PASCO CI-6729 (1X) CONDUCTIVITY SENSOR - Manual (Page 10)

6
Conductivity Sensor 012–06485B
- +
After a fairly short period of time most of the ions in solution will move
to the electrodes and the current flow through probe will begin to
decrease. This process is called polarization (Figure 4). Polarization
causes the conductivity probe’s output to be low if not corrected.
One method used to reduce the effects of polarization is to alternate the
polarity of the voltage applied to the electrode. Polarization of the
electrode and the associated build up of oxidation/reduction by-
products is prevented if the voltage is alternated quickly enough.
Conductivity Measurements
The conductivity of an electrolytic solution is dependent on several
factors, including the concentration of the solute, the degree of dis-
sociation of the solute, the degree of dissociation of the molecules
present in the solution, the valiancy of the ion(s) present in the
solution, the mobilities of ions that are formed upon dissociation, and
the temperature of the solution. In general, ion mobilities remain
constant over a specific concentration range, depending on the solute
in solution. Within the specific concentration range, the conductivity
of a solution will increase in proportion to an increase in solute
concentration.
Since temperature is a measure of the average kinetic energy of an atom,
ion, or molecule, any change in temperature of a solution will strongly
affect the mobility of the species present in the electrolyte. Therefore,
the conductivity of the solution will also change. As a result, the
conductivity of a solution whose concentration is known is always
quoted for a specific temperature. Several tables in the Appendix
illustrate the temperature dependence of conductivity for specific
electrolytic solutions.
Concentration or total dissolved solid (TDS) measurements can also be
accurately determined by correlating the conductivity of a solution with
reference tables or graphs. TDS measurements are generally used to
estimate the salt content of water samples. A rule of thumb used to
estimate TDS in mg/L (ppm) is to divide the measured conductivity in
microsiemens per centimeter (
µS/cm) by two. Table 4 in the Appendix
is a conversion chart for TDS measurements involving NaCl and
CaCO
3
. Additional references of conductivity data are also listed in the
Appendix.
Figure 4
Polarized Electrode
➤ Approximate conductivities
(at 25
°°°°°C) and impurity
concentrations (in ppm) of
various water samples are given
below.
Sample Conductivity
Pure H
2
0 0.055 µS/cm (0.027 ppm)
Distilled H
2
0 0.5 µS/cm (0.206 ppm)
City H
2
0 50 µS/cm (25 ppm)
Ocean H
2
0 53 mS/cm (35,000 ppm)