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Operating Principles IĆ13
Sample Path
Cathode Contact
Anode Contact
A constant voltage, called a polarizing voltage, is maintained between the anode
and the cathode. As dissolved oxygen from the sample passes through the
membrane into the electrolyte solution, it is reduced at the cathode.
O
2
+ 2H
2
O + 4e
–
! 4OH
–
The circuit is completed at the anode, when the Ag is oxidized.
4Ag ! 4Ag
+
+ 4e
–
The amount of reduced oxygen is directly proportional to the number of electrons
gained at the cathode. Therefore, by measuring the change in current (electron
flow) between the anode and the cathode, the amount of oxygen in the electrolyte
solution is determined.
16
The 850 and 860 systems analyze blood samples for sodium (Na
+
), potassium
(K
+
), chloride (Cl
–
), and calcium (Ca
++
) in addition to pH and the blood gases.
These systems report two additional parameters, the anion gap and a value for
calcium adjusted to pH of 7.40. Refer to the section Other Reported Parameters,
page I-25, for a discussion of these parameters.
The sensors used for electrolytes are based on ion-selective electrode (ISE)
technology. Each sensor has a membrane that is highly selective for a specific ion.
Figure I-8 identifies components of the electrolyte sensors. The illustration shows
two sensors. One represents the K
+
, the Cl
–
, and the Ca
++
sensors, which have
similar components. The other represents the Na
+
sensor.