Polarography is an electroanalytical technique that determines the concentration of specific analytes in a solution by measuring the current flowing in an electrochemical cell as a function of the applied voltage. The principle of polarography is based on the behavior of electrodes at varying voltages, which allows for the quantitative analysis of various substances.
Principle of polarography
1. Electrode Behavior
- Polarography commonly uses a dropping mercury electrode (DME) or a hanging mercury drop electrode (HMDE). It employs a small drop of mercury at the end of a capillary tube as the working electrode.
- The mercury electrode coats with a thin layer of mercury salt, typically mercurous sulfate (Hg2SO4), forming a stable amalgam with analytes.
- Applying an appropriate potential (voltage) to the mercury electrode causes the oxidation or reduction of electroactive species present in the solution.
2. Analyte Interaction
– Analytes in the sample solution interact with the mercury electrode through various electrochemical reactions, including oxidation or reduction, depending on the nature of the analyte.
– Analytes form complex amalgams or compounds with the mercury electrode during the measurement.
3. Mercury Drop Formation
In polarography, we allow a small drop of mercury to fall and break away from the capillary tube, creating a new mercury surface.
This fresh mercury surface is critical for achieving a stable and reproducible potential for the electrochemical reactions.
4. Voltage Sweep
We apply a potential to the mercury electrode in a controlled and continuously changing manner. Initially, we set the potential at a specific value, and then we sweep or change it at a known rate.
The voltage sweep allows for the controlled and gradual oxidation or reduction of electroactive species.
5. Current Measurement
– As the potential changes, the current passing through the electrochemical cell is continuously measured.
– The current is directly related to the concentration of the electroactive species and the rate of electrochemical reactions taking place at the mercury electrode.
6. Analytical Information
– The resulting polarogram is a plot of current (y-axis) against applied voltage (x-axis).
– Peaks or waves in the polarogram correspond to the different electroactive species present in the solution.
– The magnitude and shape of the peaks provide information about the concentration and nature of the analytes.
7. Quantitative Analysis
– The concentration of analytes in the sample solution can be quantitatively determined by measuring the current at the peak or the half-wave potential and using calibration curves.
Applications
Polarography is widely used to determine various analytes, including metal ions, organic compounds, and gases, in fields such as environmental analysis, pharmaceuticals, food chemistry, and industrial quality control.