Biomedical Engineering Reference
In-Depth Information
Therefore, the concentration of either ion can be used to express the acidity or alkalinity of
a solution. All neutral solutions have a pH of 7.0.
The measurement of blood pH is fundamental to many diagnostic procedures. In normal
blood, pH is maintained under tight control and is typically around 7.40 (slightly basic). By
measuring blood pH, it is possible to determine whether the lungs are removing sufficient
CO
2
gas from the body or how well the kidneys regulate the acid-base balance.
pH electrodes belong to group of potentiometric sensors (i.e., electrochemical sensors
producing a voltage). These sensors generate a small potential difference without the need
to polarize the electrochemical cell. A pH electrode essentially consists of two separate elec-
trodes: a reference electrode and an active (indicator) electrode, as shown in Figure 10.32.
The two electrodes are typically made of an Ag/AgCl wire dipped in a KCl solution and
encased in a glass container. A salt bridge, which is essentially a glass tube containing an
electrolyte enclosed in a membrane that is permeable to all ions, maintains the potential
of the reference electrode at a constant value regardless of the solution under test. Unlike
the reference electrode, the active electrode is sealed with hydrogen-impermeable glass
except at the tip. The reference electrode may also be combined with the indicator electrode
in a single glass housing.
The boundary separating two solutions has a potential proportional to the hydrogen ion
concentration of one solution and, at a constant temperature of 25
o
C, is given by
V
H
þ
þ
C
¼
59 mV
log
10
½
ð
10
:
21
Þ
where
C
is a constant. Since pH is defined as
pH
H
þ
Þ
the potential of the active pH electrode, V, is proportional to the pH of the solution under
test and is equal to
¼
log
10
½
ð
10
:
22
V
¼
59 mV
pH
þ
C
ð
10
:
23
Þ
FIGURE 10.32
Principle of a pH electrode.
