Reading the first paragraph in Wikipedia's I found one sentence particularly interesting:

Some gases, such as hydrogen chloride ($\ce{HCl}$), under conditions of high temperature or low pressure can also function as electrolytes.[clarification needed] - https://en.wikipedia.org/wiki/Electrolyte

note: as mentioned in comments the "clarification needed" has since been removed.

Question: Can there be a gas-phase electrolyte? If so, is there any more in depth or academic resource that:

  1. describes the nature and behavior of a gas-phase electrolyte
  2. descrbies gas-phase HCl as one?
  • 4
    $\begingroup$ They don't. That's why "clarification..." was added, but it should be just deleted. Probably it was a hint about plasma, but it's considered a state of matter distinct from gas. $\endgroup$
    – Mithoron
    Jul 2 '21 at 0:54
  • $\begingroup$ I suspect that this is abuse of language. HCl can be electrolysed when fed to a reactor as a gas phase reagent, see eg sciencedirect.com/science/article/pii/S0013468619316512 That would not make HCl a "gas phase electrolyte". The Wikipedia page definitely needs clarification. $\endgroup$
    – Buck Thorn
    Jul 3 '21 at 5:01
  • 1
    $\begingroup$ Buck Thorn, seems that the clarification label has been removed, albeit not the original claim. $\endgroup$ Jul 3 '21 at 5:20
  • 1
    $\begingroup$ @BuckThorn I made some edits to the question; maybe "No. Electrolysis can happen at a gas-solid interface but that doesn't make the process electrolysis proper. is all that's needed as an answer? $\endgroup$
    – uhoh
    Jul 3 '21 at 7:35
  • 1

I think, it is a result of the word having multiple definitions in different contexts. Specifically, the original definition in context of chemistry

Electrolyte is a solid, causing electrical conductivity when dissolved in a liquid.

Later works extended it to non-solids, of course, but the root "lyte" (AFAIK, Greek for "earth") remained.

However, in context of technical application it is clearly different, since, for example, it is common to hear a term 'solid electrolyte'. In addition to that, electrolytes are solutions that can be electrolized, but there are many things that can be electrolized, such as molten salts. "molten salt electrolyte", "solid electrolyte", "glass electrolyte" (sic!) are terms you can find. In this context a different definition is in effect

Electrolyte is a medium with ionic conductance.

This includes:

  • solutions of eletrolytically dissociating compounds
  • many molten salts
  • some auto-dissociating liquids
  • solids with ionic conductance

Assuming the second definition, if there is a gas medium with significant concentration of ions, it could fit. Normally, however, gases are molecules, and if the gas has high conductivity, it is usually mostly due to free electrons and falls under domain of plasma physics.

The possible borderline case is supercritical fluids. Well above pressure and temperature of critical point you can't really talk about 'gas' and 'liquid' since there is no phase transition between them. The density of the medium, however, might be high enough to stabilize positive and negative ions. Indeed, "supercritical electrolytes" are a thing in literature, though usually term "supercritical electrolyte solutions" is used. I didn't find anything about individual $\ce{HCl}$ specifically, but, for example, $\ce{H2O-HCl}$ mixtures at high temperature and pressure might qualify. Similarly, there are works on electrochemistry in supercritical $\ce{CO2}$


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