Polarography

Polarography is a specialized type of voltammetry used to study chemical substances in liquids. It belongs to the field of electroanalytical methods. This technique uses a working electrode made of liquid mercury to measure electrical signals. Scientists use it to find the concentration of specific molecules, which are called electroactive species. By measuring these species, researchers can understand the chemical makeup of a solution.

The process relies on a dropping mercury electrode, or DME. In this setup, mercury flows through a thin glass capillary. A small drop forms at the end of the tube and grows over time. As the drop grows, the scientist changes the electrical potential of the electrode. The potential is the electrical push applied to the mercury. This change happens linearly over time. While the drop grows, the device measures the resulting electrical current. When the drop becomes too heavy, it falls away from the capillary. A new drop then begins to form from a larger mercury pool.

There are different ways to perform these measurements. In classical polarography, the current is measured continuously as the drop grows. This creates a plot of current versus potential that looks like a wave. This shape is called a sigmoidal curve. The flat part of this curve is the diffusion-limited current. This happens because diffusion becomes the main way material moves to the electrode. More advanced versions of the technique produce sharp peaks instead of waves. These peaks allow for better resolution of different chemical species. Some advanced methods can even reach detection limits as low as 10^-9 M.

Classical polarography has some technical limitations. As the mercury drop grows, its surface area increases rapidly. This causes a large amount of capacitive current. Capacitive current is the electricity used to charge the interface between the mercury and the liquid. This charging effect can interfere with the measurement of the faradaic current. The faradaic current is the signal produced by the actual chemical reaction. Because the potential also changes during the drop's life, the signal can be noisy. This noise usually limits detection to concentrations of 10^-5 or 10^-6 M.

To solve these problems, scientists developed better techniques using electronic potentiostats. One improvement is tast polarography, which only measures current at the very end of the drop's life. Another method is differential pulse polarography. This method applies short potential pulses onto the main scan. These pulses usually have amplitudes between 10 and 50 mV. The pulses last for 20 to 50 ms. By measuring the current before and after these pulses, scientists can subtract the capacitive current. This provides a 100 to 1000-fold improvement in the detection limit.

The history of polarography is tied to major scientific breakthroughs. Czechoslovak chemist Jaroslav Heyrovský invented the technique in 1922. His work was so significant that he won the Nobel prize in 1959. Another important figure was the Slovak chemist Dionýz Ilkovič. He derived the Ilkovic equation to help quantify results. This equation relates the diffusion current to the concentration of the substance being studied. It includes several specific variables. These include the mass flow rate of mercury and the number of electrons exchanged. It also accounts for the diffusion coefficient of the substance.

Mercury is an excellent material for these electrodes for several reasons. First, it offers a very wide voltage window. It can operate from approximately +0.2 V to -1.8 V against a reversible hydrogen electrode. This makes it ideal for studying electroreduction reactions. Second, mercury is a liquid, so the electrode surface is always reproducible. Third, the surface is very easy to clean. A fresh, clean surface is created every time a new drop forms.

Polarography has had a massive impact on scientific literature. According to the SciFinderN database, there are over 45,000 publications related to the topic. Most of these are journal articles, while there are only 992 patents. The technique was a primary tool for analytical chemistry until the 1990s. Even though newer methods have replaced it, it remains important for specific tasks. For example, differential pulse anodic stripping voltammetry can be used in marine studies. This helps scientists characterize how metals and organic matter interact in the ocean.