Selection of our books indexed in the Book Citation Index in Web of Science™ Core Collection (bkci)

Figure 4. Sensing principle of Clark-type pO
2
sensor
In this oxidation reaction, silver from the anode electrode is firstly oxidized to silver ions, and
electrons are liberated to the anode. These silver ions are immediately combined with chloride
ions to form a kind of compound precipitant silver chloride 
AgCl
which precipitates on the
surface of anode. The transient current between the anode and the cathode in the external
circuit produced by this reaction is directly proportional to the number of O
2
molecules
constantly reduced on the surface of the cathode. The electrodes in the polarographic cell are
immersed in an electrolyte solution of potassium chloride and surrounded by an O
2
-permeable
or polypropylene membrane that permits gases to diffuse slowly into electrode. Thus, by
Advances in Bioengineering
186

---

measuring the change in current between the cathode and the anode, the amount of oxygen
that is dissolved in the solution can be determined.
2.2.2. Carbon dioxide sensor for blood
Electrodes for measuring partial pressure of carbon dioxide CO
2
in blood are based on
measuring the pH as illustrated in figure 5. The measurement is based on the observation that
it forms a weakly dissociated carbonic acid (H
2
CO
3
) that subsequently forms free hydrogen
and bicarbonate ions when CO
2
is dissolved in water according to the following reaction:
CO
2
+ H
2
O ↔ H
2
CO
3
↔ H
+
+ 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 electrode that is proportional to the negative
logarithm of the concentration of the carbon dioxide pCO
2
in the plasma.