Polarography is an electroanalytical technique that measures the current flowing in an electrochemical cell as the applied voltage is gradually varied. The method is based on the use of a polarizable electrode, most traditionally the Dropping Mercury Electrode (DME). Polarography is especially useful for analyzing ions that can undergo reduction at the mercury surface. It is a powerful tool in pharmaceutical, environmental, and industrial analysis due to its high sensitivity, selectivity, and ability to detect trace concentrations of analytes.
A polarogram, the plot of current versus applied voltage, provides information about both qualitative identification and quantitative estimation of electroactive species in a solution.
Half-Wave Potential
The half-wave potential (E1/2) is a key characteristic parameter in polarography. It represents the applied potential at which the polarographic diffusion current reaches half of its limiting value. Since E1/2 is unique for each reducible ion under specific conditions, it is used for qualitative identification of analytes.
Concept of Half-Wave Potential
During a polarographic experiment, as the applied voltage increases, the current initially rises slowly and then steeply until it reaches a plateau called the limiting current. The midpoint of this transition region corresponds to the half-wave potential. It is independent of concentration and influenced by the nature of the electrode, supporting electrolyte, pH, and complexation.
Significance of Half-Wave Potential
- Serves as a qualitative identifier for electroactive ions.
- Unaffected by analyte concentration, making it a stable reference point.
- Helps distinguish between multiple ions that may produce overlapping polarographic waves.
- Useful in complexation and stability constant studies.
Principle of Polarography
Polarography works on the principle that the current flowing through an electrochemical cell is dependent on the reduction or oxidation of ions at the working electrode. In classical polarography using DME, mercury drops continuously from the capillary tube, creating a fresh, reproducible surface. This ensures stable diffusion conditions and minimizes contamination or passivation.
As the potential is varied linearly, ions near the electrode surface are reduced or oxidized, producing a change in current proportional to their concentration. The mass transport is dominated by diffusion, which gives rise to diffusion-controlled limiting current.
Polarographic Currents
The total current in a polarographic system is composed of several components:
- Residual current: A small background current due to impurities or electrode reactions.
- Migration current: Caused by ionic movement toward the electrode due to electric field; minimized by supporting electrolyte.
- Diffusion current: The most important component, generated due to concentration gradient.
- Limiting current: The current plateau reached when the rate of diffusion becomes the limiting step.
The Ilkovic Equation
The Ilkovic equation describes the relationship between diffusion current (id) and the properties of the dropping mercury electrode:
id = 607 n D1/2 m2/3 t1/6 C
Where:
- n: Number of electrons involved
- D: Diffusion coefficient
- m: Mercury flow rate
- t: Drop time of mercury
- C: Concentration of analyte
This equation highlights that diffusion current is directly proportional to analyte concentration, making polarography a highly effective quantitative method.
Nature of the Limiting Current
The limiting current is achieved when the supply of electroactive species to the electrode surface is purely diffusion-controlled. At this stage, increasing the applied voltage does not increase current further. The limiting current is used for quantitative analysis.
Factors influencing limiting current include temperature, viscosity of solution, electrode area, and concentration of analyte.
Instrumentation
A typical polarographic setup includes:
- Working electrode (Dropping Mercury Electrode or Rotating Platinum Electrode)
- Reference electrode (e.g., Calomel or Ag/AgCl)
- Auxiliary electrode (counter electrode)
- Polarograph instrument
- Electrolysis cell
1. Polarography with Dropping Mercury Electrode (DME)
The DME is the classical electrode used in polarography. Mercury drops fall at fixed intervals, continuously presenting a clean, renewed surface. Advantages of DME include:
- Highly reproducible surface area
- Low contamination risk
- Wide negative potential range for reduction reactions
However, mercury toxicity and disposal issues limit its modern use.
2. Polarography with Rotating Platinum Electrode (RPE)
The RPE is a solid electrode that rotates at controlled speeds to maintain steady hydrodynamic conditions. Advantages include:
- No mercury usage
- Better control of mass transport
- Suitable for oxidation reactions
It provides stable currents and is widely used in modern electrochemical analysis.
Recent Advanced Methodologies in Polarography
(A) Oscillographic Polarography
Uses rapidly varying (oscillating) potentials, producing oscillograms suitable for studying fast redox processes. It provides higher sensitivity and faster analysis.
(B) Chronopotentiometry
In this method, a constant current is applied and potential is measured as a function of time. It is used for kinetic studies and determining diffusion coefficients.
(C) AC Polarography
Applies alternating current superimposed on a DC voltage. It improves sensitivity and separates faradaic from non-faradaic currents.
(D) Pulse Polarography
Pulse polarography applies periodic voltage pulses and measures current at specific intervals. It provides extremely low detection limits and is widely used for trace metal analysis.
Applications of Polarography
- Determination of trace metals such as cadmium, lead, zinc, and copper
- Analysis of pharmaceutical ingredients and impurities
- Study of redox mechanisms and reaction kinetics
- Determination of complex formation and stability constants
- Environmental analysis of water and soil samples
- Food analysis and determination of additives
- Clinical analysis for metal ions in biological fluids
Detailed Notes:
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