Polarization
Polarization has a number of meanings but, in chromatography, it usually refers to the condition of the electrodes used in an electrical conductivity detector. If two electrodes are immersed in a conducting liquid and a DC (direct current) potential is applied to the electrodes, ions are neutralized at the electrode surface and deposits are formed. These deposits may consist simply of bubbles of hydrogen and oxygen, metals if inorganic salts are present, or, with organic salts, organic acids or bases. These deposits interfere with the performance of the detector and the electrodes are said to have become ‘polarized’. This problem is solved by employing AC (alternating current) potential to the electrode which continually reverses the electrolytic effect and, thus, prevents deposits and polarization.
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Electrical-Conductivity
conductivity detector measures the conductivity of the mobile phase. There is
usually background conductivity which must be backed-off by suitable electronic
adjustments. If the mobile phase contains buffers, the detector gives a base
signal that completely overwhelms that from any solute usually making detection
impossible. Thus, the electrical conductivity detector a bulk property
detector. and senses all ions whether they are from a solute or from the mobile
phase.
In order to
prevent polarization of the sensing electrodes, AC voltages must be used and so
it is the impedance not the resistance of the electrode system
that is actually measured. From a physical chemistry stand point the
conductivity of a solution is more important than its resistance. However, it
is the resistance (impedance) of the electrode system that determines the
current across it.
The resistance
(R) of any conductor varies directly as its length (L) and inversely as its
cross sectional area (a).
HPLC-Detectors Electrical-Conductivity
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Electrical-Conductivity
amplifier. The output from
the amplifier is either digitized, and the binary number sent to a computer for
storage, or the output is passed directly to a potentiometric recorder. The
detector actually measures the electrical resistance between the electrodes
which by suitable non-linear amplification, can be made to provide an output
that is linearly related to solute concentration. It is essential that an AC
voltage is used across the electrodes to measure the cell impedance to avoid
electrode polarization. The frequency of the AC potential across the electrodes
is usually around 10 kHz
Figure
21. An Electrical Conductivity Detector Sensing Cell
 
HPLC Electrical-Conductivity