Potentiometry

What is potentiometry-an introduction

1. 1 POTENTIOMETRY
Table of Contents
1. History………………………………………………………………………………………………………………………………………….2
2. Introduction………………………………………………………………………………………………………………………………...2
2.1. Electrochemical Cell….…………………………………………………………………………………………………………..2
3. Potentiometry…………………………………………………………………………………………………………………………… …3
3.1. Definition…………………………………………………………………………………………………………………………… …3
3.2. Principle…………………..………………………………………………………………………………………………………… …3
4. Instrumentation………………………………… …………………………………………………………………………………………4
4.1. Potentiometric Analysis…………………………………………………………………………………………………… ……4
4.2. Types of Potentiometers………………………………………………………………………………………………… ……..5
4.3. Working of Potentiometer (Constant Current Potentiometer)………………………………… ……………..6
5. Applications…………………………………………………………………………………………………………………………… …….6
6. References…………………………………………………………………………………………………………………………… ………7

2. 2 POTENTIOMETRY
1. History
Potentiometry Timeline
Shown below are major milestones in the development of Potentiometry.


2. Introduction
2.1. Electrochemical cell
Potentiometry is one type of electrochemical analysis methods. Electrochemistry is a part of
chemistry, which determines electrochemical properties of substances. An electrical circle is required
for measuring current and potential (voltage) created by movement of charged particles. Galvanic cell
(Fig. 1) serves as a sample of such system.









Electrochemical cell consists of two solutions connected by salt bridge and electrodes to form
electrical circle. Sample cell on figure 1 consists of solutions of ZnSO4 and CuSO4. Metallic Zn and Cu
electrodes are immersed in respective solutions. Electrodes have two contacts:
Galvanometer

3. 3 POTENTIOMETRY
a) Through wires connected to voltmeter;
b) Through solutions and salt bridge.
Salt bridge consists of a tube filled with saturated salt solution (e.g. KCl solution). The ends of the tube
are capped with porous frits that prevent solutions from mixing, but permit movement of ions.
Three distinct charge transfer processes are described for the system in Fig 1:
1. Electrons move in electrodes and wires from zinc electrode to copper electrode.
2. Ions move in solutions:
a) In solution on the left, zinc ions move away from the electrode and sulfate ions move towards it.
b) In solution on the right, copper ions move towards the electrode and negatively charged ions
(sulfate) away from it.
c) In salt bridge positive ions move right and negative ions left.
3. On the surfaces of electrodes electrons are transferred to ions or vice versa:
a) Zinc electrode dissolves: Zn → Zn
+
+ 2e
-

b) Metallic copper is deposited on the electrode surface: Cu
2+
+ 2e
-
Cu
Three processes mentioned above form a closed electrical circle making the flow of electrical current
possible.
Potential on an electrode depends on the ions present in the solution and their concentration. This
way electrochemical cells can be used to determine ions and their concentration in solution. The
dependence of potential between electrodes from concentration of ions is expressed by Nernst
equation (Eq. 1).

3. Potentiometry
3.1. Definition
It is the method of electroanalytical chemistry and is used to measure the electric potential of
different substances or metallic systems, pH, activity co-efficient and concentration of solute.
3.2. Principle
Potentiometric methods of analysis are based upon measurements of the potential of electrochemical
cells under conditions of zero current.
The measured potential may be used to determine the the concentration of some component of the
analyte solution. The potential difference between the cathode electrode potential and the anode
electrode potential is the potential of the electrochemical cell (Eq.2).

4. 4 POTENTIOMETRY











The typical cell for potentiometric analysis can be depicted as:
Reference electrode⏐salt bridge⏐analyte solution⏐indicator electrode

Eref Ej Eind
Eref (electrolyte reference) is independent of the concentration of analyte or any other ions in the
solution.
Eind (electrolyte indicator) depends upon the activity of the analyte.
Equation: Ecell = Eind − Eref + Ej (Eq.2.)
(For most electroanalytical methods, the junction potential so small that can be neglected.)
The salt bridge prevents components of analyte solution from mixing with those of reference solution.
Two potentials develop on both sides of salt bridge across the junction which tend to cancel one
another if same number of cations and anions move to opposite sides of salt bridge. Net Ej is,
therefore, usually less than a few millivolts. This uncertainty in junction potential limits the accuracy
of potentiometric analysis.
4. Instrumentation
4.1. Potentiometric Analysis
 Reference electrode
– A half-cell with an accurately known electrode potential, Eref, that is independent of the
concentration of the analyte or any other ions in the solution
– Always treated as the left-hand electrode.



Fig.1. Typical Electrocanalytical Cell (Danial Cell)

5. 5 POTENTIOMETRY
 Ideal Reference Electrode
– Has a potential that is accurately known, constant and completely insensitive to the
composition of the analyte solution
--Different types of reference electrodes are present:
 Standard Hydrogen Electrode
 Calomel Reference Electrode
 Silver/Silver Chloride Reference Electrode
 Indicator electrode
– Which is immersed in a solution of the analyte, develops a potential, Eind, which depends
on the activity of the analyte.
– Is selective in its response
 Ideal Indicator Electrode
– Responds rapidly and reproducibly to changes in the concentration of an analyte ion (or
groups of analyte ions).
--Different types of indicator electrodes are present:
 Metallic Electrodes
o Metal Electrodes of the 1st Kind
o Metal Electrodes of the 2nd Kind
o Metal Electrodes of the 3rd Kind
o Redox Electrodes
 Membrane Electrodes
o Ion Sensitive Electrodes (ISE)
o Molecular selective electrodes (MSE)
 Salt bridge
– Preventing components of the analyte solution from mixing with those of the reference
electrode
– A potential develops across the liquid junctions at each end of the salt bridge.
– Potassium chloride is an ideal electrolyte for the salt bridge because the mobilities of
the K + ion and the Cl ion are nearly equal.

4.2. Types of Potentiometer
1. Constant Resistance Potentiometers
The constant resistance potentiometer in which a variable current is fed through a fixed resistor.
These are used for measurements in the millivolt and microvolt range.
2. Microvolt Potentiometers
This is a form of the constant resistance potentiometer but is made to minimize the effects of
constant resistance and thermal EMF. These are for readings of 1000 nV.
3. Thermocouple Potentiometer
A thermocouple is a device used extensively for measuring temperature. Learn how the device
works here. A thermocouple is comprised of at least two metals joined together to form two
junctions. One is connected to the body whose temperature is to be measured; this is the hot or

6. 6 POTENTIOMETRY
measuring junction and the other junction is connected to a body of known temperature; this is
the cold or reference junction. Therefore the thermocouple measures unknown temperature of
the body with reference to the known temperature of the other body.
4.3. Working of Potentiometer (Constant Current Potentiometer)
In the circuit (Figure 2), the two ends of uniform resistance wire (R1) is connected to Voltage DC
supplier which further divides the voltage. The potentiometer is first calibrated by setting the
wiper (needle) at that spot on the R1 wire which matches to the voltage of a standard cell so
that
A standard electrochemical cell with known EMF is used i.e. Weston Standard Cell of voltage
1.0183. The Voltage Source is then used to remove the zero error from Galvanometer which
indicates that the voltage on R2 is equal to standard cell voltage. An unknown DC voltage is
connected in series with galvanometer across a variable-length section R3 of the resistance wire.
The moving of the needle (wiper) indicates that the current is flowing. As soon as needle (wiper)
stop moving, this indicates that no current is flowing into or out of it as shown by galvanometer.
The voltage across the selected R3 section of wire is then equal to the unknown voltage. The final
step is to calculate the unknown voltage from the fraction of the length of the resistance wire that
was connected to the unknown voltage.
If the length of the R1 resistance wire is AB, where A is the (-) end and B is the (+) end, and the
movable wiper is at point X at a distance AX on the R3 portion of the resistance wire when the
galvanometer gives a zero reading for an unknown voltage, the distance AX is measured or read
from a preprinted scale next to the resistance wire. The unknown voltage can then be
calculated:
5. Applications of Potentiometry
I. Clinical Chemistry
Ion-selective electrodes are important sensors for clinical samples because of their selectivity for
analytes in complex mediums. The most common analytes are electrolytes, such as Na
+
, K
+
, Ca
2+
,
H
+
, and Cl
-
, and dissolved gases such as CO
2
.
II. Environmental Chemistry
For the analysis of CN
-
, F
-
, NH3, and NO3
-
in water and waste water. One advantage of an ion-
selective electrode is its ability to incorporate into a flow cell for the continuous monitoring of
waste-water streams.
III. Potentiometric Titrations
For determining the equivalence point of an acid–base titration and is done for acid–base,
complexation, redox, and precipitation titrations, as well as for titrations in aqueous and
nonaqueous solvents.
IV. Agriculture
For determining NO
3
, NH
4
, Cl, K, Ca, I, CN in soils, plant material, fertilizers and feedstuffs.
V. Detergent Manufacture
For determining Ca, Ba, F and studying effects on water quality.

7. 7 POTENTIOMETRY
VI. Food Processing
For determining NO3, NO2 in meat preservatives.
Salt content of meat, fish, dairy products, fruit juices, brewing solutions.
F in drinking water and other drinks.
Ca in dairy products and beer.
K in fruit juices and wine making.
Corrosive effect of NO3 in canned foods

6. References
1. Cremer, M, Z. Biol. 1906, 47, 562.
2. Buck, R.P. and Lindner, E. Anal. Chem. 2001, 73, 88A.
3. Frant, M.S., Analyst, 1994, 119, 2293.
4. Frant, M.S. and Ross, J.W., Science, 1966, 154, 1553.
5. Ross, J.W., Science, 1967, 156, 1378.
6. Simon, W., Swiss Pat., 479870, 1969.
7. Frant, M.S. and Ross, J.W., Science, 1970, 167, 987.
8. Potentiometry, Ch.15 http://jan.ucc.nau.edu/~jci5/courses/chm320/Lecture%2026%20Ch15.pdf
9. Electrochemistry (potentiometry), Meditsiiniline keemia/Medical chemistry LOKT.00.009,
http://tera.chem.ut.ee/~koit/arstpr/pot_en.pdf
10. What is a Thermocouple & How Does it Work?, Haresh Khemani • edited by: Lamar
Stonecypher, 2013, http://www.brighthubengineering.com/manufacturing-technology/53682-
what-is-a-thermocouple-how-thermocouple-works/
11. Kenyon.edu Dept of Physics. Thermodynamics: Thermocouple Potentiometer.
12. Potentiometry, http://memo.cgu.edu.tw/hsiu-po/Analytical%20Chem/Lecture%207.pdf
13. Definition of Potentiometry, http://www.chemicool.com/definition/potentiometry.html