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Electrical conductivity
1. Chapter 9
Electrical conductivity
1 Introduction
As in the case of metallic conductors, electrical current can flow through a solution of an
electrolyte also. For metallic conductors: current is carried by electrons, chemical
properties of metal are not changed and an increase in temperature increases
resistance. The characteristics of current flow in electrolytes in these respects are
different. The current is carried by ions, chemical changes occur in the solution and an
increase in temperature decreases the resistance.
Electrical conductivity (EC) is a measure of the ability of water to conduct an electric
current and depends on:
Concentration of the ions (higher concentration, higher EC)
Temperature of the solution (high temperature, higher EC)
Specific nature of the ions (higher specific ability and higher valence, higher EC)
Conductivity changes with storage time and temperature. The measurement should
therefore be made in situ (dipping the electrode in the stream or well water) or in the
field directly after sampling. The determination of the electrical conductivity is a rapid
and convenient means of estimating the concentration of ions in solution. Since each
ion has its own specific ability to conduct current, EC is only an estimate of the total ion
concentration.
2 Equations and dimensions
Ohm's law defines the relation between potential (V) and current (I). The resistance (R)
is the ratio between V and I:
V
R= (1)
I
The resistance depends upon the dimensions of the conductor, length, L, in cm, cross-
sectional area, A, in cm2
and the specific resistance, p, in ohm.cm, of the conductor:
L
R=p x (2)
A
In the present case our interest is in specific conductance or electrical conductivity
(which is the preferred term), the reciprocal of specific resistance, k, in 1/ohm.cm or
Siemens per centimetre, S/cm, which can be thought of as the conductance offered by
1 cm3
of electrolyte:
1 L 1
k= = — x — (3)
p A R
The resistance of the electrolyte is measured across two plates dipped in the liquid and
held at a fixed distance apart in a conductivity cell. The ratio L/A for the cell is called cell
constant, Kc, and has the dimensions 1/cm. The value of the constant is determined by
measuring the resistance of a standard solution of known conductivity:
Kc= R.k ( 4 )
2. 3 Unit of measurement and reporting
In the international system of units (SI) the electrical conductivity is expressed in
Siemens which is the reciprocal of resistance in ohm. The older unit for conductance
was mho. Report conductivity as milli Siemens per meter at 25°C (mS.m"1
). See table
for conversions.
4 Apparatus
An apparatus called a conductivity meter that consists of a conductivity cell and a meter
measures conductivity. The conductivity cell
consists of two electrodes (platinum plates)
rigidly held at a constant distance from each
other and are connected by cables to the
meter. The meter consists of a Wheatstone
bridge circuit as shown in the figure. The
source of electric current in the meter applies
a potential to the plates and the meter
measures the electrical resistance of the
solution. In order to avoid change of apparent
resistance with time due to chemical
reactions (polarisation effect at the
electrodes) alternating current is used. Some
meters read resistance (ohm) while others
read in units of conductivity (milli-Siemens
Ri R2 per meter). Platinised electrodes must be in
good condition (clean, black-coated) and require replating if readings of the standard
solution become erratic. Replating should be done in the laboratory. The cell should
always be kept in distilled water when not in use, and thoroughly rinsed in distilled water
after measurement.
5 The cell constant (calibration)
The design of the plates in the conductivity cell (size, shape, position and condition)
determines the conductivity measured and is reflected in the so-called cell constant (Kc),
Typical values for Kc are 0.1 to 2.0. The cell constant can be determined by using the
conductivity meter to measure the resistance of a standard solution of 0.0100mol/L
potassium chloride (KCI). The conductivity of the solution (141.2 mS/m at 25°C)
multiplied by the measured resistance gives the value of Kc, Equation 4. The cell
constant is subject to slow changes in time, even under ideal conditions. Thus,
determination of the cell constant must be done regularly.
3. 6 Temperature correction
Conductivity is highly temperature dependent. Electrolyte conductivity increases with
temperature at a rate of 0.0191 mS/m°C for a standard KCI solution of 0.0100M.
For natural waters, this temperature coefficient is only approximately the same as that
of the standard KCI solution. Thus, the more the sample temperature deviates from
25°C the greater the uncertainty in applying the temperature correction. Always record
the temperature of a sample (+0.1 °C) and report the measured conductivity at 25°C
(using a temperature coefficient of 0.0191 mS/m°C)
Most of the modern conductivity meters have a facility to calculate the specific
conductivity at 25°C using a built in temperature compensation from 0 to 60°C. The
compensation can be manual (measure temperature separately and adjust meter to
this) or automatic (there is a temperature electrode connected to the meter).
7 Conductivity factor for different ions
Current is carried by both cations and anions, but to a different degree. The conductivity
due to divalent cations is more than that of mono-valent cations. However, it is not true
for anions. The conductivity factors for major ions present in water are listed below.
Table 2 Conductivity Factors for ions commonly found in water
Ion
Conductivity Factor
fjS/cm per mg/L
Cations
Ca 2+
2.60
Mg2+
3.82
K+
1.84
Na+
2.13
Anions
HCO3- 0.715
cr 2.14
S04'- 1.54
NO3- 1.15
The conductivity of a water sample can be approximated using the following relationship
EC = E (C, X f,)
in which
EC = electrical conductivity, pS/ cm
Ci = concentration of ionic specie i in solution, mg / L
fi = conductivity factor for ionic specie i
4. Example 1
Given the following analysis of a water sample, estimate the EC value in ij S/cm and
mS/m.
Cations: Ca2+
= 85.0 mg/L, Mg2+
= 43.0 mg/L, K+
= 2.9 mg/L, Na+
= 92.0 mg/L
Anions: HC03" =362.0 mg/L, Cl"=131.0 mg/L, S04
2
"=89.0 mg/L, N03"=20.0 mg/L
Calculate the electrical conductivity of each ion using the data given in Table 3.
Table 3 Ion specific conductivity's
Ion
Cone.
mg/L
Factor
pS/cm per mg/L
Conductivity
pS/cm
Ca'+
85.0 2.60 221.0
Mg*+
43.0 3.82 164.3
K+
2.9 1.84 5.3
Na+
92.0 2.13 196.0
HC03" 362.0 0.716 258.8
c r 131.0 2.14 280.3
SO42
" 89.0 1.54 137.1
N03 20.0 1.15 23.0
[Total 1285.8
Electrical Conductivity = 1285.8 fjS/cm = 1285.8 X 0.1 = 128.58
mS/m (Table 1).
8 Use of EC measurement
• Check purity of distilled or de-ionised water
Table 4 Gradation of water for laboratory use.
Gradation of Water Use of water EC (mS/m)
Type I use at detection limit of method <0.01
Type II routine quantitative analysis <0.1
Type III washing and qualitative analysis <1
• Relations with many individual constituents and TDS can be established.
The relationship between TDS (mg/L) and EC (pS/cm) is often described by a
constant, that varies according to chemical composition: TDS = A x EC, where A
is in the range of 0.55 to 0.9. Typically the constant is high for chloride-rich
waters and low for sulphate rich waters.
• Check deterioration of samples in time (effect of storage)
If EC is checked at time of sampling and again prior to analysis in the laboratory,
the change in EC is a measure for the 'freshness' of the sample.
5. Example 2
For the water sample given in the example 1, calculate TDS and the corresponding
constant 'A'.
Ion
Cone.
Ion
Mg/L
Ca^+
85.0
43.0
K+
2.9
Na+
92.0
HC03 362.0
CI 131.0
S04"- 89.0
N03" 20.0
I = 824.9
TDS in the sample = 824.9 mg/L. EC value = 1285.8 pS/cm.
TDS
824.9
= A x EC
= Ax 1285.8
= 0.64
6. OPERATION OF CONDUCTIVITY METER
Measurement of the conductivity
1 .Connect the conductivity electrode to the measuring instrument.
2.Press the < 0>key. (display test appears briefly on the display, after this, the
measuring instrument automatically switches to the measuring mode.)
3.Select the parameter (TDS, Salinity, Conductivity) by pressing <M>key.
4. Immerse the electrode in the water sample.
5. Press <AR> Key to activate auto read.
6. Press <Run/Enter> Key to start the auto read measurement, AR flashes on the
display until a stable measured value is reached, this can be terminated at any time with
<Run/Enter> Key.
Storing the data
1.Press the <STO>key in the measuring mode (display number with the number of the
next free memory location).
2. Press <Run/Enter>key.
3. Enter the ID number with <V >< A>.
4.Terminate the save with <Run/Enter>key.
To see the data memory
1 Press <RCL>Key.(display SEr disp)
2.Press <Run/Enter>key (display number at which data store)
3.Press <Run/Enter>key (display identification number).
4.Press <Run/Enter>key (display day, month)
5.Press <Run/Enter>key (display time)
6.Press <M>key to return in measuring mode.
Calibration
1.Connect the conductivity Electrode to the measuring instrument.
2.Immerse the electrode in the electrolyte solution provided with the instrument.
2.Press the <Cal>in the measuring mode (display Cell).
3. Press <Run/Enter>key (display CAL and cell constant value ,it should be 0.450-
0.500Cm"1
)
note :- At this point ,this procedure can be terminated with<M>.
Precautions
1. Always keep the electrode dry before and after use with absorbent paper.
Need to Calibrate after a fixed interval.
