I often hear that water gets purified by being in a silver vessel, which sounds plausible because of bactericidal feature of silver. What doesn't sound plausible, though, is the way it's explained: that silver releases ions into the water. Since silver is a noble metal, why would any "reaction" at all occur with something as neutral as water? Is the above explanation nonsense? Does the "disinfection" of water happen only on contact with the metal?
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3$\begingroup$ Silver forms many compounds with little effort (think silver nitrate, or even just tarnishing in air). Why do you assume it is not reactive? $\endgroup$– soandosCommented Apr 25, 2012 at 19:09
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$\begingroup$ I thought so because it is a noble metal $\endgroup$– EvgeniCommented Apr 26, 2012 at 14:33
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3$\begingroup$ This is a typical example for beginners "black or white" thinking. Noble metals are more or less noble. Have a look in Nernst's equation. $\endgroup$– GeorgCommented Apr 26, 2012 at 19:56
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3 Answers
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Silver is not as inert as gold. Tarnish is the name we give to the phenomenon when silver metal is oxidized and becomes a salt. Surfaces made of silver tend to disinfect themselves pretty quickly. As for disinfecting water poured into a silver cup, I imagine that would take a little longer since you have to wait for silver to diffuse away from the surface and into the solution. But even very trace levels of silver can have strong antimicrobial effects.
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$\begingroup$ This doesn't explain the bactericidal property of silver. $\endgroup$– lesderidCommented Apr 25, 2012 at 19:56
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14$\begingroup$ That is not the question that is being asked. The question seems to accept that property of silver as a given, but then asks how the silver does that if it is inert. This answer simply corrects the fallacy that the OP made with regard to the inertness of silver. $\endgroup$– soandosCommented Apr 25, 2012 at 21:07
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1$\begingroup$ It should also be noted that at one point in time, there were quite a lot of silver-based antiseptics in use (e.g. Argyrol, a complex of colloidal silver in egg protein). $\endgroup$– user95Commented Apr 26, 2012 at 6:27
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$\begingroup$ What does diffuse into water? Ions or "stable" atoms? $\endgroup$– EvgeniCommented Apr 26, 2012 at 14:40
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1$\begingroup$ In an oxygenated environment, Ag+ ions are actually more stable than silver metal. The reason why silver can exist as metal at all is because the oxidation reaction is very slow for a solid piece of metal, even though tarnish is thermodynamically favorable. So Ag+ would do the diffusing, not Ag(0). $\endgroup$– PatCommented Apr 26, 2012 at 16:29
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As far as I know, a chunk of solid silver will not spontaneously react with water. But if you pass an electrical current through silver electrodes immersed in water, the silver will be oxidized according to the following equation:
$$\ce{2H2O(l) + 2Ag(s) -> 2Ag+(aq) + H2(g) + OH- (aq)}\qquad E^\circ=-1.63\ \mathrm V$$
That will get you the ions you need. Incidentally, Russia's MIR space station and the ISS use electrolytically dissolved silver ions to purify drinking water.
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2$\begingroup$ They use the elecrolytically dissolved silver solution for water purification (though I'm not sure if they use water or another oxidizing agent). $\endgroup$ Commented Apr 25, 2012 at 19:26
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As to the above answers I also want to include the mechanism of action of silver as an antimicrobial agent. The exact mechanism of action of silver as an antimicrobial agent is not known and the current hypothesis is silver will converted to silver ions and this positively charged ions will attack the cell membrane, DNA or proteins which are negatively charged thus inhibiting the growth of microorganisms.
See also this link.
