Biomedical Engineering Reference
In-Depth Information
FIGURE 10.33
Principle of a
p
CO
2
electrode.
The value of
is usually compensated for electronically when the pH electrode is calibrated
by placing the electrode inside different buffer solutions with known pH values.
C
10.4.3 Carbon Dioxide Sensors
Electrodes for measurement of partial pressure of CO
2
in blood or other liquids are based
on measuring the pH, as illustrated in Figure 10.33. The measurement is based on the obser-
vation that when CO
2
is dissolved in water, it forms a weakly dissociated carbonic acid
(H
2
CO
3
) that subsequently forms free hydrogen and bicarbonate ions according to the follow-
ing chemical reaction:
HCO
3
As a result of this chemical reaction, the pH of the solution is changed. This change
generates a potential between the glass pH and a reference (e.g., Ag/AgCl) electrode that
is proportional to the negative logarithm of the
H
þ
þ
CO
2
þ
H
2
O
$
H
2
CO
3
$
p
CO
2
.
10.5 BIOANALYTICAL SENSORS
Biomolecules such as enzymes, antibodies, and microorganisms, as well as animal and
plant cells, have been used as biological sensing elements. Among these, microorganisms
offer unique advantages by their ability to detect a wide range of chemical substances, ame-
nability to genetic modification, and broad operating pH and temperature range, making
them ideal as biological sensing materials.
