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In a class of electrolysis, my instructor told me that Hg forms Na-Hg in the electrolysis of dilute NaCl aqueous solution. For this reason, sodium cations are reduced in the cathode instead of hydronium ions. However, he did not give me any reason for this whatsoever . So, I would like to know why Hg creates amalgam whereas other metals like Pt don't form them ?

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    $\begingroup$ It has to something with the hydrogen overpotential of Hg electrode. $\endgroup$ – M. Farooq May 17 at 23:21
  • $\begingroup$ Well,what do you mean by "hydrogen overpotential" ?And what has it got to do with Hg? $\endgroup$ – Habib May 17 at 23:31
  • $\begingroup$ Umm, proper question would be rather why Na is reduced with Hg electrode and is this even really the case. While Na might perhaps be reduced also with other electrodes, it obviously won't become part of amalgamate, as only solutions in Hg are called like that. $\endgroup$ – Mithoron May 18 at 0:13
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    $\begingroup$ Please see “Castner–Kellner process” in wikipedia. Definitely sodium can be electrolyzed from brine. The reason it works is the overpotential for hydrogen reduction at a mercury cathode. In other words, there is a kinetic hindrance to hydrogen reduction and sodium forms an alloy with mercury, so back-reaction with the aqueous solution is impeded. Traditionally, mercury alloys with other metals are called amalgams. Liquid gallium, for example, also readily forms alloys with many metals, but gallium has not been known since ancient times. $\endgroup$ – Ed V May 18 at 0:24
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    $\begingroup$ In the case of sodium ion reduction at the liquid mercury cathode, the sodium atoms get “dissolved” in the liquid mercury. So they move around like in a ordinary solution. But this keeps them from contacting the brine, so they do not get to react with the brine. Those at the interface of brine and mercury can so react, producing hydrogen gas and NaOH. For platinum, for example, the sodium would, at best, form a plating: the atoms could not get sheltered, as it were, by getting into the interior of the platinum. So it is a matter of getting the sodium away from the brine and mercury does it. $\endgroup$ – Ed V May 18 at 0:56
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If you heat metallic sodium with let’s say gold / silver / mercury / platinum etc.. it will form an amalgam.

I have tried this with a tiny bit of gold some years ago.

I did heat Na in an inert argon atmosphere to several hundred degrees in an quartz tube and drop the gold inside.

It did take only fractions of a second till the amalgam was formed.

The result is a green looking amalgam when it is hot - when it cools down it is a brown solid substance.

Given the fact that gold is a pretty inert substance, I guess that sodium will do the same with Pt, Rh, Ir, Ru, Os, Ag, Hg ...

I guess during electrolysis you have metallic Na formed. I don’t mean junks of Na but Na atoms and some of them may live long enough to amalgamate with the Hg.

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    $\begingroup$ Not every metal amalgamates. See chemistry.stackexchange.com/q/117633/79678. $\endgroup$ – Ed V Jun 23 at 15:02
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    $\begingroup$ The question was not about Na amalgam nor amalgams in general, but specifically why it or metallic Na is formed at all during NaCl electrolysis with Hg as the cathode. And yes, other metal than mercury obviously cannot form amalgams as they are not mercury. $\endgroup$ – Poutnik Jun 23 at 15:11

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