Potentiometric Methods of Analysis.pptx

1. Instrumental Analysis Week 16 Dr. Mohamed Babiker
Week 16
Potentiometric Methods of Analysis
In a potentiometric measurement two electrodes are used. These
consist of the indicator or sensing electrode, and a reference
electrode.
Electroanalytical measurements relating
potential to analyte concentration rely on
the response of one electrode only (the
indicator electrode).
The other electrode, the reference
electrode is independent of the solution
composition and provides a stable
constant potential. The open circuit cell
potential is measured using a potential
measuring device such as a
potentiometer, a high impedance
voltameter or an electrometer.
A
B
Reference
electrode
Indicator
electrode
Solution containing
analyte species
Potential reading
Device (DVM)

2. 2
There are two commonly used instruments for making potentiometric
measurements.
The potentiometer is a device which is normally used for the
measurement of potentials in low resistance circuits and as a result is
only rarely applied.
The pH meter, which is a voltmeter, is a voltage measuring device
designed for use with high resistance glass electrodes and can be
used with both low and high resistance circuits. During a measurement
the voltage is converted to a current for amplification via an ac circuit
and these are therefore high input impedance devices. (Impedance in
an ac circuit is similar to resistance in a dc circuit). Due to the high
input resistance very little current flows during the measurement,
typically 10-13 to 10-15 A, hence the chemical equilibrium remains
relatively undisturbed and the criteria for applying the Nernst equation
are retained. For convenience when making pH measurements, the
voltage reading can be converted directly to pH units.
The Potentiometer and pH Meter

3. 3
Fundamentals of Potentiometric Measurement:
The potential of the indicator electrode is related to the activities of one or
more of the components of the test solution and it therefore determines
the overall equilibrium cell potential Ee .
H2(g)
H+(aq)
Pt
Ee
A(aq)
B(aq)
H2 in
reference
electrode
SHE
indicator
electrode
electron
flow
salt bridge
test analyte
redox couple
Pt

4. 4
Under ideal circumstances, the response of the indicator electrode to
changes in analyte species activity at the indicator electrode/ solution
interface should be rapid, reversible and governed by the Nernst
equation.
The electron transfer (ET) reaction involving the analyte species should
be kinetically facile and the ratio of the analyte/product concentration
should depend on the interfacial potential difference via the Nernst
equation.
The net cell potential at equilibrium is given by the expression across
where Eind denotes the potential of the indicator electrode, Eref denotes
the reference electrode potential and Ej is the liquid junction potential
which is usually small.
 Ej
Ecell  Eind  Eref

5. There are two ways in which the output from potentiometric
measurements can be used analytically:
 Directly- Direct Potentiometry
 Relatively- Potentiometric Titrations
Direct potentiometry provides a rapid and convenient method of
determining the activity of a variety of cations and anions. The
technique requires only a comparison of the cell potential developed
between the indicator and reference electrodes, when immersed in
the analyte solution compared to that developed when immersed in
one or more standard solutions of known analyte concentration The
best example of this, is of course, the measurement of pH using a
typical pH meter calibrated against two buffer solutions.
Quantitative Applications of Potentiometry
5

6. Direct Potentiometry- Glass pH Electrodes
Typical pH meter
Typical commercial
Glass electrode
The glass pH electrode is used almost universally for pH
measurements and can be found in a range of environments
including hospitals, chemical plants, and forensic
laboratories. Its attraction lies in its rapid responses, wide pH
range, functions well in physiological systems and is not
affected by the presence of oxidising or reducing species.
pH   log10 aH 

7. All pH electrodes require calibration prior to use. This usually takes the
form of a two point calibration using appropriate buffer solutions. For
example to calibrate the electrode for acidic measurements it is usual to:
 Use a pH = 7.0 buffer (typically a phosphate buffer)
 A pH = 4.0 buffer (typically phthalate solutions)
For alkaline measurements the recommended buffers are:
 A pH = 7.0 buffer
 A pH=10.0 buffer.
All of these buffers are generally purchased from the manufacturers and
are based on the NIST (National Institute of Standards and Technology)
certified standard buffers. Prior to calibrating the pH electrode it is
important to adjust the temperature to compensate for temperature
effects. Some pH meters include a temperature probe which allows for
automatic temperature compensation (ATC).
Direct Potentiometry- Calibrating pH Electrodes
7

8. Potentiometric Titrations
A potentiometric titration involves measurement of the potential of a
suitable indicator electrode as a function of titrant volume. The
measurement is based on the titrant volume that causes a rapid change
in potential near the equivalence point. Potentiometric titrations provide
data that are more reliable than data from titrations that use chemical
indicators. They are particularly useful with colored or turbid solutions.
 Determination of acid with base (pH).
 Determination of Cl–
with AgNO3
(AgCl, Ag+
ISE).
 Determination of Al3+
with NaF
(AlF6
3–
, F–
ISE).
 Determination of Fe2+
with K2Cr2O7
(Fe3+
, Pt).
8

9. Detection of the end point
A direct plot of potential as a function of reagent volume generates a
titration curve. The inflection point in the steeply rising portion of the
curve, is taken as the end point.
9

10. Differential curves:
In weak solutions and titrations involving slow reactions, the titration
curve may be flatter and difficult to interpret. The problem can
simplified by using equipment that records the first or second
differential, or both of the titration curve.
Calculate the change in potential
per unit volume of titrant (i.e.,
ΔE/ΔV). If the titration curve is
symmetrical, the point of
maximum slope coincides with
the equivalence point.
The first derivative is approximated as
10

11. Types of Potentiometric Titration
Depending on the type of the reactions involved to which potential
measurement can be applied for end point detection, potentiometric
titrations can be classified into followings:
1. A cid-Base Titration
2. Complexometric Titration
3. Oxidation-Reduction Titration
4. Precipitation Titration
In all potentiometric titration the change in electrode potential upon the
addition of titrant are noted by the volume of titrant added. At the end
point the rate of change of potential is maximum, the potentiometric
end point has been applied to all types of chemical reaction. It can be
used with colored or opaque solution.
11

12. A cid-Base Titration
The neutralization of acid or base is always accompanied by the
changes the concentration of H+
and OH–
ions.
In these reactions hydrogen electrode is used as indicator electrode
and N-calomel electrode as a reference electrode.
A known volume of the acid titrant is kept in a beaker with continuous
stir; the hydrogen and N- calomel electrode are connected by the salt
bridges and connected to a potentiometer which record the EMF of the
solution, into the beaker.
After the addition of base from burette the values of EMF are plotted
against volume of titrant added and a curve are obtained, the point
where E.M.F increased rapidly is the end point.
12

13. Volume Of
Base (mL)
pH
2 3.24
4 3.54
6 3.78
8 3.98
10 4.20
12 4.47
14 4.75
16 5.20
18 6.60
19.3 10.50
21 11.80
22.5 12.30
23.5 12.35
1. Plot the titration curve find the end
point .
2. Plot the first derivative curve and
locate the end point.
The following sample data models the
titration of a weak acid with a strong base.
Example
13

14. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
0
2
4
6
8
10
12
14
Series1
Volume of Base (ml)
pH
pH vs Volume
1. Plot the titration curve find the end point
14

15. Volume (mL) pH ΔV ΔpH ΔpH/ΔV
2 3.24
4 3.54 2 0.3 0.15
6 3.78 2 0.24 0.12
8 3.98 2 0.2 0.1
10 4.2 2 0.22 0.11
12 4.47 2 0.27 0.135
14 4.75 2 0.28 0.14
16 5.2 2 0.45 0.225
18 6.6 2 1.4 0.7
19.3 10.5 1.3 3.9 3
21 11.8 1.7 1.3 0.765
22.5 12.3 1.5 0.5 0.333
23.5 12.35 1 0.05 0.05
2. Plot the first derivative curve and locate the end point.
15

16. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0
0.5
1
1.5
2
2.5
3
3.5
Volume of Base (ml)
First derivative curve (ΔpH/ΔV VsV)
ΔpH/ΔV
End point = 18.7ml
16

17. 17
There are number of advantages offered by potentiometric indicators
over visual indicators to follow the progress of titrimetric reactions and
detect end- points. These are:
 Ability to function is highly coloured solutions;
 Ability to find multiple end-points when samples contain more than
one titratable species. For instance, a sample containing both
weak and strong acids or polyprotic acids (eg: orthophosphoric
acid H3PO4) where there is a significant difference between the K
values of the titratable protons. See example (9.1) on the next slide
 Offers opportunities for automation for both detection of end-points
and for the analysis of multiple samples dispensed from auto-
samplers.
Advantages of potentiometric over visual indicators

18. Assignment
Plot the data
to Determine the end
Point.