Siemens Rapidlab 1200, Operator'S Manual, Page: 48 / 448

1-28 Rapidlab 1200 Operator’s Guide: System Overview and Intended Use
02087462 Rev. V
Each electrode, which acts as a half-cell with a half-cell potential, contains an inner
reference element immersed in an internal electrolyte solution. The measuring electrode is
designed to respond to changes in the concentration of the specific analyte being measured
in the sample solution. The electrode develops a half-cell potential that is directly related
to the concentration or activity of the specific analyte. The reference electrode provides a
steady, unchanging potential to the cell. Both electrodes are connected to the measuring
device. With the current in the cell at zero, the potential developed by the electrochemical
cell is determined by calculating the difference in potential between the measuring
electrode and the reference electrode.
E
cell
= E
meas
- (E
ref
+ E
lj
)
where
E
cell
= electrochemical cell potential
E
meas
= measuring electrode half-cell potential
E
ref
= reference electrode half-cell potential
E
l
j = liquid junction potential
The liquid junction potential (E
lj
), a small but significant voltage, develops at the liquid
junction between the reference electrode, which contains a solution of saturated potassium
chloride, and the sample solution. This potential occurs because of the different rates at
which chemical species diffuse across the boundary between 2 liquids. This difference in
rates results in a charge separation that gives rise to the liquid junction potential. Although
the potential formed is small, it must be considered when measuring cell potential.
1
System sensors are designed to measure a specific substance in a sample. 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. For
example, an Ion Selective Electrode (ISE) contains a specifically 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.