1.
CHAPTER NINE
9. CONDUCTOMETRY
Conductometry measures electrolytic conductivity or
resistance of the solution located between two electrodes.
This property of the solutes is the result of the ion
movement under the influence of the electric field applied.
Only ionizable molecules (electrolytes) conduct electric
current. The magnitude of the conductivity depends on
the amount of ions presented in the solution.
The electrolyte kind, its concentration and temperature
affects both conductivity and specific conductance.
1
instrumental analysislecturenote

2.
Conductance is the extrinsic property while conductivity is
the intrinsic property.
This means that conductance is the property of an object
dependent of its amount/mass or physical shape and size,
while conductivity is the inherent property of the material
that makes up the object.
No matter how the object changes in terms of
shape/size/mass, as long as it is made of the same material
and the temperature remains the same, its conductivity does
not change.
2
instrumental analysislecturenote

3.
Conductance (L) is the reciprocal of the electric resistance R:
𝐿 =
1
𝑅
(1)
Conductance unit is Ω-1
(also called Siemens, S). Conductance of aqueous electrolyte solutions
is, as rule in the range of mS or µS. it is the ability of the solution to conduct electricity. The
conductance of the solution is the sum of the conductances of all of the ions that are in the
solution.
𝐿 = 𝐿𝑖 (2)
3
instrumental analysislecturenote

4.
In order to do conductance measurements, two metal electrodes (Pt) should be immersed in the
solution and connected to an alternating current (AC) voltage source (Figure 9.1). Under the effect
of the potential difference between electrodes, each ion in solution moves to the electrode of
opposite charge. According to Ohm’s law, the electric current in this circuit (including the
solution) is:
𝐼 =
𝐸
𝑅
= 𝐿𝐸 (3)
L depends on both solution properties (i.e. conductivity, k) and geometrical parameter of the cell
(i.e. electrode area, A, and distance between electrodes, l, Figure 9.1) according to the following
equation:
𝐿 = 𝑘
𝐴
𝑙
(4)
4
instrumental analysislecturenote

5.
Theratio(A/l) (incm)istermedasthe cell constant andcan bedeterminedbymeasuringthe
conductance(L)ofasolutionwithaknownconductivity.Conductivity(inΩ−1
cm−1
orderivate
units)dependsonsolutionproperties,suchasionconcentrations,ionchargesandionmobilities.
ItisworthnotingthatH+
andOH-
haveamuchhighermobilitycomparedtootherions(Table1).
5
instrumental analysislecturenote

6.
Figure 9.1. (A) Schematics of conductance measurement setup. (B) The conductometric
probe. 1, 2, and 3 are platinized platinum sheets; 1 and 3 are electrically connected
and form the two parts of a split electrode.
6
instrumental analysislecturenote

7.
Table 1. Mobility of ions in water at 25 oC.
Ion Mobility (m2/(s.V))o Ion Mobility (m2/(s.V))o
H+ 36.30 x10-8 OH- 20.50 x10-8
K+ 7.62 x10-8 SO4
2- 8.27 x10-8
NH4
+ 7.61 x10-8 Br- 8.13 x10-8
La3+ 7.21 x10-8 I- 7.96 x10-8
Ba2+ 6.59 x10-8 Cl- 7.91 x10-8
Ag+ 6.42 x10-8 NO3
- 7.40 x10-8
Ca2+ 6.12 x10-8 ClO4
- 7.05 x10-8
Cu2+ 5.56 x10-8 F- 5.70 x10-8
Na+ 5.19 x10-8 CH3CO2
- 4.24 x10-8
Li+ 4.01 x10-8 7
instrumental analysislecturenote

8.
Conductivity is widely used for estimating the overall ion content in
various sample of practical interest.
But conductivity values cannot indicate the concentration of a specific
ion in the sample.
Ion concentration can be determined by means of conductometric
titration.
The reaction between the ion of interest and the added reagent (i.e.
neutralization, precipitation, or formation of complex compound) brings
about a strong modification of solution conductivity.
The reagent should be added in the form of a standard solution and
the conductance (or conductivity) is measured as a function of the
added volume. This procedure is a conductometric titration.
With a properly selected reaction, the end point of the titration is
indicated by a particular point on the titration curve (i.e. conductance
vs. added reagent volume).
8
instrumental analysislecturenote

9.
Conductometric Titrations
The principle of conductometric titration is based on the fact that
during the titration, one of the ions is replaced by the other and
invariably these two ions differ in the ionic conductivity with the result
that conductivity of the solution varies during the course of titration.
The equivalence point may be located graphically by plotting the
change in conductance as a function of the volume of titrant added.
The main advantages to the conductometric titrationare its
applicability to very dilute, and coloured solutions and to system that
involve relative incomplete reactions.
 For example, which neither a potentiometric, nor indicator method
can be used for the neutralization titration of phenol (Ka = 10–10) a
conductometric endpoint can be successfully applied.
9
instrumental analysislecturenote

10.
Application: Acid-base titration, especially at trace levels. Relative
precision better than 1% at all levels. There are also few
disadvantages with this technique.
As you know the conductance is a non-specific property,
concentration of other electrolyte can be troublesome.
The electrical conductance of a solution is a measure of its currents
carrying capacity and therefore determined by the total ionic
strength.
It is a non-specific property and for this reason direct conductance
measurement are of little use unless the solution contains only the
electrolyte to be determined or the concentrations of other ionic
species in the solution are known.
 Conductometric titrations, in which the species in the solution are
converted to non-ionic for by neutralization, precipitation, etc. are of
more value.
10
instrumental analysislecturenote

11.
Some Typical Conductometric Titration Curves are:
1. Strong Acid with a Strong Base, e.g. HCl with NaOH: Before NaOH
is added, the conductance is high due to the presence of highly
mobile hydrogen ions.
When the base is added, the conductance falls due to the
replacement of hydrogen ions by the added cation as H+ ions react
with OH− ions to form undissociated water.
This decrease in the conductance continues till the equivalence point.
At the equivalence point, the solution contains only NaCl.
After the equivalence point, the conductance increases due to the
large conductivity of OH- ions (Figure 9.2).
11
instrumental analysislecturenote

12.
Figure 9.2. Conductometric titration of a strong acid (HCl) vs. a strong base (NaOH)
12
instrumental analysislecturenote

13.
• 2. Weak Acid with a Strong Base, e.g. acetic acid with NaOH: Initially
the conductance is low due to the feeble ionization of acetic acid.
• On the addition of base, there is decrease in conductance not only
due to the replacement of H+ by Na+ but also suppresses the
dissociation of acetic acid due to common ion acetate.
• But very soon, the conductance increases on adding NaOH as NaOH
neutralizes the un-dissociated CH3COOH to CH3COONa which is the
strong electrolyte.
• This increase in conductance continues raise up to the equivalence
point.
• The graph near the equivalence point is curved due the hydrolysis of
salt CH3COONa.
• Beyond the equivalence point, conductance increases more rapidly
with the addition of NaOH due to the highly conducting OH− ions
(Figure 9.3).
13
instrumental analysislecturenote

14.
Figure 9.3. Conductometric titration of a weak acid (acetic acid) vs. a strong base (NaOH)
14
instrumental analysislecturenote

15.
3. Strong Acid with a Weak Base, e.g. sulphuric acid with
dilute ammonia:
Initially the conductance is high and then it decreases due
to the replacement of H+. But after the endpoint has been
reached the graph becomes almost horizontal, since the
excess aqueous ammonia is not appreciably ionized in the
presence of ammonium sulphate (Figure 9.5).
15
instrumental analysislecturenote

16.
Figure 9.5. Conductometric titration of a strong acid (H2SO4) vs. a weak base (NH4OH)
16
instrumental analysislecturenote

17.
4. Weak Acid with a Weak Base: The nature of curve before the
equivalence point is similar to the curve obtained by titrating weak
acid against strong base. After the equivalence point, conductance
virtually remains same as the weak base which is being added is
feebly ionized and, therefore, is not much conducting (Figure 9.6).
• Figure 9.6. Conductometric titration of a weak acid (acetic acid) vs. a weak
base (NH4OH)
17
instrumental analysislecturenote

18.
5. Mixture of a Strong Acid and a Weak Acid vs. a Strong Base or a
Weak Base: In this curve there are two break points.
 The first break point corresponds to the neutralization of
strong acid. When the strong acid has been completely
neutralized only then the weak acid starts neutralizing.
The second break point corresponds to the neutralization of
weak acid and after that the conductance increases due to
the excess of OH− ions in case of a strong base as the titrant.
However, when the titrant is a weak base, it remains almost
constant after the end point similar to Figure 9.6 (Figure 9.7).
18
instrumental analysislecturenote

19.
Figure 9.7. Conductometric titration of a mixture of a strong acid (HCl) and a weak acid
(CH3COOH) vs. a strong base (NaOH) or a weak base (NH4OH)
19
instrumental analysislecturenote

20.
Calculations
Corrections
During the titration process, the sample gets diluted by added titrant solution. A correction should
be therefore applied to each conductance reading. If it assumed that the conductance is proportional
to the total ion concentration, C (k = a proportionality constant):
𝐿 = 𝑘𝐶 = 𝑘
𝑛
𝑉
(5)
Here, n is the total number of ion moles and V is solution volume.
The conductance at a given moment of the titration is:
𝐿𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 = 𝑘𝑛/(𝑉0 + 𝑉
𝑎 ) (6)
where 𝑉0 is the initial volume of the titrated sample and 𝑉
𝑎 stands for the added volume of titrant.
In the absence of dilution, the corrected conductance assumes the corrected value which is:
𝐿𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝑘𝑛/𝑉0 (7) 20
instrumental analysislecturenote

21.
From Equations (6) and (7) it results:
𝐿𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝐿𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑
(𝑉0+𝑉𝑎 )
𝑉0
(8)
This correction should be applied to each measured values and corrected conductance should be
plotted against 𝑉
𝑎 in order to get the titration curve.
Calculating analysis result
The amount of analyte in the whole sample can be calculated by means of the well-known
equation:
𝑚𝑋 = 𝑉
𝑒 𝐶𝑀𝑋
𝑉𝑠
𝑉0
(9)
Here, 𝑉
𝑒 is the equivalence volume (in liter), C is the molar concentration of titrant
and 𝑀𝑋 is the mol weight of the analyte (g/mol), 𝑉0 is the volume of the titrated
aliquot (sample) and 𝑉
𝑠 is the overall volume of the sample.
21
instrumental analysislecturenote

22.
Applications of Conductometric Titrations
Acid-base titrations redox titrations are known to us in which commonly indicators are used to
locate the end point e.g., methyl orange, phenolphlthalene for acid base titrations and starch
solutions for iodemetry type redox process. However, electrical conductance measurement can be
used as a tool to locate the end point.
 This method can be used with much diluted solutions.
 This method can be used with colored or turbid solutions in which end point cannot be
seen by eye.
 This method can be used in which there is no suitable indicator.
Has many applications, i.e. it can be used for acid base, redox, precipitation, or complex titrations
 Determination of sulphur dioxide in air pollution studies
 Determination of soap in oil
 Determination of accelerators in rubber
 Determination of total soap in latex
 Specific conductance of water
22
instrumental analysislecturenote

23.
23
instrumental analysislecturenote

24.
24
instrumental analysislecturenote