Which HT HP reference electrode should I use - Part III Pd/H2 electrode
Cormet has three alternative reference electrodes for high temperature high pressure applications. All of them can be installed in an autoclave or a flow through cell. The differences are related to the operation environment.
A palladium hydrogen electrode is a very suitable internal reference electrode for high temperature aqueous work for several reasons. The mechanism of hydrogen evolution reaction has been extensively studied and is now reasonably well understood. In addition, the species (H2, H+) are stable over the temperatures of interest and they do not contaminate the environment where the measurements are carried out. Palladium was chosen as an electrode material instead of platinum because the effect of dissolved oxygen on the behavior of palladium potential is much smaller.
An assembled Pd/H2 electrode for the autoclave operation.
The electrode reaction for the hydrogen electrode is
The Nernst equilibrium potential for this reaction can be written as
the subscript T refers to the operating temperature,
pHT to the thermodynamic pH value at that temperature,
F is Faradays constant,
R the gas constant and
fH2,T the hydrogen fugacity.
An accurately calculated value of ET requires precise knowledge of the hydrogen fugacity (i.e. pressure) and the high temperature pHT values. If some other electroactive species (oxidising species) are present in the solution, the observed potential may be a mixed potential that is not reproducible.
The palladium - hydrogen electrode is acting as Reversible Hydrogen Electrode (RHE). Essential to the proper operation of the palladium electrode is to understand how the electrode response depends on the hydrogen fugacity, pH and on the temperature.
When properly calibrated and when operated in the absence of any oxidising species, the electrode potential of the palladium electrode follows the equilibrium line H+/H2. Increasing the pH decreases the electrode potential with (2.303 RT/F) V/pH unit. Increasing the temperature increases the negative slope of the pH dependence.
In practice, it has been found that the hydrogen fugacity (equation 2) has only a secondary influence on the Pd-electrode potential. This can be due to the fact that the hydrogen fugacity at the surface of the Pd-electrode is rather constant, because of the continuous generation of hydrogen at the surface. At the same time, the water chemistry and the applied hydrogen pressure must remain stable.
Pd/H2 electrode sensor head.
PROS AND CONS
Being an internal solid state electrode, Pd/H2 electrode is easy to operate:
Operator does not have to consider the KCl electrolyte dilution or the electrode contamination as with a Ag/AgCl electrode.
The electrode does not leak any chlorides in the environment.
The electrode maintenance is limited to the wiping off the possible precipitates on the palladium wire surface.
The highest operation temperature of the Pd/H2 electrode (at least 350°C) is higher than that of the Ag/AgCl electrode (300°C).
The challenges of Pd/H2 electrode operation are related to the operation temperature, testing solution pH and the chemical operation environment.
The electrode response depends on the testing solution pH at high temperature. Knowing the HT pH of a clean water is trivial, but if you have a complex solution you will have to measure or calculate the HT pH before the operation.
The environment must remain stable during the operation. Therefore, buffered testing solutions are preferred.
The electrode operation is based on a continuous polarization. The testing solution conductivity must adequate.
The operation environment should not include any oxidizing species.
The cathodic reaction must really be the hydrogen evolution. The testing solution must be relatively simple to avoid any competitive electrochemical reactions.
As a conclusion, I recommend the Pd/H2 electrode to be used in environment, where the chemistry is simple, where the chloride contamination is an issue and where there are no oxidizing species present. Typical example: stable PWR primary side water chemistry.