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 PdH_0.6

The electrochemical behaviour of a palladium electrode in equilibrium with H₃O⁺ ions in solution parallels the behaviour of palladium with molecular hydrogen

{\tfrac {1}{2}}\mathrm {H} _{2}=\mathrm {H} _{ads}=\mathrm {H} _{abs}

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.

E=E^{0}+{RT \over F}\ln {a_{\mathrm {H} ^{+}} \over ({\frac {p_{\mathrm {H} 2}}{p^{0}}})^{1/2}}

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.

References

^ "Operation principle of Pd/H2 reference electrode" (PDF). Archived from the original (PDF) on 2011-07-20. Retrieved 2009-11-07.
^ A palladium-hydrogen probe electrode for use as a microreference electrode
^ Palladium-hydrogen electrodes for coulometric titration