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DrX Whiz Niraj
Polarography: Principle & Ilkovic Equation
Master Voltammetry! Complete notes on the Principle of Polarography, the famous Ilkovic Equation, and the Construction of the Dropping Mercury Electrode (DME).
Polarography is a type of Voltammetry where the current flowing through an electrochemical cell is measured as a function of the applied potential. The unique feature of Polarography is that the working (indicator) electrode used is a Dropping Mercury Electrode (DME).
What does a Polarogram tell us?
The graph plotted between the Current (Y-axis) and the Applied Potential (X-axis) is called a Polarogram. It gives two major pieces of information:
- Qualitative Analysis: The Half-Wave Potential (E½) is characteristic of a specific metal ion. It tells us "Which" metal is present.
- Quantitative Analysis: The Diffusion Current (Id) is directly proportional to the concentration of the analyte. It tells us "How much" metal is present.
The Ilkovic equation is the fundamental equation in polarography. It gives the mathematical relationship between the Diffusion Current (Id) and the concentration of the electroactive substance.
Where:
- Id = Average diffusion current (in microamperes, µA).
- 607 = A constant combining Faraday's constant, Pi, and density of mercury.
- n = Number of electrons transferred in the redox reaction.
- D = Diffusion coefficient of the analyte (cm²/sec).
- C = Concentration of the analyte (millimoles per liter).
- m = Mass flow rate of mercury through the capillary (mg/sec).
- t = Drop time of mercury (seconds).
Conclusion: The equation proves that Diffusion current (Id) is directly proportional to Concentration (C).
Dropping Mercury Electrode (DME)
It is the most common working electrode in polarography. Liquid mercury continuously drops from a very fine glass capillary tube at a rate of 1 drop every 2-6 seconds.
- Advantages: Provides a highly reproducible, completely fresh and clean surface for every drop. It has a high hydrogen overvoltage, allowing analysis of reducible metals in acidic solutions.
- Disadvantages: Mercury is highly toxic. It oxidizes easily, so it cannot be used for anodic (oxidation) processes at highly positive potentials.
Rotating Platinum Electrode
Consists of a short piece of platinum wire sealed into the side of a glass tube. It is rotated by an electric motor at a constant speed (usually 600 rpm).
- Why use it? Developed to overcome the limitations of the DME. Since platinum does not oxidize as easily as mercury, it can be used for anodic (oxidation) reactions.
- Advantages: Rotating it makes diffusion very fast, leading to higher sensitivity than a stationary electrode.
- Trace Metal Analysis: Highly sensitive for detecting heavy metal impurities (Lead, Copper, Zinc, Cadmium) in pharmaceutical raw materials.
- Dissolved Oxygen: It is the standard method for determining the amount of dissolved oxygen in solutions and natural waters.
- Vitamin & Antioxidant Analysis: Used for the quantitative estimation of Vitamin C (Ascorbic acid) and Riboflavin.
- Drug Analysis: Quantitative determination of drugs containing reducible groups like nitro, azo, and carbonyl groups (e.g., Chloramphenicol, Diazepam).
Practical Alert: Mercury Toxicity
While the Dropping Mercury Electrode is excellent for analysis, Mercury vapors and its salts are highly neurotoxic. Improper handling of DME in labs can lead to chronic mercury poisoning. Today, many modern labs replace DME with Hanging Mercury Drop Electrodes (HMDE) or solid electrodes to minimize mercury waste.