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.
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.
Key Components:
Working Electrode: Usually a dropping mercury electrode (DME) or static mercury drop electrode (SMDE).
Reference Electrode: Maintains a stable, known potential.
Counter Electrode: Completes the circuit.
Sample Solution: Contains the analyte and supporting electrolyte.
Potentiostat: Controls potential and measures current.
Steps in Polarographic Experiment:
Electrodes are immersed in the sample solution.
A linearly changing potential is applied to the working electrode.
Redox reactions occur at specific potentials, generating a current.
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.