Bayer HealthCare Rapidlab 800 - Manual (Page 622)

IĆ2 800 Series Operator's Manual
Figure IĆ1.āăPotentiometric Cell
Voltmeter
Sample Solution
Reference Electrode
Measuring Electrode
Liquid Junction
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. It 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
lj
= 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 two 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.
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