rearrange once to turn to silver sulfide and then again to go back to silver metal. An SEM of the surface might be very interesting.

[Mangetout]

I would love to see some microscope images (of polished silver alone would be fascinating)

[richs_1]

The tarnish is not oxidation of the silver. It is a black compound formed by the reaction of silver with sulfur in the air.
The cleaning method using hot water, baking soda, and aluminum foil causes an electrochemical reaction between the aluminum and the silver sulfide (the tarnish). The aluminum is much more reactive than the silver, and “steals” the sulfur from the tarnish compound, reducing it to pure silver again:
3Ag2S + 2Al => 6Ag + 2 Al2S3.
In effect, the sulfur is transferred from the silver to the aluminum. No H2S gas (which has a rotten egg smell) is produced. All of the silver remains on the object.
In the process a small electric current passes between the siiver and the foil, which facilitates the reaction. That is why it is called an “electro-” chemical reaction.
The aluminum sulfide is left as a coating on the foil, but as it does not adhere well, it may appear as small flakes (generally yellow in color.
I hope this helps explain the process. It works very well.

[Colombo]

I agree with the above chemistry, but Aluminum Sulfide decomposes in water to form Aluminum oxides and hydroxides, and Hydrogen Sulfide gas. The aluminum and silver, with the electrolye between form a battery. When you touch part of the silver to the aluminum, you short the battery and current flows, allowing the reaction to proceed.

[DSYoungEsq]

Zombie thread.

[Some_Call_Me.Tim]

@richs

The tarnish is not oxidation of the silver. It is a black compound formed by the reaction of silver with sulfur in the air.
The cleaning method using hot water, baking soda, and aluminum foil causes an electrochemical reaction between the aluminum and the silver sulfide (the tarnish). The aluminum is much more reactive than the silver, and “steals” the sulfur from the tarnish compound, reducing it to pure silver again:
3Ag2S + 2Al => 6Ag + 2 Al2S3.
In effect, the sulfur is transferred from the silver to the aluminum. No H2S gas (which has a rotten egg smell) is produced. All of the silver remains on the object.
In the process a small electric current passes between the siiver and the foil, which facilitates the reaction. That is why it is called an “electro-” chemical reaction.
The aluminum sulfide is left as a coating on the foil, but as it does not adhere well, it may appear as small flakes (generally yellow in color.
I hope this helps explain the process. It works very well.

By the way, “oxidation” of silver in chemistry refers to any process that removes electrons from the silver- it’s not strictly restricted to reactions with the element oxygen.

So, if sulfur reacts with silver metal to form AgS, the silver is being oxidized by the sulfur. I realize this is confusing, but that’s what the word oxidation means in this context.