68 Fundamentals of Sensors for Engineering and Science
unity. There, a
A
' [C
A
]. In either situation, Equation 2.85 can be substi-
tuted into Equation 2.84 to relate electrode potential to the concentration
of a species in solution.
This method can be used to measure the concentration of hydrogen ions
in solution. This, in turn, determines the pH of the solution through the
expression
pH = −log
10
a
H
+
. (2.86)
An electrochemical potential also can be established across a membrane
that is permeable to particular molecule, such as zirconia oxide to oxygen
and palladium to hydrogen. This forms the basis of certain gas sensors.
Amperometric
The ChemFET sensor is one example of an amperometric sensor. This
sensor behaves similarly to an electrical circuit MOSFET ...