Polarography
Polarography is an analytical technique used to study chemical solutions through electrolysis, involving both organic and inorganic materials. Developed by Czech chemist Jaroslav Heyrovsky in the early 20th century, this method employs special electrodes to apply varying voltages to a solution, allowing researchers to identify the elements present and their concentrations. The technique primarily utilizes a dropping mercury electrode (DME), where measured drops of mercury form one electrode, while a stationary electrode acts as the counterpart. By gradually increasing the voltage, researchers can observe current-voltage relationships to distinguish between different solutions.
Despite its effectiveness, polarography has faced scrutiny due to the toxic nature of mercury, prompting the development of nontoxic alternatives; however, mercury remains favored for its accuracy. Polarography's applications extend into modern research, including use by NASA for gas analysis. The technique continues to be a cost-effective and valuable tool in chemical analysis, with new studies emerging annually, showcasing its relevance in contemporary scientific inquiry.
Polarography
Polarography is a way of analyzing a chemical solution through electrolysis. These solutions can be organic (derived from living material) or inorganic (not from living material) in nature. The process uses a set of special electrodes to apply different voltages of electrical current to the sample. By increasing the amount of voltage in a set way, the researcher can determine what element the solution is made of and how concentrated it is.
Background
The technique of polarography was developed by Czech chemist Jaroslav Heyrovsky. The Prague native studied chemistry at the University College in London and worked in a military hospital as a radiologist and dispensing chemist during World War I (1914–1918). At the same time, he pursued advanced degrees, finishing his doctorate in 1921.
Heyrovsky's practical experience and advanced degrees gave him the background he needed to invent the technique of polarography in 1922. The discovery came in part from a comment made by one of the examiners for his doctorate exam. While talking about the electrocapillarity effect of mercury—or the effect an electric current has on mercury's surface—Heyrovsky and his examiner, Bohumil Kucera, were discussing a discrepancy the examiner had found between his own results while testing with a dropping mercury electrode and those recorded in many classic experiments of the past.
The question posed by Kucera intrigued Heyrovsky, and he accepted an invitation to work on the problem in Kucera's laboratory at the Chemical Institute of the Charles University in Heyrovsky's hometown of Prague. At the same time, Heyrovsky continued work he had begun on his own that involved attempting to determine the electrode potential of aluminum. This is done by exposing an electrode made of the element, in this case, aluminum, to a liquid solution that contains ions of the same element.
As part of the process of trying to resolve the discrepancy noticed by Kucera, Heyrovsky used two electrodes made of mercury to pass a current through a solution. As he increased the voltage by small amounts, he took note of the point at which the current did not increase, or the limiting value. Each solution reaches this point at a different voltage; Heyrovsky determined and recorded the amount of current used for different solutions. This method of passing a gradually increasing current through an unknown solution could then allow any researcher to identify the makeup of the solution.
A few years after discovering his method, Heyrovsky built a device to perform these tests. His polarograph was first demonstrated in 1924–1925 and became the prototype for the manufacture of similar devices. He published his findings in Chemike Listy, a chemistry journal in Czechoslovakia, in October of 1922. Within ten years, polarographs came into widespread use.
Heyrovsky continued to improve on his device. By 1938, he had developed an enhanced version that could process larger samples. It could also be used to analyze compound solutions that included more than one element. For his discoveries, Heyrovsky was awarded the Nobel Prize in chemistry in 1959. Polarography continues to have some applications in the analysis of chemical solutions in the twenty-first century.
Overview
Heyrovsky's initial experiment used a galvanometer connected to a circuit that included two electrodes immersed in a solution. One of the electrodes was a dropping mercury electrode, or DME. The DME was invented by Heyrovsky's doctorate examiner Kucera. It consists of measured drops of mercury—a metal in liquid form—that are dispensed from a thick glass tube known as a capillary. The mercury would fall through the solution to the bottom of the container holding it and form a second electrode, sometimes known as the stationary electrode. Heyrovsky then passed a measured amount of electrical current from a direct current generator through the mercury drops into the solution and down to the stationary electrode.
The first galvanometer Heyrovsky used was not sensitive enough to detect the differences in current between the solutions. Eventually, he connected a more sensitive device known as a mirror galvanometer. By recording the current-voltage curves indicated by this galvanometer, Heyrovsky was able to determine how to tell apart solutions containing different elements.
While polarography became a widely used technique for identifying solutions, the process was eventually called into question. In the later part of the twentieth century, concerns arose about the use of mercury. Mercury is a shiny silver liquid metal that can be very difficult to capture and contain. It is also highly toxic when it enters the human body through ingestion or through an open cut or sore, with a potential to cause serious damage to the liver, kidney, and nerves. Nontoxic alternatives for the electrodes were developed, but mercury remains the preferred material for polarography electrodes because it provides the most accurate results. In some situations, however, researchers use another device called a spectrometer or a procedure to separate the solution so that the type and amount of element that it contains can be discovered.
Polarography remains in use in the twenty-first century. The US National Aeronautics and Space Administration (NASA) continues to use the technology in gas polarographic hydrogen sensors. Although there are concerns about its use in some circumstances, polarography incorporating DME and stationary mercury sensors remains a cost-effective way to analyze a wide variety of organic and inorganic solutions. As a result, new studies using polarography as part of the research methodology are published every year, and the technique remains a viable way for chemists to conduct sophisticated analysis.
Bibliography
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