Redox and Combination Sensors

This sensor was developed for monitoring oxygen, hydrogen, and redox potential in aqueous solutions at temperatures extending above the critical temperature of water. The sensor is based on a combination of two electrodes that are structurally combined into a single unit a redox insensitive yttria stabilized zirconia (YSZ) membrane pH electrode, of the type described in the previous section, and a Pt electrode. The potential of this latter electrode is, of course, sensitive to oxygen and hydrogen concentration and to pH. The potential of the YSZ membrane can be represented as 

Where fn2 is the fugacity of hydrogen. Similarly, the potential of the reversible oxygen electrode can be written as follows  

Importantly, Eqs. (62)-(64) contain the same dependence of the electrode potential on pH. Hence, the potentid difference between the YSZ electrode and Pt electrode in both hydrogenated and oxygenated environments will be pH independent, under ideal conditions, provided that the electrodes are at equihbrium. For hydrogenated environments, we obtain by subtracting Eq. (63) from (62)  

From this last equation, it is apparent that we can estimate the hydrogen fugacity in the solution, independently of the solution pH, as  

Likewise, subtracting Eq. (64) from Eq. (62) we obtain for oxygenated environments  

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