Palladium-hydrogen electrode

The Palladium-Hydrogen electrode (Abbreviation: Pd/H₂) is one of the common reference electrodes used in electrochemical study. 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.

Additional recommended knowledge

Recognize and detect the effects of electrostatic charges on your balance

Don't let static charges disrupt your weighing accuracy

Guide to balance cleaning: 8 simple steps

Two phases can coexist in palladium when hydrogen is absorbed:

• alpha-phase at hydrogen concentration <0.3 atom percent
• beta-phase at hydrogen concentration corresponds 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
1/2H₂ = H_ads = H_abs
Thus the equilibrium is controlled in one case by the partial pressure of fugacity of molecular hydrogen and in other case - by activity of H⁺-ions in solution.

When palladium is electrochemically charged by hydrogen, the existence of two phases is manifested by a constant potential of aprox. +50 mV vs 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

• E. Gileadi, E.Kirowa-Eisner, J.Penciner Experiments in interfacial electrochemistry. Institute of Chemistry, Tel-Aviv Univ., part 2, pp. 247-249