4
CONDUCTIVITY
4.1 General Principles
In general, conductivity refers to the ability of a substance to conduct heat or electricity – it is the
opposite of resistance. In the context of this subject, it will refer the ability of a solution to conduct
electricity. In a solution, the conduction between electrodes is carried out by movement of ions to the
electrodes: anions to the positive electrode, and cations to the negative. The ability of a solution to
conduct is determined by a number of factors, including:

the number of ions – the more ions, the greater the conductivity

the nature of the ions – certain ions have greater conductivity per mole than others due to charge
and size

the dimensions of the electrodes – the greater the area of the electrodes, the greater the
conductivity
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the distance between the electrodes – the closer together the electrodes, the greater the
conductivity

temperature – the higher the temperature, the greater the conductivity
While conductivity is the property, conductance (G) is the actual measured value. The unit of
conductance is the Siemen (S).
4.2 Conductivity Apparatus
Various types of conductivity cells are commercially available, their actual design being dependent on
the use to which they will be put. They all have in common a pair of platinum electrodes.
The simplest and most commonly used cell is a dip
cell, shown in Figure 4.1. The bottom of the cell is open
and there are also holes in the sides to allow ready flow of
the sample liquid into the space between the electrodes.
The cell is very simply to maintain: it can be stored
dry, and only needs rinsing at the end of use. The
electrode surfaces should not be cleaned with anything
abrasive, because this can affect their performance. If
necessary, cleaning of the electrode is done with a
potassium dichromate/sulfuric acid mixture.
The manufacturer will generally supply the cell
constant for the electrode. This is a numerical value
which describes the physical properties (surface area and
distance between electrodes). The cell constant can be
determined by calibration, and should not change by more
than 5% over the lifetime of the cell, unless it is
FIGURE 4.1 Dip-type conductivity cell
physically damaged. The cell affects the measured
conductance value, because of its physical dimensions. This will be discussed more in the next
section.
A conductance meter, to which the cell is attached, is designed around the electrical circuit known as
a Wheatstone bridge. This is a combination of resistors where the resistance of one half is defined by
the cell, and the other half by a pair of variable resistors. The bridge is adjusted manually or
electronically so that the resistance of each half is equal. This indirectly determines the resistance of
the cell. The power supply provides alternating (AC) current to avoid the development of currents
that would cause a change in the solution composition.
4.4 Conductometric Titrations
This is the conductivity equivalent of the potentiometric titrations from Chapter 2. In this case, a
conductivity cell is used to monitor changes in the solution conductance, and allow us to determine
the endpoint volume.
Conductance
The method relies on the conductance of the reactants and products of the titration reaction
being significantly different, so that the trend in conductance either side of the endpoint is different.
A typical conductometric titration curve is shown in Figure 4.2.
endpoint volume
Titrant volume
FIGURE 4.2 Typical conductometric titration curve (with endpoint detection method shown)
Enough values are obtained on either side of the equivalence point zone to accurately locate the
equivalence point volume. Unlike potentiometric titrations, here there is no need to take values
carefully at the equivalence point (or even close to it) as the linear regions either side of the
equivalence point define the end point through their point of intersection, as shown in Figure 4.2. In
fact, values very close to the end point don’t help in obtaining the result. For a normal titration, data
points 1 mL apart are totally adequate for accurate endpoint detection. There is no need to change
addition volumes throughout the titration, making it much easier to actually perform than a
potentiometric titration.
Each measured conductance value needs to be corrected for dilution as a result of addition of
the titrant, as shown in Equation 4.4. If this is not done, the sharp changes in conductance will be
somewhat masked by the dilution, and the graph will be distorted.
Gcorr 
Gobs( V  v )
V
Eqn 4.4
where Gobs and Gcorr are the measured and corrected conductances, V is the initial solution volume
(before addition and including water) and v is the volume of added titrant.
Advantages and disadvantages
Conductivity and voltage measurements are direct competitors as endpoint detection methods.
Compared to indicator-based methods, they have similar merits and problems (see Chapter 2). But
compared to each other, which is better? Well of course, the answer is that each has its own
advantages and disadvantages.
The advantages of conductometric titrations, compared to potentiometric titrations, are:




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they are easy to perform (no special care needs to be taken around the end point,
the same electrode is suitable for a range of analyses (acid/base and precipitation)
the electrode is simple to maintain (compared to indicator and reference electrodes which can
dry out, and be easily damaged),
the response is faster (especially around endpoint),
endpoint detection is easier (especially for weak acids or bases),
they work better in very dilute solutions, and
they work better in turbid solutions.
The disadvantages of conductometric titrations, compared to potentiometric titrations, include:

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they don’t work well in more concentrated solutions or in solutions with high background
conductivity of non-reactive species,
they aren’t useful for complexometric and redox titrations,
corrections are necessary for volume dilution effects, and
graphing of all data is necessary for endpoint detection (can’t use titration to a certain value, or
first derivative methods).
In summary, the use of conductometric titrations could be greater, due to their comparative
advantages over potentiometric titrations, particularly for analysis of weak acids and bases. Why they
aren’t more widely used possible stems from the need for graphing and the great availability of pH
electrodes.
Acid-Base Titrations
The high ionic conductances of H+ and OH- make conductometric monitoring of an acid-base titration
an ideal method of detecting the endpoint. Loss or gain of either these species will cause a significant
change in solution conductance, and therefore a clear endpoint from the curve. Figure 4.3 shows
representations of the titration curves for acid/base titrations.
(a)
G
(b)
G
Titrant volume
Titrant volume
FIGURE 4.3 Acid-base conductometric titration curves. (a) strong acid/strong base (b) weak acid/strong base
or weak base/strong acid.
Precipitation titrations
These are equally suited to conductometric titration, since the reaction relies of the removal of an ion
from solution. In this case the spectator ion – the one that simply adds to the solution without being
part of the reaction – should have a low conductivity where possible, so that its presence does not
mask the changes.