Solubility in Inorganic Materials

In the absence of oxygen, the reaction depicted by (2.5) is reversible. However, when oxygen is present, a competing oxidation takes place at the Pd surface, making the overall reaction irreversible. 

Hydrogen is a very important species and various hydrogen sensors based on reactions (2.5) through (2.7) have been commercialized. They are discussed in more detail later. 

An entirely different selectivity principle known as phase equilibrium comes into play in high-temperature ionic conductors. Many important gases dissolve in ionic solids at elevated temperatures. However, the solubility is rather sharply defined for the gas and the solid by the lattice parameters and the size of the gas molecule. The best example is the solubility of oxygen in zirconium dioxide. When Z1O2 is doped with yttrium ions, it exhibits a high mobility for the O anion. The solubility and anion mobility then become the basis for several electrochemical gas sensors, using yttria-stabilized zirconia (YSZ). 

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