Voltammetry

Voltammetry and polarography1. NORMAL PULSE VOLTAMMETRY

2. DIFFERENTIAL PULSE VOLTAMMETRY

By Shobana.N.SQueen Mary’s College, Chennai

Voltammetry

It is an Electro- analytical technique.

It gives information about the analyte.

We can even measure the amount of current by varying the voltage.

The change in current with the varying voltage gives the plot and is known as uoltammogram

There is a minimum potential required to initiate an oxidation or reduction reaction at an electrode.

Instrumentation

It is a three electrode system.

1. Working electrode ; 2. Reference electrode and

3. Auxiliary electrode.

(1) working electrode;

(2) auxiliary electrode;

(3) reference electrode

Working Electrode

Ranges from small mercury drop to flat platinum disc.

Platinum disc

DME

Other commonly used electrode materials gold, platinum and glassy carbon.

Glassy carbon Gold electrode

Depending on the choice of working electrode, the type of voltammetry is decided.

Example :

We use Dropping Mercury Electrode (DME) in Polarography technique

We use Platinum electrode in Cyclic Voltammetry

We use Glassy Carbon as electrode in Linear Sweep Voltammetry.

Reference Electrode

Usually Standard electrode is used as Reference electrode.

Its potential is constant.

It provides potential to the Working electrode.

Common reference electrode are :

Calomel electrode and Ag/AgCl electrode

Auxiliary Electrode

It is usually a thin platinum wire.

It serves merely to carry the current flowing through the cell.

Usually redox reaction occur simultaneously at the

auxiliary electrode.

Type of Voltammetry

LINEAR SWEEP VOLTAMMETRY

STAIRCASE VOLTAMMETRY

CYCLIC VOLTAMMETRY

SQUAREWAVE VOLTAMMETRY

ANODIC STRIPPING VOLTAMMETRY

CATHODIC STRIPPING VOLTAMMETRY

ABSORPTIVE STRIPPING VOLTAMMETRY

ALTERNATING CURRENT VOLTAMMETRY

POLAROGRAPHY

ROTATED ELECTRODE VOLTAMMETRY

NORMAL PULSE VOLTAMMETRY

DIFFERENTIAL PULSE VOLTAMMETRY

CHRONOAMPEROMETRY.

Linear Sweep Voltammetry

In linear sweep voltammetry (LSV) a fixed potential range.

the voltage is scanned from a lower limit to an upper limit.

In LSV measurements the current response is plotted as a function of voltage rather than time.

The scan begins from the left hand side of the current/voltage plot where no current flows.

As the voltage is swept further to the right (to more reductive values) a current begins to flow and eventually reaches a peak before dropping

Normal Pulse Voltammetry

Normal polarography has been replaced by various forms of pulse polarography.

It uses a series of potential pulses.

Here the pulse time (tp) is 50ms.

Each potential has different amplitude

Differential Pulse Voltammetry

If potential pulse is applied periodically to the Linear Sweep Voltammetry, then it is known as Differential Pulse Polarography.

Hence it is denoted as the differential of linear sweep voltammetry.

The current is measured twice per cycle.

Here the pulse time (tp) is 17ms.

The difference in the two currents gives rise to the peak-shaped voltammogram.

consistent enhancement of the signal is achieved.

Detection limit is as low as 10-8 M

Each potential step has the same amplitude

Polarography

Polarography is the branch of voltammetry in which a dropping mercury electrode (DME) is used as the working electrode.

Conventional DC

Wide cathodic potential range and a renewable surface

Hence widely used for the determination of many reducible species

Reduction begins at sufficiently negative potential

voltage applied to the polarisable electrode is increased over the range of 0-2V

It is known as incomplete electrolysis.

Jaroslav Heyrovsky

He was the inventor of Polarographic technique.

He received Nobel Prize in 1959

He is known as father of electro analytical

method.

Polarography has a special characteristic :

1. Use of polarized electrode.

2. Use of depolarized electrode.

POLARIZED ELECTRODE : DME

DEPOLARIZED ELECTRODE : SCE

Dropping Mercury Electrode

It is the working electrode in one of the voltammetry technique known as polarography

Advantages of DME

Clean Surface generated

Rapid constant current achievement

Even metal with high negative standard voltage systems can be studied without hydrogen.

Amalgam formation more favourable.

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