Polarography Principle Instrumentation and Application

Polarography in Pharmaceutical Analysis: Principle, Instrumentation, Equations, and Applications

Introduction
In the field of pharmaceutical analysis, electroanalytical techniques provide precise and sensitive methods for drug determination. One of the most important methods is polarography, introduced by Jaroslav Heyrovský (Nobel Prize, 1959). It involves the measurement of current as a function of an applied potential using a dropping mercury electrode (DME).

This technique is highly valuable for identifying, quantifying, and studying the electrochemical behavior of drugs and impurities.

Principle of Polarography

The principle of polarography is based on the fact that when a gradually increasing potential is applied between a dropping mercury electrode (working electrode) and a reference electrode, the analyte undergoes reduction or oxidation at the mercury surface.

• The resulting diffusion current is proportional to the concentration of the electroactive species in the solution.
• A plot of current vs applied voltage is called a polarogram.
• Each substance shows a characteristic half-wave potential (E½), which is used for identification.

Equations in Polarography

Ilković Equation

This is the most fundamental equation in polarography, describing the diffusion current:

id = 708nD1/2 m2/3 t1/6 C

Where:

• (id) = diffusion current (µA)
• (n) = number of electrons involved
• (D) = diffusion coefficient (cm²/s)
• (m) = mercury flow rate (mg/sec)
• (t) = drop time (sec)
• (C) = concentration of analyte (mM)

This equation shows that diffusion current is directly proportional to concentration, making polarography quantitative.

Half-Wave Potential Equation

The position of the wave on a polarogram is defined by the half-wave potential (E½):

E = E1/2 + 0.0591/n Log i/id-i

Where:

• (E) = applied potential
• (E{1/2}) = half-wave potential (characteristic of the analyte)
• (i) = instantaneous current
• (id) = diffusion current
• (n) = number of electrons transferred

Half-wave potential (E½) is constant for a given substance and helps in qualitative identification of drugs.

Instrumentation of Polarography

A polarographic apparatus is designed to measure current–voltage relationships of electroactive species in a solution. The essential components include a polarographic cell, electrodes, a potentiostat, and a current measuring system.

1. Polarographic Cell

• The container (usually glass) in which the drug sample solution is taken along with a supporting electrolyte.
• The supporting electrolyte ensures:
  • High conductivity of the solution.
  • Minimization of migration current (movement due to electric field).
  • Stabilization of the current–voltage curve.

2. Dropping Mercury Electrode (DME) – Working Electrode

• The heart of the polarograph.
• Consists of a thin capillary tube through which mercury flows, forming uniform mercury drops at the tip.
• Each drop falls after a few seconds, exposing a fresh, uncontaminated electrode surface for each measurement.
• Advantages:
  • High reproducibility.
  • Wide negative potential range (up to –2 V).
  • Eliminates surface contamination issues.
• Function: Site where reduction/oxidation of the analyte takes place.

3. Reference Electrode

• Provides a stable, known reference potential against which the working electrode potential is measured.
• Commonly used:
  • Saturated Calomel Electrode (SCE)
  • Silver/Silver Chloride Electrode (Ag/AgCl)
• Function: Ensures accurate measurement of electrode potential.

4. Auxiliary (Counter) Electrode

• Made of platinum wire or other inert material.
• Completes the electrical circuit by allowing current to pass.
• Function: Provides a path for electrons, preventing polarization of the reference electrode.

5. Power Supply (Potentiostat)

• Supplies a controlled and gradually varying potential between the working and reference electrode.
• Function: Controls the voltage applied to the system so that the current response can be measured at each potential.

6. Current Measuring Device

• Records the tiny currents (microampere range) generated by electrochemical reactions.
• In traditional systems: a galvanometer was used.
• In modern instruments: digital electronics and computers record data automatically and generate a polarogram (current vs. voltage plot).

Summary of Instrumentation:

• Working Electrode (DME): Reaction site for redox process.
• Reference Electrode (SCE/Ag-AgCl): Provides constant reference potential.
• Auxiliary Electrode: Completes the circuit.
• Potentiostat: Controls applied voltage.
• Current Detector: Records current and produces polarogram.

Applications of Polarography in Drug Analysis

1. Qualitative Analysis

• Half-wave potential (E½) is used for identifying electroactive drugs.
• Example: Vitamin C, riboflavin, sulfonamides, antibiotics.

2. Quantitative Analysis

• Using Ilković equation, drug concentration can be determined.
• Example: Assay of ascorbic acid, adrenaline.

3. Detection of Trace Metals

• Sensitive to parts-per-million (ppm) levels.
• Used to detect heavy metals like Pb, Cd, Zn, Cu in pharmaceutical formulations.

4. Study of Drug Reaction Mechanisms

• Provides insight into redox reactions and electron transfer pathways of drugs.

5. Mixture Analysis

• Different drugs in a mixture can be resolved due to their different half-wave potentials.

6. Quality Control in Industry

• Used in checking drug purity and metallic impurity levels to meet pharmacopeial standards.

Conclusion

Polarography is a powerful electroanalytical technique for pharmaceutical analysis, based on current–voltage measurements at a dropping mercury electrode. The Ilković equation enables quantitative estimation, while the half-wave potential equation helps in qualitative identification. With applications in drug assay, impurity detection, and mechanism studies, polarography remains a crucial tool in pharmaceutical quality control and research.

• Polarimetry Principle Instrumentation and Applications