Physics:Palladium-hydrogen electrode
The palladium-hydrogen electrode (abbreviation: Pd/H2) is one of the common reference electrodes used in electrochemical study.[1] Most of its characteristics are similar to the standard hydrogen electrode (with platinum). But palladium has one significant feature—the capability to absorb (dissolve into itself) molecular hydrogen.[2][3]
Electrode operation
Two phases can coexist in palladium when hydrogen is absorbed:
- alpha-phase at hydrogen concentration less than 0.025 atoms per atom of palladium
- beta-phase at hydrogen concentration corresponding to the non-stoichiometric formula PdH0.6
The electrochemical behaviour of a palladium electrode in equilibrium with H3O+ ions in solution parallels the behaviour of palladium with molecular hydrogen
- [math]\displaystyle{ \tfrac{1}{2} \mathrm{H}_2 = \mathrm{H}_{ads} = \mathrm{H}_{abs} }[/math]
Thus the equilibrium is controlled in one case by the partial pressure or fugacity of molecular hydrogen and in other case—by activity of H+-ions in solution.
- [math]\displaystyle{ E=E^0 + {RT \over F}\ln {a_{\mathrm{H}^+} \over (\frac{p_{\mathrm{H}2}}{p^0})^{1/2}} }[/math]
When palladium is electrochemically charged by hydrogen, the existence of two phases is manifested by a constant potential of approximately +50 mV compared to the reversible hydrogen electrode. This potential is independent of the amount of hydrogen absorbed over a wide range. This property has been utilized in the construction of a palladium/hydrogen reference electrode. The main feature of such electrode is an absence of non-stop bubbling of molecular hydrogen through the solution as it is absolutely necessary for the standard hydrogen electrode.
See also
References
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