Biomedical Engineering Reference
In-Depth Information
accompanied by the injection/ejection of H . The possible reactions involved in the
electrode processes are:
Hydration of iridium oxide
Ir 2 O 3
3H 2 O
2Ir(OH) 3
(7)
Charge transfer and injection/ejection of hydrogen ion
2H
2e
2Ir(OH) 3
2IrO 2
H 2 O
(8)
Depending on the fabrication techniques and deposition parameters, the pH sensitive
slope of IrOx electrodes varies from near-Nernstian (about 59 mV/pH) to super-Nernstian
(about 70 mV/pH or higher). Since the compounds in the oxide layers are possibly
mixed in stoichiometry and oxidation states, most reported iridium oxide reactions
use x , y in the chemical formulas, such as Ir 2 O 3
x H 2 O and IrOx(OH) y . Such mixed
oxidation states in IrOx compounds may induce more H ion transfer per electron,
which has been attributed to causing super-Nerstian pH responses [41].
Considering the H dependent redox reaction between two oxidation states of
the iridium oxide as the basis of the pH sensing mechanism, the electrode potential
changes to the hydrogen ion concentration are expressed by Nernstian equation:
2.303 RT/F (log[H ])
E o
E
E o
61.54 pH
(9)
where E o is the standard electrode potential with the value of 729 mV vs Ag/AgCl ref-
erence electrode. The potential/pH slope is expected to be
61.5 mV/pH at 37ºC.
10.3.5 Ag/AgCl reference microelectrodes
Reference electrodes provide a standard for the electrochemical measurements. For
potentiometric sensors, an accurate and stable reference electrode that acts as a half-
cell in the measurement circuit is critical to providing a stable reference potential and
for measuring the change in potential difference across the pH sensitive membrane as
the pH concentration changes. This is especially important in clinical applications such
as pH measurements in the blood, heart, and brain, where the relevant physiological
pH range is restricted to a very small range, usually less than one unit.
The three main requirements for a satisfactory reference electrode, given by Ives
and Janz [107], are reversibility (non-polarizability), reproducibility, and stability.
Hydrogen electrodes have been chosen as the primary reference electrode due to their
excellent reproducibility [54]. The electrode is represented schematically as
Pt/Pt black|H 2 (1 atm), 1 M H 2 SO 4
(10)
which consists of a platinum foil with electroplated Pt black to catalyze the hydrogen
electron transfer reaction. The standard potential of the hydrogen electrode is conven-
tionally set to zero at all temperatures, thus establishing a hydrogen scale of standard
potential. However, this electrode is impractical in routine usages, thus secondary ref-
erence electrodes, such as calomel, sulphate, and silver/silver chloride (Ag/AgCl) are
used [80].
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