I saw that a gold ring decolourised as it got in contact with mercury . Why does this happen ? Is there any way to reverse this?
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6$\begingroup$ en.wikipedia.org/wiki/Amalgam_(chemistry) :3 $\endgroup$– paracetamolCommented Jun 25, 2017 at 8:11
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4$\begingroup$ If the ring was "decolourised" by mercury (amalgamating with the gold) then 1) The ring is gold plated, and 2) No, there's no simple way to "put back" the gold on the ring. O:) $\endgroup$– paracetamolCommented Jun 25, 2017 at 8:22
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2$\begingroup$ Can't I use nitric acid? Won't it atleast remove the grey colour ? The ring is actually gold and not gold plated. $\endgroup$– Chitra LekhaCommented Jun 25, 2017 at 9:18
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2$\begingroup$ @paracetamol I'm not sure why you think this is a sign the ring is gold-plated: mercury will damage any gold surface plated or not. $\endgroup$– matt_blackCommented Jun 26, 2017 at 8:19
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$\begingroup$ @matt That the amalgam would stick to the surface of the ring slipped my mind >_<. Hence I thought "discoloration of ring" = "Part of the gold surface is lost" :P $\endgroup$– paracetamolCommented Jun 26, 2017 at 16:43
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4 Answers
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The grey colour is an amalgam of mercury and gold.
Mercury forms amalgams with many other metals. Some are used as chemical reagents in laboratory chemistry as they have different properties than the original metals involved. Gold amalgam is much greyer than gold. Silver amalgam has been used in dentistry.
Mercury has been used in the extraction of gold in mining as it can extract gold from low grade ore. But the recovery process involves distilling off the mercury which is dangerous and polluting.
Chemists used to work with mercury a lot and, when they did, they were recommended to remove their wedding rings or other gold jewellery as the mercury would damage the gold.
Once the amalgam is formed there is no easy way to reverse the process short of resmelting the ring. If the contact with mercury has been brief, you might be able to polish it to remove the amalgam layer, but this won't be easy.
answered Jun 25, 2017 at 9:58
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$\begingroup$ Does immersing in acids or heating upto 350 degrees in help anyway?like mercury alone vaporises and gol remains intact. $\endgroup$ Commented Jun 25, 2017 at 10:05
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8$\begingroup$ @Chit "Does immersing in acids or heating upto 350 degrees in help anyway?" Uh...are you sure you want your ring back in one piece? ;) $\endgroup$ Commented Jun 25, 2017 at 10:10
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$\begingroup$ As gold does not melt at that temperature, why can't this method be used. Would it make the ring brittle? $\endgroup$ Commented Jun 25, 2017 at 10:12
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29$\begingroup$ If you're working with mercury, and your wedding ring is vulnerable, then you have a bigger problem than your wedding ring—you're working with mercury using your bare hands! $\endgroup$ Commented Jun 25, 2017 at 13:19
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4$\begingroup$ I'm no expert in metallic toxicology, but I'm pretty sure the claim that you could drink liquid mercury "without much harm" is completely bogus. It is true that elemental liquid mercury is less toxic than other forms, and that vapor represents the largest risk, but that hardly means it is "safe". (Plus, the liquid form does vaporize at room temp.) I don't know if I'm just unusual, but it is rare that my hands are completely free of cuts, so even if absorption through intact skin is very slow, I'd say the typical lab worker is still putting herself at undue risk for not taking precautions. $\endgroup$ Commented Jun 30, 2017 at 9:47
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Once when I was doing an experiment I had the same experience; my ring was decolourised. I was afraid at first but we could reverse this process to achieve the gold colour once again.
There is no reaction between $\ce{Au}$ and $\ce{Hg}$; they form a mixture similar to dissolving $\ce{NaCl}$ in water. When the gold and mercury are mixed they make an amalgam (this process is called amalgamation). It is also used to allocate pure gold by colluvies.
Before I said that there is no reaction between gold and mercury, therefore there is an easy way to allocate the gold again. We know that the boiling point of gold is higher than that of mercury ($\ce{Hg}$: $\pu{356.7 ^\circ C}$ and $\ce{Au}$: $\pu{2700 ^\circ C}$). Therefore when you heat the amalgam the mercury will boil off and the gold surface will be restored. But don't do this in the house, give it to a professional goldsmith. (After my experience, I went to a gold shop, they heated my ring, and it became just like it was before.)
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$\begingroup$ Actually mercury was used for volume measurement using displacement principle. If it were done in a muffle furnace would it work (I don't have the idea of trying it out , asked just out of curiosity) $\endgroup$ Commented Jun 25, 2017 at 16:38
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$\begingroup$ did they just heat it to bring it back. After that was the layer smooth and still intact. $\endgroup$ Commented Jun 25, 2017 at 17:16
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1$\begingroup$ It is a little inaccurate to describe amalgamation as the same as salvation: it is a little more complex than that. $\endgroup$ Commented Jun 25, 2017 at 19:41
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3$\begingroup$ The boiling point of gold is irrelevant, here. If it //melts// your ring is lost already. Hopefully, 1064.18 °C for melting gold (wikipedia) is still far beyond the 356°C for boiling mercury. Not that it makes it a better idea to do at home. $\endgroup$– PypeBrosCommented Jun 26, 2017 at 12:59
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Mercury is a stable element . It has the ability to amalgamate most metals. Whenever the grey alloy is heated, based on their boiling point with mercury having the smaller, it will boil first leaving the precious metal, gold behind.
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The gold surface is simply "wet" by mercury and the gold is not decolorized. A tiny amount of mercury can quickly swarm across a gold surface making it appear that the color has changed almost instantly but the effect is superficial. When mercury was a common plaything, people would often coat silver coins with mercury. Mercury has the same effect on silver as it does on gold and the coins would become super shiny with a slick feel. After a few days, the mercury would either evaporate or oxidize leaving the coins badly tarnished. This is why it was common to find strangely dark looking silver coins in circulation. Mercury will leave the surface of a gold object in a few weeks, if not days, if left alone and the gold surface will return to normal. Heating the gold speeds the process but the gold is unharmed.
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6$\begingroup$ Nope, mercury doesn't just coat the surface, it forms an amalgam and that process isn't easy to reverse. $\endgroup$ Commented Jun 25, 2017 at 19:42
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$\begingroup$ Thanks all for your active responses, actually the contact of the ring with mercury was very less. And it was just the top layer coated by amalgam . $\endgroup$ Commented Jun 26, 2017 at 11:23
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$\begingroup$ Because the ring is starting to get back its original color after three full days. Actually no attempt was made to remove the layer like heating or dipping in acid. But when I saw the ring after 3 days(the ring is not mine) the layer was gone mostly.And I beleive if I leave it for some more days it will naturally get back its own color and lustre(doubtful about lustre) $\endgroup$ Commented Jun 26, 2017 at 11:32