Bayer HealthCare Rapidlab 800 - Manual (Page 623)

Operating Principles IĆ3
System sensors are designed to measure a specific substance in a sample. To better
understand the ability of a sensor to measure specific substances, consider
ion-selective electrode (ISE) technology. Many of the sensors, like the pH sensor,
are designed with this technology. For the purpose of measuring a variety of
analytes in solution, sensors must have the ability to measure specific analytes in
solution. This ability is known as the recognition mechanism. An ISE contains a
specially designed membrane that provides sensor selectivity. Selectivity is the
ability of the sensor to interact with a specific ion in solution. The membrane
separates an inner, reference element, which is immersed in a fixed electrolyte
solution, from the sample.
During analysis, a membrane potential develops as a result of the interaction of the
analyte (ion) at the membrane. The membrane potential is related to the amount of
substance being measured in the sample. The half-cell potential in the sensor
consists of the inner reference element potential plus the membrane potential.
Figure IĆ2.āăIonĆSelective Electrode and Reference Electrode
Inner element of
the reference electrode.
Potassium Chloride
Solution
Ion-selective
Membrane
Liquid junction potential
develops here.
ISE Inner Reference
Element
A fixed electrolyte
solution surrounds
the inner element.
Voltmeter
The equation for calculating electrochemical cell potential can be expanded to
include the inner reference element and the membrane potential of the ISE.