Voltammetry and Potentiometry

Pranay KrishnanM.Sc EVS

SIES Nerul College of Arts, Science and Commerce.

Navi Mumbai,Maharashtra,India.

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.

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.

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.

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:

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.

• 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 film3. Many metal ions can be reversibly reduced into it. • Other commonly used electrode materials are gold,

platinum, and glassy carbon.

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.

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.

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.

3 electrode system

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.

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.

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.

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

Basic instrument for Voltammetry.

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.

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.

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.

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.

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

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.

• 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.

• 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.

Applications

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

Concentration and Activity

• Activity, (ai): measures the effective concentration of an ion (i) taking into account interactions with ions that may mask it

ai = i [i]

Where = activity coefficient (a function of ionic strength) and [i] = the molar concentration.

• is a correction factor.

References

• Textbook of Voltammetry and Potentionmetry

– IGNOU.• Wikipedia – An Introduction to Voltammetry

and Potentiometry.• Electrochemical methods for Environmental

Analysis – DBC.