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I know that while electrolysing a solution of brine, if we use mercury cathode, we get sodium amalgam instead of having hydrogen gas liberated. Even the Castner-Kellner process uses this to produce sodium hydroxide.

However, wouldn't the mercury electrodes be used to produce commercial elemental sodium? I feel it's much easier to electrolyse aqueous NaCl than to melt NaCl. Then why don't they use aqueous NaCl decomposition with mercury cathode to get Na metal?

Edit: Its seems even more topsy-turvy now. Regular electrolysis of Brine itself produces NaOH, then why bother with the mercury?

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    $\begingroup$ Perhaps because the production of Na is a small subset of the production of Cl, so the process is optimized for the latter, not the former. $\endgroup$
    – Jon Custer
    Apr 13, 2017 at 11:38
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    $\begingroup$ Two thoughts: 1. Are you trying to make elemental sodium in the presence of water? 2. Hg(Na) amalgam is not the same thing as elemental sodium. $\endgroup$
    – Zhe
    Apr 13, 2017 at 13:44
  • $\begingroup$ @Zhe Yes Sodium amalgam isn't the same as sodium, but I'm sure that it can be separated from mercury, since mercury boils easily, unlike sodium. I think sodium amalgam is more stable in water, and it can be isolated $\endgroup$ Apr 13, 2017 at 14:58
  • $\begingroup$ Are you going to distill it? Once the mixture is highly concentrated in sodium, the distillation of the remaining mercury is probably non-trivial. $\endgroup$
    – Zhe
    Apr 13, 2017 at 15:29
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    $\begingroup$ Elemental mercury is a liquid, but most other forms, including amalgams, are not. Also, although elemental Hg is a liquid, it has a relatively low vapor pressure / high boiling point (357C) (that doesn't mean it can't be distilled off, it's just not as easy as you might think). $\endgroup$
    – airhuff
    Apr 13, 2017 at 18:18

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Among other things, sodium amalgam is not really sodium dissolved in mercury in the manner that an aqueous sugar solution is sugar dissolved in water. Evaporate the water from the sugar solution and you get back the sugar, but sodium amalgam forms intermetallic compounds instead.

Deiseroth et al.[1] give a review; even in the 1990s several such compounds were known:

A summary of recent results concerning the binary system NaHg is given. Characterized by six thermodynamically stable solids: $\ce{NaHg4, NaHg2, NaHg (α,β,γ), Na3Hg2, Na8Hg3 (α, β, γ)}$ and $\ce{Na3Hg(α,β)}$, their crysta[l] structures could be solved by single crystal X-ray studies, except those of $\ce{NaHg4}$ and $\ce{β-NaHg}$.

So, attempting to purify the amalgam by the usual physical means will give intermetallic compounds instead of elemental sodium. Sodium is but one of many metals whose amalgams with mercury form various intermetallic compounds, and so to get the pure metal you need to use a product other than the amalgam.

Reference

  1. Hans-Jörg Deiseroth, Erik Biehl, Matthias Rochnia (1997). "Sodium amalgams: phase diagram, structural chemistry and thermodynamic data, a summary of recent developments", Journal of Alloys and Compounds, 246, 80-90, ISSN 0925-8388, https://doi.org/10.1016/S0925-8388(96)02460-7.
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    $\begingroup$ Also, of course, a process that avoids the use of mercury would minimise the emissions of mercury. There is also a reason why non mercury-using alternatives to the Castner Kellner process were developed. $\endgroup$
    – matt_black
    Sep 29, 2023 at 23:18

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