Johann von Baeyer | Robert Bunsen | August Wilhelm von Hofmann
Born in Göttingen on 31 March, 1811, the German Robert Wilhelm Bunsen is considered one of the greatest, and is certainly one of the best known, chemists of all time. He is best known for the eponymous Bunsen burner used in school chemistry laboratories around the world. However, contrary to popular belief, he had little to do with the invention of this device, although he did much to popularise it.
After graduating from the University of Göttingen in 1831, he visited Berlin, Paris and Vienna. In 1833, Bunsen became Privatdozent1 at Göttingen, and in 1836 he succeeded Wöhler as professor at the Technical School in Kassel. In 1839, he was appointed associate professor, and in 1841 professor of chemistry at the University of Marburg. In 1846, he took part in a scientific expedition to Iceland, following which he spent a year as professor of chemistry at Breslau (now Wroclaw in Poland). In 1852, Bunsen accepted the chair of chemistry at the University of Heidelberg - a post he held for 37 years until his retirement in 1889.
During his career, Bunsen's work ranged over a wide field, and several other famous chemists received training in his laboratories. These include the Russian chemist Dmitri Mendeleev, Victor Meyer and Friedrich Kekulé. One of his PhD students was Fritz Haber, who was later to devise the process for synthesising ammonia from its elements - used in the manufacture of fertilisers.Bunsen first came to prominence due to his work investigating the constituent radicals of organic chemistry. He was following up the work of such luminaries as Berzelius, Liebig and Dumas, who were all convinced of the existence of groups of elements (or 'radicals') as constituents of organic compounds. Bunsen chose to investigate cacodyl compounds2, and succeeded in proving the existence of the radical C4H12As2 (C2H6As). By heating the chloride C2H6AsCl with zinc, Bunsen obtained the spontaneously flammable free cacodyl C4H12As2, nowadays written as
Bunsen was a highly talented experimenter rather than a theoretical chemist. The breadth of his interests can be deduced by considering some of his inventions. These include the ice calorimeter (to measure the specific heat capacity of metals), a grease-pot photometer (to measure light intensity), a filter pump, and the Bunsen cell (a battery to isolate pure metal elements by electrolysis). He was also involved in analysing the waste gases of blast furnaces, and also improved the methods of mineralogical analysis by means of dry tests and flame colourations. In 1834, Bunsen discovered that hydrated iron (III) oxide could be used as an antidote to arsenic poisoning, and this is still used today.
The Birth of Spectroscopy
One of Bunsen's most significant achievements was the work he carried out at Heidelberg with the physicist Gustav Robert Kirchhoff, concerning the examination of the coloured flames from metal salts. To understand Bunsen's contribution, it is necessary to outline what was known up until that point.
As early as 1758, the German chemist AS Sigismund had shown that the salts of sodium and potassium can be distinguished by the yellow and lavender colours they impart, respectively, to a flame. Furthermore, in 1814 a young Bavarian instrument-maker, Josef Fraunhofer3, had shown that the Sun's spectrum is traversed by a host of dark lines, which came to be known as 'Fraunhofer lines'.
Then, in 1822, the English astronomer Sir John Herschel showed that when coloured light from burning salts is passed through a prism, a spectrum of bright lights separated by dark spaces is also obtained. His friend William Henry Fox Talbot had also been looking at the coloured flames generated by burning various substances, possibly with Herschel. In 1826, Fox Talbot published a paper in which he stated that:
A glance at the prismatic spectrum of a flame may show it to contain substances which it would otherwise require a laborious chemical analysis to detect.
Fox Talbot continued with this investigation, and in 1834 wrote that some lithium, which he had obtained from Faraday, and strontium, both of which burned with a red light, looked the same until examined with a spectroscope; then:
the prism betrays between them the most marked distinction which can be imagined.
He then continues to describe the single line of lithium and a collection of lines from strontium. Meanwhile, in 1833, another Englishman, William Hallowes Miller, continued Herschel's work by examining sunlight after it had passed through various gases in the laboratory, and found that additional dark lines appeared. These lines were obviously due to the absorption of various wavelengths present in sunlight. From this it was deduced that the Fraunhofer lines must be due to absorption by gases in the outer layers of the Sun.
Another step was taken in 1855 when Foucault showed that the very close pair of dark lines in the solar spectrum, to which Fraunhofer had assigned the letter 'D', coincided exactly with the pair of bright yellow lines given out by metallic sodium in the laboratory. Thus a general belief arose, encapsulated by Bunsen in a letter to Henry Roscoe, that:
a means has been found to determine the composition of the Sun and fixed stars with the same accuracy as we determine sulphuric acid, chlorine, etc, without chemical reagents.
A further advance was made in 1858 by Balfour Stewart, working in Heidelberg, when he showed that the power of a substance to radiate heat is equal to its power to absorb heat radiation. This was quickly applied to the power of a body to radiate light waves as well. It was important because it meant a body radiates at exactly the same wavelengths as it absorbs. In 1860, Kirchhoff, also in Heidelberg, made the same discovery independently. So the stage was set for finding the composition of the Sun and stars, as had been suggested by Bunsen (and others, such as Lord Kelvin).
The Kirchhoff-Bunsen Spectroscope
In 1859, at Heidelberg, Bunsen and Kirchhoff began to systematically investigate the colours and positions of the bright spectral lines emitted by elements. Kirchhoff's mind was more speculative than Bunsen's, and he was familiar with the research of Newton, Fraunhofer and Clausius. He showed Bunsen that, instead of merely looking through coloured glass to distinguish between similarly coloured flames, he should use a prism to separate the light into its constituent rays. In the memoir in which they reported their findings, they described the construction of a 'spectroscope' for the examination of coloured flames. Therefore, they introduced into practical chemistry a new instrument of chemical analysis and a powerful aid to the discovery of new elements.
Kirchhoff and Bunsen found that the spectral lines occupied definite positions, corresponding to definite wavelengths of light, and depend only on the metal present in the vaporised salt. The positions of the spectral lines were not altered by the presence of other (contaminating) salts. They wrote:
The positions which they [the spectral lines] occupy in the spectrum are due to a chemical property as invariable and of as fundamental a nature as the atomic weight, and can therefore be determined with almost astronomical accuracy.
So it became possible to analyse spectroscopically a mixture of salts by examining the colours imparted to a flame by the mixture. The method of spectroscopy showed itself to be superior to the ordinary (wet) methods of chemical analysis used hitherto, which depended largely on the formation of differently coloured precipitates, since, in this case, the colours of the different precipitates could interfere with one another.
Spectroscopic analysis is also infinitely more sensitive than wet chemical analysis. Kirchhoff and Bunsen showed that it was easily possible to detect the presence of less than one three-millionth of a milligram of sodium salt, and the lithium in 20g of sea water. Thus spectroscopic methods can be used to detect elements which may be present in extremely small amounts, or at great dilution.
Kichhoff and Bunsen demonstrated this very quickly after their invention of the spectroscope when they discovered a new alkali metal present in the mineral water of Durkheim. Writing to Sir Henry Roscoe on 10 April, 1860, Bunsen said:
I have obtained full certainty, by means of spectrum analysis, that besides Ka, Na and Li, a fourth alkali metal must exist... Where the presence of this body is indicated, it occurs in such minute quantity that I almost give up hope of isolating it, unless I am fortunate to find a material which contains it in larger amount.
Later the same year, in another letter to Roscoe dated 6 November, 1860, Bunsen wrote:
I have been very fortunate with my new metal. I have got 50g of the nearly chemically pure chloro-platinic compound. It is true that this 50g has been obtained from no less than 40 tons of the mineral water... I am calling the new metal caesium (from caesius = blue) on account of the splendid blue line in its spectrum.
Caesium was the first element to be discovered by means of the spectroscope. A few months later, on 23 February, 1861, Kirchhoff and Bunsen reported the discovery of yet another alkali metal. It was detected in the mineral lepidolite, and was characterised by two red lines towards the extreme end of the visible spectrum. They wrote:
The magnificent dark-red colour of these rays led us to give this element the name rubidium, from rubidus, which, with the ancients, served to designate the deepest red.
So the Kirchhoff-Bunsen spectrometer revolutionised chemical analysis and became of supreme importance to the discovery of new elements.
Returning to the Bunsen burner, Bunsen required a high-temperature gas flame that burned with near invisibility for his experiments on spectroscopy. In 1885, he modified a burner which had been designed by his young technician, Peter Desdega, at Heidelberg. This was a simple metal tube connected to a gas supply, sitting on a circular base with adjustable air inlets around the base of the tube. It enabled the fuel:air ratio to be controlled to attain the highest flame temperatures; if correctly set, the hottest point should be just above the flame, where the temperature can exceed 1,500°C.
Bunsen's work and character are encapsulated in the words of one of his pupils, HE Roscoe, who wrote:
As an investigator he was great, as a teacher he was greater, as a man and friend he was greatest.
Bunsen died on 16 August, 1899.