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'Mercury' was included in one of the possible answers for this exam question:

Name a metal that an inert electrode could be made from.

My answer was 'platinum' which was also a possible answer, but I’m curious how mercury, which is a liquid metal, could be used as an electrode during electrolysis?

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    $\begingroup$ You might like to have a look at Castner-Kellner cell $\endgroup$
    – Zenix
    May 20, 2020 at 15:15
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    $\begingroup$ Polarography! Please look up the field of polarography in wikipedia! Look up the “dropping mercury electrode”. $\endgroup$
    – Ed V
    May 20, 2020 at 15:45
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    $\begingroup$ Also look up “Hanging mercury drop electrode” in wikipedia! And @Alchimista I “inherited” one of those mercury contaminated labs when the department’ electrochemist retired. There was even mercury droplets in a cabinet drawer. $\endgroup$
    – Ed V
    May 20, 2020 at 16:00
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    $\begingroup$ See saturated calomel electrode $\endgroup$
    – Nilay Ghosh
    May 20, 2020 at 16:00
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    $\begingroup$ By the way, mercury is not inert, relative to platinum or glassy carbon electrodes, for example. But the over-voltage for hydrogen ion reduction is important for aqueous solution work, and the mercury surface is harder to foul than Pt or Au, etc. With the dropping mercury electrode (DME), you get a fresh growing drop of Hg every few seconds or so, so the surface is relatively clean and you do not have to clean and polish the electrodes. $\endgroup$
    – Ed V
    May 20, 2020 at 16:06

3 Answers 3

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The dropping mercury electrode (DME) is a working electrode made of mercury and used in polarography. Experiments run with mercury electrodes are referred to as forms of polarography even if the experiments are identical or very similar to a corresponding voltammetry experiment which uses solid working electrodes. Like other working electrodes these electrodes are used in electrochemical studies using three electrode systems when investigating reaction mechanisms related to redox chemistry among other chemical phenomena.

Source: dropping mercury electrode

Massive thanks to Ed V for advice in the comments prompting my research

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    $\begingroup$ (+1) Good work! Glad to see you answer your question! $\endgroup$
    – Ed V
    May 20, 2020 at 16:36
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    $\begingroup$ @EdV thank you very much for your help $\endgroup$
    – Aryan Beezadhur
    May 20, 2020 at 19:05
  • $\begingroup$ If I understand it correctly, the mercury in polarography is not an inert electrode - it directly participates in the redox chemistry. On the other hand, the platinum electrode acts as a catalyst for gases to undergo redox reactions (and as a conductor). $\endgroup$
    – Karsten ♦
    May 21, 2020 at 15:17
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    $\begingroup$ I disagree about the mercury participating (via redox itself) in the typical applications. This P.A.R. application note is quite good: ameteksi.com/-/media/ameteksi/download_links/documentations/… $\endgroup$
    – Ed V
    May 21, 2020 at 15:49
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Related only - but posted as an answer as, while not 100% what you asked about, it's related enough to be worth noting.

In the past some angle detection or motion detection switches used rolling mercury as a bridge between two metal contacts.

The term "Mercury switch" was commonly used. The Mercury is necessarily inert relative to the associated metallic contacts on the environment used. Substantial currents can be handled in some designs. See references below for details. Thase switches are much less common now due to the hazards of Mercury contamination and the increased availability of electronic based alternatives.

Mercury Jet contacts have also been used in devices where minimising contact force is important.

Wikipedia Mercury Switch

Many images here each linked to a webpage.

Search What metals are used as contacts in a mercury switch?

  • Often Platinum, but also Iron, Molybdenum and more.
    Never Gold! :-)

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Related / interesting:

Material for liquid metal contacts

Mercury metal oscillator !!! & inverter driven by one.

Links suggested by uhoh:

NIST NanoFab Tool: Mercury Probe

  • The Four Dimensions CV92A mercury probe system provides non-destructive and rapid measurement of current-voltage and capacitance-voltage curves to derive resistivity, breakdown voltage/field and film thickness of thin dielectric films without having to metallize the wafer and fabricate capacitors, as is required by traditional probe stations. The capacitor top contact (”gate”) is simply formed by a mercury contact. The accuracy of the measurements is guaranteed by the precisely defined contact area.

MERCURY C-V PROFILING

  • MCV systems allow to eliminate the need for costly metal and poly deposition processes by using a pneumatically controlled, non-damaging probe design and a top-side mercury contact. The system features an extremely stable contact area and uses only a small quantity of mercury to make highly repeatable C-V and I-V measurements for process development and process monitoring applications.

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Mercury switch - source Wikipedia

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    $\begingroup$ @EdV Thanks. You checked up on me :-). $\endgroup$
    – Russell McMahon
    May 21, 2020 at 2:18
  • $\begingroup$ +1 also nist.gov/laboratories/tools-instruments/… and semilab.com/category/products/mercury-c-v-profiling-0 $\endgroup$
    – uhoh
    May 21, 2020 at 9:55
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    $\begingroup$ @uhoh Thanks - I added your references to my answer $\endgroup$
    – Russell McMahon
    May 21, 2020 at 11:56
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    $\begingroup$ @uhoh I did wonder how you were aware of them. | I too have been known to use "In a previous lifetime" :-). 23 years in corporate employ and for the last 27 years having fun under my own steam. (That makes me old, right ? :-) ). $\endgroup$
    – Russell McMahon
    May 21, 2020 at 12:29
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    $\begingroup$ @uhoh Borobadur - several reconstructions - earliest early 20th century. Latest 1975-1982 -wikipedia - they essentially dismantled the whole edifice, all stones marked for location, laid it all out on the surrounding plain, rebuilt the foundation with drainage and strengthening and then rebuilt it. [!!!]. Have you met the "Borobadur Boat". - imge in the stone of the craft(2?) used to settle Madagascar from Borneo before Magellan sailed by? [!!! again]. The majority people of Madagascar are thus ethnically 'Indonesian'. $\endgroup$
    – Russell McMahon
    May 21, 2020 at 23:29
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An old method of electrolytic production of sodium from water solution uses a mercury cathode. It is "inert" in regard to not taking part in the electrolysis itself. It dissolves sodium and prevents it from reacting back with water.

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  • $\begingroup$ @NilayGhosh Just for interest - kathode is the German language version. (Possibly also Dutch?) $\endgroup$
    – Russell McMahon
    May 21, 2020 at 7:12
  • $\begingroup$ Yep. But I speak neither German nor Dutch, nor any other language that has "th" in "cathode". I made a honest and genuine spelling mistake. $\endgroup$
    – fraxinus
    May 21, 2020 at 7:21

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