Polarography

• Polarography is an electroanalytical technique introduced by Jaroslav Heyrovsky in 1922.

• It measures the current-potential (I-V) relationship by applying a gradually increasing voltage to a working electrode in a solution containing the analyte.

• The current results from redox reactions at the electrode surface.

illustration of Polarography, showing the setup with the working electrode in a solution

Principle of Polarography

• Polarography is based on measuring current as a function of applied potential to study the electrochemical behavior of chemical species.

• The technique focuses on redox reactions at the surface of a working electrode when the potential is varied.

ey Components:

1. Working Electrode: Usually a dropping mercury electrode (DME) or static mercury drop electrode (SMDE).

2. Reference Electrode: Maintains a stable, known potential.

3. Counter Electrode: Completes the circuit.

4. Sample Solution: Contains the analyte and supporting electrolyte.

5. Potentiostat: Controls potential and measures current.

Steps in Polarographic Experiment:

1. Electrodes are immersed in the sample solution.

2. A linearly changing potential is applied to the working electrode.

3. Redox reactions occur at specific potentials, generating a current.

4. The current is plotted against potential to form a polarogram, which shows peaks or steps indicating the concentration and behavior of analytes.

Ilkovic Equation

• The Ilkovic equation relates the diffusion current measured in a polarographic experiment to the concentration of the electroactive species (analyte) in solution.

• It was derived by Slovak chemist Dionýz Ilkovič in 1934.

id = 2.69 x 10^5 n D^1/2 C d

where:

• id = diffusion-limited current (in microamperes, µA)

• n = number of electrons transferred in the redox reaction

• D = diffusion coefficient of the analyte (in cm²/s)

• C = concentration of the analyte (in mol/cm³)

• d = diameter of the mercury drop (in cm)

• This equation is particularly applicable to a Dropping Mercury Electrode (DME), where the mercury drop grows and detaches from the electrode at regular intervals.

• The Ilkovic equation assumes steady-state conditions with diffusion as the primary mass transport mechanism, and negligible effects from migration and convection.