Biomedical Engineering Reference
In-Depth Information
Arrhenius, Bronsted, and Lewis. Arrhenius was the fi rst person to give a defi nition of
an acid and a base in 1887. According to Arrhenius, acids were substances that con-
tained hydrogen and yielded hydrogen ions in aqueous solutions; bases contained the
OH group and yielded hydroxide ions in aqueous solutions. Bronsted-Lowry in 1923
introduced the familiar concept of the conjugate acid-base pair and developed the
more generalized concept that an acid is a hydrogen ion or proton donor, and that a
base is a hydroxide ion or proton acceptor. An even broader theory of the acid-base
theory was introduced in 1923 by Lewis, who extended the concept to defi ning an acid
as an electron acceptor and a base as an electron donor. In the body where all the fl uids
are basically water solutions, the concepts of Arrhenius and Bronsted are preferred.
In 1909, Danish biochemist Soren Sorenson fi rst introduced the pH concept. pH is
defi ned by the negative logarithm of the hydrogen ion activity:
pH
log
a
H
(1)
where
a
H
is the hydrogen ion activity. For most purposes, the difference between the
hydrogen ion activity and the hydrogen ion concentration for dilute aqueous solutions
can be ignored. Therefore, it is more convenient to defi ne pH as
log[H
]
pH
(2)
where [H
] is the hydrogen ion concentration in mol/L.
A schematic diagram of a typical pH electrode system is shown in Fig. 10.1. The
cell potential, i.e. the electromotive force, is measured between a pH electrode and a
reference electrode in a test solution. The pH electrode responds to the activity or con-
centration of hydrogen ions in the solution. The reference electrode has a very stable
half-cell potential. The most commonly used reference electrodes for potentiometry
are the silver/silver chloride electrodes (Ag/AgCl) and the saturated calomel electrodes
(SCE).
This cell voltage measurement is made with a pH/mV meter under equilibrium con-
ditions, i.e. at open circuit potential with zero current. The pH/mV meter is actually
a high input impedance voltmeter. The fi rst commercial pH meter (Beckman G) was
developed by Beckman in the 1930s. The meter was built with a vacuum tube and
housed in a walnut box. It was designed for testing citrus juice [36]. Taking advantage
of the fast progress made in IC circuit technology, today's advanced digital pH meters
are much smaller in size, have more functions, and provide reliable and accurate pH
measurements.
The properties of a pH electrode are characterized by parameters like linear response
slope, response time, sensitivity, selectivity, reproducibility/accuracy, stability and bio-
compatibility. Most of these properties are related to each other, and an optimization
process of sensor properties often leads to a compromised result. For the development
of pH sensors for
in-vivo
measurements or implantable applications, both reproducibil-
ity and biocompatibility are crucial. Recommendations about using ion-selective elec-
trodes for blood electrolyte analysis have been made by the International Federation of
Clinical Chemistry and Laboratory Medicine (IFCC) [37]. IUPAC working party on pH
has published IUPAC's recommendations on the defi nition, standards, and procedures
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