Solid-state electrochemical sensors for measuring the degrees of acidity or alkalinity (in terms of pH values) of liquid solutions are being developed. These sensors are intended to supplant older electrochemical pH sensors that include glass electrode structures and reference solutions. The older sensors are fragile and subject to drift. The present developmental solid-state sensors are more rugged and are expected to be usable in harsh environments.
Like the older electrochemical pH sensors, the present sensors are based on a differential- electrode measurement principle. Each sensor includes two electrodes, made of different materials, in equilibrium with the solution of interest. The electrode materials are chosen so that the electric potential of one electrode is sensitive (or more sensitive) to the pH of the solution of interest while the electric potential of the other electrode is insensitive (or less sensitive) to the pH of the solution. One measures the difference between the potentials on the two electrodes and deduces the pH from the known relationship between that difference and the pH.
One of the electrodes of a pH sensor of the present type is an iridium wire that has been partially oxidized to have a surface layer of iridium oxide about 15 μm thick. The other electrode is a rhodium foil that has been similarly treated to impart a surface layer of rhodium oxide about 5 μm thick.
In calibration tests, the dependence of the electric potential of the iridium/iridium oxide electrode upon pH was found to closely approximate that predicted by the Nernst equation, at a slope between –57 and –59 mV/pH. The dependence of the electric potential of the rhodium/ rhodium oxide electrode upon pH was found to be sub-Nernstian, at a slope of about –26 mV/pH. Hence, in constructing a pH sensor, iridium/iridium oxide was used for the sensing (more-sensitive) electrode and rhodium/rhodium oxide for the reference (less-sensitive) electrode. When the difference between the potentials of the two electrodes was measured as a function of pH, the slope was found to be about –30 mV/pH (see figure). This slope is well within the range of typical instrumentation used in converting DC signals to digital data for recording.
This work was done by William West, Martin Buehler, and Didier Keymeulen of Caltech for NASA's Jet Propulsion Laboratory.
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