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What if one or both electrodes are in gas phase? Use PV=nRT? Also the definition of hydrogen electrode gets a little sketchy if we use PV=nRT to write it's concentration, the electrode potential doesn't come out 0 that way.

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  • $\begingroup$ What do you mean by doesn't come out 0? The Hydrogen electrode potential is assigned an arbitrary zero in order to have a reference point. It doesn't have a zero value under any circumstances. In order to answer your question anyway, we use pressure values as proxies for the activity of gases, not concentration. $\endgroup$ Jan 16 '17 at 5:50
  • $\begingroup$ So we use pressure values instead of concentration even in cases where ionic and gas phase both are present? $\endgroup$
    – jatin
    Jan 16 '17 at 19:08
  • $\begingroup$ That is correct. Pressure for gaseous reactants and concentrations for dissolved ones. $\endgroup$ Jan 17 '17 at 4:50

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