Born into an academic family and well-versed in many disciplines, Bunsen made chemistry his field of choice. After receiving his doctorate at the age of nineteen, Bunsen went on a partially government-funded tour throughout Europe, where he met many chemists and engineers. His travels provided him with a network of contacts that he used throughout his career. Upon his return and after teaching at several universities, he settled into a professorship at the University of Heidelberg in 1852, where he stayed until his retirement in 1889.
Bunsen was an avid experimentalist. He spent much of his time in the laboratory trying to discover the composition of chemical substances. His early research concerned the properties of arsenic and its compounds. In particular, the chemical composition of a certain class of chemicals, called cacodyls, was unknown. His experiments showed that cacodyls were oxides of arsenic. But Bunsen’s work with arsenic nearly cost him his life—he nearly killed himself through arsenic poisoning and lost the sight in one eye during his experiments with cacodyls.
Arsenic is the twentieth most abundant element in Earth’s crust, averaging a concentration of approximately 2 ppm. Arsenopyrite (FeAsS) is its most common mineral. Arsenic occurs widely in nature, and most abundantly in sulfide ores and the products of volcanic eruptions. Arsenic concentrations in rock and soil are highly variable; the highest concentrations are in hydrothermal sulfide mineralization areas. Arsenic has two common oxidation states: +5, the predominant one and the less thermodynamically stable +3. Arsenic has twenty-three isotopes; of these, one (mass 75) is stable. The other isotopes have very short half-lives.
Trace amounts of arsenic occur in groundwater; it may cause human cancers at concentrations in drinking water of about 300 ppb. The U.S. Environmental Protection Agency (EPA) has proposed lowering the maximum allowable arsenic concentration in U.S. drinking water from 50 to 5 ppb. The latter lower limit is still controversial. The properties of arsenic sulfides were known to physicians and “professional prisoners” in the fifth century B.C.E. Albertus Magnus (1193–1280) is credited with having isolated elemental arsenic by heating auripigment (As2S3 ) with soap.
Beneficial effects of arsenic compounds have been known for a very long time. Arsenic was important in the development of metallurgy at the beginning of the Bronze Age and later as a pigment and as an incendiary warfare ingredient. Since ancient and classical times, arsenic formulations have been prescribed to cure diseases. Historically arsenic compounds were alchemical ingredients and the art of secret poisoning was a part of the social and political life of many societies. Arsenic toxicity resulted in the deaths of painters who mixed arsenic pigments.
Between 1850 and 1950 humans were habitually exposed to arsenic in medicine, food, air, and water. Consumer products of the period that contained arsenic included pigments, medicated soaps, embalming solutions, adhesive envelopes, glass, fly-powder, and rat poison. Currently arsenic is a part of wood preservatives, some pesticides, non- ferrous alloys, and semiconductor manufacture. Arsenic may be released into the environment from metal smelting and coal burning.
As his research advanced to the studies of gases and alkali metals, Bunsen recognized the importance of developing new methods to analyze and identify chemical substances. The importance of quantitative analysis was realized in the late eighteenth century. Chemists needed to probe further into a substance’s composition in order to help explain the physical world. Bunsen recognized this need and worked to develop new instruments for this purpose. For example, he invented new types of galvanic and carbonzinc electrochemical cells, or batteries, to isolate barium and sodium. He also constructed a new type of ice calorimeter that measured the volume, rather than the mass, of melted water. This allowed Bunsen to measure a metal’s specific heat in order to find its atomic weight.
Bunsen’s most lasting contribution to chemistry though was spectroscopy, which he developed in collaboration with the German physicist Gustav Kirchhoff. Bunsen became interested in analyzing the colors given off by heating chemicals to the point that they glowed. He heard that Kirchhoff was involved in similar work, and in 1854, Kirchhoff joined Bunsen at the University of Heidelberg. When Kirchhoff suggested that they observe the light being emitted from the elements by dispersing the light with a prism, the science of spectroscopy was born. When viewed through a prism, they found that the light was broken down into a series of lines, called spectral lines. Bunsen and Kirchhoff determined that the light emitted by each substance had its own unique pattern of spectral lines—a discovery that led to the spectroscopic method of chemical analysis.
It was during the process of developing spectroscopy that the Bunsen burner came into being. Bunsen realized that the spectral patterns observed were being contaminated by the light coming from the burner they were using to heat the elements. He modified the burner he was working with by mixing air into the gas before burning in order to obtain a high temperature, nonluminous flame.
Using the new burner, Bunsen and Kirchhoff were able to clearly see the spectra of all the chemicals they were studying. Together, they catalogued the spectra of all the known elements. This aided chemists enormously, because by identifying their spectral patterns, chemists could determine the composition of any unknown substance. In the process of cataloguing the spectra of the elements, Bunsen and Kirchhoff discovered two new elements that they named after the colors of their spectral lines: cesium (blue) and rubidium (red). Using Bunsen and Kirchhoff’s new analytical technique and the spectroscope they next developed, many new elements were subsequently discovered. But spectroscopy not only opened the door to the further analysis of earthly substances, the composition of the stars could also now be deduced.
Bunsen was a very modest man, despite being honored by some of Europe’s most prestigious scientific institutions. In 1853 he was elected to the Chemical Society in London and to the Academie des Sciences in Paris. He was named a fellow of the Royal Society of London in 1858 and received its Copley Medal in 1860. Bunsen and Kirchhoff were together awarded the first Davy Medal in 1877 for their development of spectroscopy. On his retirement in 1889, Bunsen turned his attention to another of his lifelong interests, geology. Bunsen’s contributions to chemistry included not just the Bunsen burner, but also many other instruments that allowed the physical world to be seen in new and informative ways.