Voltmeter, Potentiometer, pH meter, Instrumentation basics, Physics,

1. Voltammetry and Potentiometry
Pranay Krishnan
M.Sc EVS
SIES Nerul College of Arts, Science and Commerce.
Navi Mumbai,
Maharashtra,India.

2. Introduction of Voltammetry
• Voltammetry is the general name given to a
group of electroanalytical methods in which
the current is measured as a function of
applied potential wherein the polarization of
the indicator or working electrode is
enhanced.
• This field has been developed from
Polarography.

3. History
• Developed by Czech chemist Jaroslav Heyrovsky in the year
1992.
• He received the noble prize in chemistry for this work.
• Early voltammetric methods experienced a number of
difficulties.
• In the 1960s and 1970s significant advances were made in
all areas of voltammetry (theory, methodology, and
instrumentation).
• This enhanced the sensitivity and expanded the repertoire
of analytical methods.
• The introduction of low-cost operational amplifiers in this
also facilitated the rapid commercial development of
relatively inexpensive instrumentation.

4. Characteristics of Voltammetry
• The common characteristic of all voltammetric
techniques involves the application of a potential (E)
to an electrode and the monitoring of the resulting
current (i) flowing through the electrochemical cell.
• In many cases the applied potential is varied or the
current is monitored over a period of time (t).
• Hence all voltammetric techniques described as some
function of E, i, and t.
• They are considered active techniques because the
applied potential forces a change in the concentration
of an electro active species at the electrode surface by
electrochemically reducing or oxidizing it.

5. Theory
• In voltammetry, the effects of the applied potential and the
behavior of the redox current are described by several well-known
laws.
• The applied potential controls the concentrations of the redox
species at the electrode surface (CO
0
and CR
0
) and the rate of the
reaction (k0), as described by the Nernst or Butler– Volmer
equations, respectively.
• The interplay between these processes is responsible for the
characteristic features observed in the voltammograms of the
various techniques.
• Nernst equation:
• Butler–Volmer equation:

7. Working (indicator) electrode:
• Provides the interface across which a charge can
be transferred or its effects felt.
• The reduction or oxidation of a substance at the
surface of a working electrode, at the
appropriate applied potential, results in the mass
transport of new material to the electrode
surface and the generation of a current.
• Basically a microelectrode whose potential is
varied linearly with time.

8. • Working electrodes are of various geometries and
materials, ranging from small Hg drops to flat Pt disks.
• Mercury is useful because:
1. It displays a wide negative potential range
2. It’s surface is readily regenerated by producing a new
drop or film
3. Many metal ions can be reversibly reduced into it.
• Other commonly used electrode materials are gold,
platinum, and glassy carbon.

9. Reference Electrodes
• Normally a standard electrode is used whose potential is
constant throughout the experiment
• Easy to assemble and maintain.
• The most commonly used reference electrodes for aqueous
solutions are the calomel electrode and the silver/silver
chloride electrode (Ag/AgCl).
• Electrodes are commercially available in a variety of sizes
and shapes.
• In most cases the reference electrode should be as close as
possible to the working electrode.
• Sometimes to avoid contamination, it may be necessary to
place the reference electrode in a separate compartment.

10. Counter (Auxiliary) electrode
• It consists of a coil of platinum wire or a pool of
mercury that simply serves to conduct electricity from
the signal source through the solution to the
microelectrode.
• In most voltammetric techniques the analytical
reactions at the electrode surfaces occur over very
short time periods. Thus, isolation of the counter
electrode from the sample is not normally necessary.
• Most often the counter electrode consists of a thin Pt
wire, although Au and sometimes graphite have also
been used.

11. Other Components
• Cells or sample holders come in a variety of
sizes, shapes, and materials.
• Type used depends on the amount and type
of sample, the technique, and the analytical
data to be obtained.
• The material of the cell (glass, Teflon,
polyethylene) is selected to minimize reaction
with the sample.

12. 3 electrode system

13. Types of Voltammetry
• Linear sweep voltammetry.
• Staircase voltammetry.
• Cyclic voltammetry.
• Squarewave voltammetry.
• Anodic stripping voltammetry.
• Cathodic stripping voltammetry.
• Adsorptive stripping voltammetry.
• Alternating current voltammetry.
• Polarography.
• Rotated electrode voltammetry.
• Normal pulse voltammetry.
• Differential pulse voltammetry.
• Chronoamperometry.

14. Linear Sweep Voltammetry
• Method where the current at a working
electrode is measured while the potential
between the working electrode and a
reference electrode is swept linearly in time.
• Oxidation or reduction of species is registered
as a peak or trough in the current signal at the
potential at which the species begins to be
oxidized or reduced.

16. Staircase voltammetry
• Derivative of linear sweep voltammetry.
• In staircase voltammetry the potential sweep
is a series of stair steps.
• The current is measured at the end of each
potential change, right before the next.

18. Cyclic voltammetry
• In a cyclic voltammetry experiment the working electrode potential
is ramped linearly versus time like linear sweep voltammetry.
• Cyclic voltammetry takes the experiment a step further than linear
sweep voltammetry which ends when it reaches a set potential..
The current at the working electrode is plotted versus the applied
voltage to give the cyclic voltammogram trace.
• Generally used to study the electrochemical properties of an
analyte in solution.
• Common materials used as electrodes include glassy carbon,
platinum, and gold.
• Electrodes are generally encased in a rod of inert insulator with a
disk exposed at one end

21. Basic instrument for Voltammetry.

22. Uses
• Quantitative determination of organic and inorganic compounds in
aqueous and non-aqueous solutions.
• Measurement of kinetic rates and constants.
• Determination of adsorption processes on surfaces.
• Determination of electron transfer and reaction mechanisms.
• Determination of thermodynamic properties of solvated species.
• Fundamental studies of oxidation and reduction processes in
various media.
• Making of corrosion proof materials.
• Production of new types of batteries that can store large quantities
of energy.

23. Environmental Applications
• Quantitative determination of pharmaceutical
compounds.
• Determination of metal ion concentrations in
water to sub–parts-per-billion levels.
• Determination of redox potentials.
• Detection of eluted analytes in high-
performance liquid chromatography (HPLC)
and flow injection analysis.

25. Advantages
• Excellent sensitivity with a very large useful linear
concentration range for both inorganic and organic
species (10–12 to 10–1 M).
• A large number of useful solvents and electrolytes, a
wide range of temperatures, rapid analysis times
(seconds).
• Simultaneous determination of several analytes, the
ability to determine kinetic and mechanistic
parameters.
• A well-developed theory and thus the ability to
reasonably estimate the values of unknown
parameters.

26. Potentiometry
Introduction
• Electromotive force developed by a galvanic cell cannot be
measured accurately by placing a simple dc voltmeter across the
electrodes, because a significant current is required for operation
of the meter.
• Since current is drawn from the cell, a variation in concentrations of
the reacting species will take place which leads to a change in the
cell voltage.
• An additional factor is the development of ohmic potential drop
due to the internal resistance of the cell which opposes the
potential due to the two electrodes of a galvanic cell.
• A truly significant value for the output of a cell potential can be
attained only if the measurement is made with a negligible passage
of current.
• A potentiometer is one type of instrument that helps us to achieve
this.

27. Types
Indirect:
• In a potentiometric titration an abrupt change in potential (E) is used for
the end-point.
• Potentiometric titrations can be automated and used where color-
changing indicators are difficult to see.
Direct:
• Measurement of analyte concentration according to Nernst equation
using potential (E)
• Potential of indicator electrode w.r.t. a reference electrode
e.g. pH meter

28. Titrations
• Neutralisation Titrations: Potentiometric
neutralisation titrations are particularly useful for the
analysis of mixture of acids or polyprotic acids or bases
because discrimination between the end-points can
often be made.
• Oxidation-Reduction Titrations: Indicator electrodes
for oxidation-reduction are generally fabricated from
platinum, gold mercury or silver. The determining
factor in the values of potential is the ratio of the
activity or concentration of the oxidised and reduced
forms of certain ion species.

29. • Precipitation titrations: Titrations involving the
precipitation reactions are not nearly as
numerous in titrimetric analysis for as those
involving redox or acid - base reactions.
• Complex Formation Titrations: Both metal
electrodes and the membrane electrodes can be
used to detect end point in reactions that involve
formation of a soluble complex. This is
particularly useful for EDTA titrations or acid -
base reactions.

30. • Differential Titrations: This titration requires
the use of two identical indicator electrodes,
one of which is well-shielded from bulk of
solution.
• Automatic Titrations: Highly useful, rapid
results but may not be very accurate.

32. Applications
Measurement of
• Concentration and Activity.
• pH.
• Hardness.

33. Concentration and Activity
• Activity, (ai): measures the effective
concentration of an ion (i) taking into account
interactions with ions that may mask it
ai = γi [i]
Where γ = activity coefficient (a function of
ionic strength)
and [i] = the molar concentration.
• γ is a correction factor.

36. References
• Textbook of Voltammetry and Potentionmetry
– IGNOU.
• Wikipedia – An Introduction to Voltammetry
and Potentiometry.
• Electrochemical methods for Environmental
Analysis – DBC.