EXPERIMENT NO. 4 : CONDUCTIVITY
Structure
4.1
Introduction
4.2
Apparatus
4.3
Reagents
4.4
Procedure
4.5
Calculations
Objective
4.1 INTRODUCTION
Conductivity is a numerical expression of the ability of an aqueous solution to carry an
electric current. This ability depends on the presence of ions, their total concentration,
mobility, valence, and relative concentrations, and on the tsmperature of measurement.
Solutions of most inorganic acids, bases, and salts are relatively good conductors.
Conversely, molecules of organic compounds that do not dissociate in aqueous solution
conduct a current very poorly, if at all.
The physical measurement made in a laboratory determination of conductivity is usually
of resistance, measured in ohms or megohms. The resistance of a conductor is inversely
proportional to its cross-sectional area and directly proportional to its length.
The magnitude of the resistance measured in an aqueous solution therefore, depends on
the characteristics of the conductivity cell used, and is not meaningful without knowledge
of these characteristics. Specific resistance is the resistance of a cube I cni on an edge. In
aqueous solutions such a measurement is rare because of the difficulties of electrode
fabrication. Practical electrodes measure a given fraction of the specific resistance, the
fraction being the cell constant C :
resistance, R,,,
c = Measured
Specific resistance, Rs
The reciprocal of resistance is conductance. It measures the ability to conduct a current
and is expressed in reciprocal ohms or mhos. A more convenient unit in water analysis is
micromhos. When the cell constant is known and applied, the measured conductance is
converted to the specific conductance or conductivity, K,,the reciprocal of the specific
resistance.
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The term "conductivity" is preferred and customarily is reported in micromhos per
centimeter (p mhos / cm). In the International System of Units (SI) the reciprocal of the
ohm is the siemens (S) and conductivity is reported as millisiemens per meter (Ms / m);
1 mS / m = 10 p mhos 1 cm. To report results in SI units divide p mhos / cm by 10.
Freshly distilled water has a conductivity of 0.5 to 2 p mhos 1 cm, increasing after a few
weeks of storage to 2 to 4 p mhos / cm. This increase is caused mainly by absorption of
atmospheric carbon dioxide, and, to a lesser extent, ammonia.
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The conductivity of potable waters in the Indian Standards ranges generally fr'om 50 to
1500 p mhos / cm. The conductivity of domestic waste waters may be near that of the
local water supply, although some industrial waters have conductivities above
10,000 p mhos / cm. Conductivity instruments are used in pipelines, channels, flowing
streams, and lakes and can be incorporated in multiple-parameter monitoring stations
using recorders.
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Laboratory measurement of conductivity is relatively accurate but less accurate means of
determining conductiyity find numerous applications such as signalling exhaustion of
ion-exchange resins and rapid determination of large changes in inorganic content of
waters and waste waters. Monitoring devices can give continuous, unattended records of
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conductivity, if they are properly installed and maintained. Most problems in obtaining
good records with monitoring equipment are related to electrode fouling and to
inadequate sample circulation.
Laboratory conductivity measurements are used to :
(a) Establish degeee of mineralization to assess the effect of the total concentration
of ions on chemical equilibria, physiological effect on plants or animals,
corrosion rates, etc.
(b) Evaluate variations in dissolved mineral concentration of raw water or waste
water. Minor seasonal variations found in reservoir waters contrast sharply with
the daily fluctuations in some polluted river waters. Waste water containing
significant trade wastes also may show a considerable daily variation.
(c) Estimate total dissolved solids in a sample by multiplying conductivity (in
micromhos per centimeter) by an empirical factor. This factor may vary from
0.55 to 0.9, depending on the soluble components of the water and on the
temperature of measurement. Relatively high factors may be required for saline
or boiler waters, whereas lower factors may apply where, considerable
hydroxide or free acid is present. Even though sample evaporation results in the
change of bicarbonate to carbonate, the empirical factor is derived for a
comparatively constant water supply by dividing dissolved solids by
conductivity.
Objective
To determine conductivity of the given water sample.
4.2 APPARATUS
(a) Self-containedConductivity Instruments :Use an instrument consisting of a source
of alternating current, a Wheatstone bridge, a null indicator, and a conductivity cell or
other instrument measuring the ratio of alternating current through the cell to voltage
across it. The latter has the advantage of a linear reading of conductivity. Choose an
instrument capable of measurement conductivity with an error not exceeding 1% or
1 pmho 1 cm, whichever is greater.
Figure 4.1 : Conductivity Meter
(b) Thermometer, capable of being read to the nearest O.l°C and covering the range 23 to
27QC.An electrical thermometer having a small thermistor sensing element is
convenient because of its rapid response.
Environmental Engineering
(c) Conductivity Cell
(1)
Platinum-electrode type : Conductivity cells containing platinized electrodes are
available in either pipet or immersion form. Cell choice depends on'expected
range of conductivity and resistance range of the instrument.
(2)
Nonplatinum-electrode type : Use conductivity cells containing electrodes
constructed from durable common metals (stainless steel among others) for
continuous monitoring and field studies. Calibrate such cells by comparing
sample conductivity with results obtained with a laboratory instrument. Use
properly designed and mated cell and instrument to minimize errors in cell
constant.
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4.3 REAGENTS
(a)
Conductivity Water :Pass distilled water through a mixed-bed deionizer and discard
first liter. Conductivity should be less than 1 pmho / cm.
(b) Standard Potassium Chloride Solution (KCl, 0.0100 M) :Dissolve 745.6 mg
anhydrous KC1 in conductivity water and dilute to 1000 mL at 25OC. This is the
standard reference solution, which at 25OC has a conductivity of 1413 pmho 1 cm. It is
satisfactory for most samples when the cell has a constant between 1 and 2. For other
cell constants, use stronger or weaker KC1 solutions listed in Table 4.1. Store in a
glass-stoppered borosilicate glass bottle.
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Table 4.1 : Conductivity of Potassium Chloride Solutions at 2S°C
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Vnce:tion
Conductivity
mho / cm I equiv.
4.4 PROCEDURE
(a)
Determination of Cell Constant :Rinse conductivity cell with at least three portions
of 0.01 M KC1 solution. Adjust temperature of a fourth portion to 25.0 ?r 0. 1°C.
Measure resistance of this portion and note temperature. Compute cell constant, C :
where,
RK ~1 = measured resistance, ohms, and
t
= observed temperature OC.
(b) Conductivity Measurement :Rinse cell with one or more portions of sample. Adjust
temperature of a final portion to 25.0 O.l°C. Measure sample resistance or
conductivity and note temperature.
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4.5 CALCULATIONS
The temperature coefficient of most waters is only approximately the same as that of
standard K C1 solution; the more the temperature of measurement deviates from 25.0°C,
the greater the uncertainty in applying the temperature correction. Report all
conductivities at 25.0°C.
(a) When sample resistance is measured, conductivity at 25OC is :
(c>
(1,000,000~
R,, [ I + 0.0191 (t - 25)]
K = -
where,
K = conductivity, pmho I cm,
C = cell constant, cm-',
R, = measured resistance of sample (in ohms),
t = temperature of measurement.
(b)
When sample conductivity is measured, conductivity at 2j°C is :
K,,, = measured conductivity, mhos at tOc,and other units are
defined as above.
[Note : If conductivity readout is in micromhos per centimeter, delete the factor
1,000,000 in the numerator.]
where,