Introduction
The dropping mercury electrode (DME) is a specialized electrode used in polarography for both quantitative and qualitative analysis of substances. It functions by continuously releasing mercury drops into the solution from a mercury reservoir through fine capillary tubes. These tubes have a diameter ranging from 0.03 to 0.05 mm, ensuring precise drop formation.
A typical drop interval ranges from 1 to 5 seconds, which allows continuous electrode surface renewal, eliminating the risk of electrode poisoning and ensuring accurate electrochemical measurements.
Construction
The DME consists of the following key components:
- Mercury Reservoir: Stores mercury and provides a constant supply for drop formation.
- Capillary Tubes: Have bore sizes between 20 to 50 mm and lengths of 10-15 cm, allowing precise drop formation.
- Rubber tubing connects the mercury reservoir to the capillary, enabling controlled mercury flow.
- Electrolysis Cell: A small glass container where the unknown solution is placed for analysis.
- Drop Time Regulation: The height of the mercury reservoir controls the drop time, typically between 1 and 5 seconds.
Working Principles of DME
- The DME can be polarized, functioning as either a cathode or an anode, while the mercury pool serves as the counter electrode.
- The counter electrode remains non-polarizable, ensuring stable measurements.
- A supporting electrolyte, such as potassium chloride (KCl), is added in 50–100 times the concentration of the sample to maintain conductivity.
- Nitrogen or hydrogen gas is bubbled through the solution to eliminate oxygen interference.
- When cadmium ions (Cd²⁺) are present in the analyte solution, they undergo reduction at the cathode.
- The applied voltage is gradually increased, and the resulting current is recorded to generate a polarogram—a graph showing the relationship between voltage and current.
- Polarogram refers to the obtained graph, while the polarograph is the instrument used for recording measurements.
Advantages
- Eliminates Electrode Poisoning: The continuous renewal of the electrode surface prevents contamination and ensures accurate readings.
- Reproducibility: A fresh mercury drop forms every few seconds, providing a stable and repeatable electrode surface.
- Wide Potential Range: Mercury’s high hydrogen overpotential allows effective analysis of many reducible substances.
- Fine Control Over Reaction Time: Each drop acts as a microelectrode, allowing precise electrochemical studies.
Disadvantages
- Mercury Toxicity: requires careful handling and disposal due to environmental hazards.
- Limited Use for Oxidation Reactions: Mercury undergoes oxidation at +0.3V (vs. SCE), restricting its application for oxidative studies.
- Sensitive to Vibrations: Vibrations can disrupt the drop formation, affecting measurement accuracy. Holding the setup vertically helps minimize this issue.
Maintenance & Handling
- Only high-purity, doubly distilled mercury should be used to ensure accurate results.
- The DME tip should be immersed in water when not in use to prevent contamination.
- To clean the electrode, it should be dipped in dilute nitric acid (HNO₃).
- Avoid disturbances by keeping the assembly in a vertical position to reduce vibrations.
Applications
- Electroanalytical Chemistry: Used in polarography to determine trace metal ions and organic compounds.
- Pharmaceutical Analysis: Detects impurities and active pharmaceutical ingredients (APIs) in drugs.
- Environmental Science: Measures heavy metal contamination in water and soil.
- Biochemical Studies: Used in enzyme activity and redox reaction analysis.
Conclusion
The dropping mercury electrode (DME) remains a valuable tool in electrochemical analysis due to its self-renewing surface, high reproducibility, and broad cathodic potential range. However, concerns over mercury toxicity have led to the exploration of alternative electrode materials in modern applications.