Potentiometry
Potentiometry is an Electroanalytical technique which involves measurement of potential of
electrochemical cells without drawing appreciable current and relating it to concentration of species.
is called. Potentiometric methods embrace two major
1. Direct Potentiometry
2. Potentiometric titration types of analysis.
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In direct Potentiometry, emf of the cell is directly related to the activity of species through
Nernst’s equation.
In potentiometric titration variation of emf of the cell with the addition of the titrant is
measured and the end point of titration is determined.
Every potentiometric measurement requires a construction of electrochemical cell consisting of a
stable reference electrode and sensitive indicator electrode and a salt bridge. The typical
electrochemical cell for potentiometric analysis may be represented as follow.
Reference electrode/salt bridge/analyte soln./Indicator electrode
Thus the different components of potentiometric cell are:
1.
2.
3.
4.
Reference electrode
Salt Bridge
Analyte
Indicator electrode
A schematic diagram of an electrochemical cell is given below:
Reference
electrode
Indicator
Electrode
The part of the cell
that is held
constant and it
potential is
constant through
the potentiometric
analysis
The part of the
cell that contains
the solution we
are interested to
analyse.
Figure 1: Atypical electrochemical cell for potentiometric measurement.
The emf of such cell is
Ecell = Eind - Eref
The reference electrode is a half cell with known electrode potential and its potential does not
change during potentiometric measurement. It is left hand electrode by convention. The
characteristics of reference electrode are
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

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It should be easily assembled.
It should have stable and constant potential (Eref)
It should be reversible and obeys Nernst equation
It should be insensitive to analyte concentration
The indicator electrode generates a potential (Eind) that depends on analyte concentration. The
indicator electrode should have following characteristics:



It should generate potential that depends on analyte concentration and obeys Nernstian
equation.
It should be selective to the analyte.
Response should be rapid and reproducible.
Salt Bridge is the third component of potentiometric cell that connects reference electrode and
indicator electrode without mixing analyte with reference electrode solution. Salt bridge generate
negligible junction potential
Direct Potentiometry
In direct Potentiometry, emf of the cell is directly related to the activity of species through Nernst
equation. Sometime the instrument is directly calibrated and the concentration of analyte is directly
read out. Application of glass electrode for determination of pH is the most widely used direct
potentiometry in which the pH-meter/glass electrode assembly is calibrated and the pH is directly
read out from the instrument. Owing to the simplicity and free from interference the glass electrode
has been most used in almost all analytical, biochemical and agricultural laboratories for pH
measurement.
The determination of pH by pH meter requires the calibration of pH meter/glass electrode assembly
that involves the following steps:
1. The glass electrode is first dipped into a buffer solution of pH = 4.
2. The temperature in the pH meter is set to the laboratory temperature.
3. Then pH meter is adjusted till the reading in the pH meter is exactly = 4.
4. Then buffer solution of pH 10 or 9.2 is taken in another beaker and rinsed glass
electrode is dipped into it and slope adjustment knob is adjusted so that pH reading
in the pH meter is equal to the buffer solution.
5. This process is known as Calibration. Once the glass electrode is calibrated, the pH
meter is ready for measuring pH of other solutions.
Calibration Method
For direct potentiometric determination concentration of ions, using indicator electrode a
calibration curve needs to be constructed which involves the following steps:
1. A series of standard solutions of known concentration are prepared.
2. To each of standard solution ionic strength buffer is added so that all the solutions have
same ionic strength.
3. The solution is transferred to a potentiometric cell and overall emf of the cell is measured
potentiometrically for each solution with indicator and reference electrodes assembly.
4. A calibration curve is then constructed by plotting emf of the cell as a function of logarithm
of concentration.
5. The solution of unknown concentration is then kept in potentiometric cell along with ionic
strength buffer and emf of the cell is measured and the concentration of the unknown
solution is determined from calibration curve.
Standard addition method
Sometime, standard addition method is employed to determine concentration of the species by
direct potentiometry. The steps involved for this method are:
1. An accurately known volume of the sample solution into the cell.
2. The emf of the cell with the reference electrode is measured.
3. Known volume of standard solution of relatively high concentration is added so that there is
negligible change in the volume of the solution, and emf of the cell is measured.
4. Then by knowing the change in emf of the cell, volume of the original sample, and volume
and concentration of standard solution one can calculate the concentration in original
solution by using Nernst equation.
Owing to the unavailability of commercial ion selective electrodes for other ions the use of direct
potentiometry was limited to glass electrode for pH measurement. In 1966, Frank and Rose
developed LaF3 single crystal doped with europium (II) as a fluoride ion selective electrode for
measurement of fluoride ion in solution. These days more than two dozen ion selective electrodes
for different ions are available commercially. Active research in the development ion selective
electrodes is going on all over the world including Nepal. Low costs silver sulphide based electrodes
have been developed in Nepal at Central Department of chemistry, TU under the leadership of Prof.
Dr. Raja Ram Pradhananga which is finding application in environmental analysis and education.
These days the uses of ion selective electrodes have been greatly increased in bio-analytical
chemistry, medicinal chemistry and many other fields. Relatively recently ion selective electrodes
have been used to study binding of sodium ion in brain tissue, diffusion of potassium and sodium
through nerves and muscles membrane, composition of cervical spinal fluid, blood coagulation and
intestinal fluid and blood carbonate/bicarbonate level.
Potentiometric Titration
Potentiometric titration is very powerful electro analytical techniques. In potentiometric
titration involving acid-base reaction, redox reaction, precipitation reaction or complex formation
reaction can be performed. In all potentiometric titration, a potentiometric cell with suitable
indicator electrode and a reference electrode is coupled with salt bridge is constructed( figure 1) and
emf of the cell is measured as a function of volume of titrant added. The end point is then
determined by all or any of the following three methods.
1. Plot EMF of the cell against volume of the titrant. At the end point there will be an
inflection.
2. Plot ∆E/∆V against V: The peak in this plot corresponds to the end point.
3. ∆E2/∆V2 against V: The point at which this second derivative curve cuts the zero axis
is the end point
.
Gran has developed an analytical method to locate the end point. This technique is known as Gran's
plot. With the Gran's plot the end point can be located more precisely without ambiguity.
1. Acid-base Titrations
Suppose we are required to
titrate HCl solution against NaOH
solution. The acid solution is taken in a
breaker, glass electrode is introduced in
it.The electrode is then coupled with
standard calomel electrode as shown in
fig.11 NaOH solution is then added to
the beaker from a burette first in larger
amounts (say 1.0 ml at a time) and then
in smaller amounts (say 0.1 ml) towards
the end point. After each addition, the
emf of the cell is recorded. The emf is
then plotted against the volume of the
alkali added. The shape of the titration
curve obtained in the case of the strong
acid versus strong base is shown in fig.
2. From the curve, the volume of the
alkali required for the equivalence point
can be obtained.
In case steepness of the curve is less pronounced, the can be obtain from the first differential
curve (fig.3(b))or second differential curve.
ii. Redox Titrations
Titration involving redox reactions can be followed potentiometrically. For example, if Fe++
solution is to be titrated against KMnO4 solution, the ferrous sulphate solution is taken in the beaker,
and a clean platinum strip is immersed into it. The electrode thus formed is coupled with a saturated
calomel electrode. KMnO4 solution is added from the burette. The emf of the cell is noted after each
addition. The end point of the titration is obtained in a manner similar to that in the case of acidbase titrations.
iii. Precipitation Titrations
Potentiometric titrations involving precipitation reaction (i.e. AgNO3 versus NaCl etc.) can also
be done more or less similarly.
iv. Complexometric titration
Potentiometric titrations involving complex formation reaction can also be done more or less
similarly. One can follow the titration of metal ion with EDTA by utilizing the indicator electrode of of
the parent metal. But unfortunately most metallic electrode do not respond in accordance with
Nernst equation and many metal electrodes are sensitive to dissolve oxygen and extraneous species
Advantages of Potentiometric Titrations
The following are the some of the advantage of potentiometric titration
1. Potentiometric titration can be performed where the analyte solution is colored, turbid or
fluorescent.
2. Potentiometric titration can be performed where no suitable indicator or color change is
difficult to determine.
3. Potentiometric titration of polyprotic acids and mixtures of strong acid and weak acid or
bases can be performed
4. Potentiometric titration can be automated with end point data stored in a computer.