The Dropping Mercury Electrode (DME) is commonly used in polarography and electrochemical techniques.
It provides a clean, renewable surface for redox reactions, offering low background current and reproducibility.
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Construction:
1. Capillary tube:
A fine glass tube with an internal diameter of 10-50 µm through which mercury flows. The tube is mounted vertically and connected to a mercury reservoir.
2. Mercury reservoir:
Holds the mercury supply and controls its flow into the capillary by adjusting the reservoir height or using a pressure regulator.
3. Electrode holder:
A supportive structure that holds the capillary tube and provides electrical connection to the mercury. It is typically made from insulating material.
4. Electrical connection:
A conductive wire establishes contact between the mercury in the capillary and the potentiostat.
Working:
1. Mercury flow:
Mercury flows through the capillary by gravity or controlled pressure, forming droplets at the tip.
2. Droplet formation and growth:
The mercury droplet grows due to surface tension until it reaches a critical size and detaches.
3. Droplet detachment:
Detachment occurs periodically (1-20 drops per second), ensuring a fresh, renewable surface for the next drop.
4. Electrochemical measurement:
During droplet formation and detachment, the potentiostat applies potential, enabling redox reactions of the analyte at the mercury surface, generating a measurable current.
5. Renewable surface:
Each droplet detachment creates a new clean surface, minimizing contamination and fouling.
Advantages:
Clean, reproducible surface.
Low background current.
Suitable for studying a wide range of redox reactions.
Limitations:
Mercury toxicity.
Limited potential range due to mercury oxidation/reduction.