Which HT HP reference electrode I should use? Part II - YSZ electrode
Yttria stabilised zirconia/metal/metal oxide (YSZMO, YSZ) electrode was developed by Niedrach, McDonald and Danielson. The electrode is used as a pH electrode and a reference electrode.
Physically, the electrode comprises a closed end yttria-stabilised zirconia tube, filled with a metal wire surrounded by metal and metal oxide powders. Typical electrode materials in Cormet’s electrodes are nickel (Ni/NiO/ZrO2) and copper (Cu/Cu2O/ZrO2).
Operation of the pH electrode is based on the zirconium oxide membrane that acts as an ionic oxygen vacancy conductor. The H+ ion activity (i.e. pH) on the liquid / ceramic interface affects the oxygen vacancy concentration of the ceramic and further the equilibrium of the Me/MenO electrode inside the zirconium tube.
The pH sensor can be used as a reference electrode at aqueous HT HP environment. The testing environment pH must be well known and stable, because YSZ’s response depends on the testing solution pH. If we discard the effects of water activity and oxygen fugacity, the response of the electrode (Cu/Cu2O in this case) is
E0Cu/Cu2O = standard equilibrium potential of Cu/Cu2O as a function of temperature,
R = gas constant = 8.314 J/K×mol
T = temperature (K)
F = Faraday’s constant = 96 480 As/mol
pHT = pH of the solution at the temperature of the environment
Theoretically, the pH sensor does not have to be calibrated. Based on our experience, the E0Me/MeO values vary somewhat between the sensors. Therefore, we prefer to calibrate the electrodes temperature range 300°C – 180°C in 0.1 M H3BO3 solution using a Ag/AgCl reference electrode.
Being a solid state electrode, YSZ 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 zirconia tube surface.
The highest operation temperature of the YSZ electrode (at least 350°C) is higher than that of the Ag/AgCl electrode (300°C).
The challenges of YSZ electrode operation are related to the operation temperature, testing solution pH and the mechanical integrity of the electrode:
The operation temperature range is limited, because the lowest operation temperature of the nickel and copper oxide electrodes is about 180°C – 200°C.
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 electrode has to be treated carefully, because the zirconia tube is brittle and fragile. Previously, some zirconia tubes used to crack during the first days in the HT HP operation, but fortunately the ceramic manufacturers have been able to improve the quality of the product.
As a conclusion, I recommend an YSZ electrode to be used in an environment, where the water chemistry is simple and where the chloride contamination is an issue. Typical example: BWR environment clean water.