Spectroscope, Kirchhoff-Bunsen

L. caesius, sky blue) Cesium was discovered spectroscopically by Bunsen and Kirchhoff in 1860 in mineral water from Durkheim. 

Rubidium was discovered as a minor constituent of lepidolite by R. W. Bunsen and G. R. Kirchhoff in 1861 only a few months after their discovery of caesium (1860) in mineral spa waters. These two elements were the first to be discovered by means of the spectroscope, which Bunsen and Kirchhoff had invented the previous year (1859) accordingly their names refer to the colour of the most prominent lines in their spectra (Latin rubidus, deepest red caesius, sky blue). 

Figure 5. The spectroscope of Bunsen and Kirchhoff. Above, figure from reference (8). Below, photo of original spectroscope on display case at Fleidelberg University. (Photo Copyright J. L. and V. R. Marshall.)... 

The year of the Karlsruhe Congress also saw the introduction of a technique which resulted in the detection of several new elements. The number of known elements had remained at 58 since 1844, since those as yet unknown were generally present in minerals in quantities too small to be revealed by the analytical techniques then in use. It was with the introduction of the spectroscope by Bunsen and Gustav Robert Kirchhoff (1824-1887) in 1860 that the detection of further elements became possible. 

Bohr s theory was received with a certain amount of scepticism by Rutherford, but it did have the advantage of explaining various features of atomic spectra. There had been numerous attempts to rationalise the lines observed in atomic emission spectra since the invention of the spectroscope by Bunsen and Kirchhoff in 1859 (Chapter 9). Little progress was made until 1885 when Johann Jacob Balmer (1825-1898), a Swiss school teacher, showed that the wavelengths of the four lines then known in the hydrogen spectrum could be expressed in terms of a simple equation. In 1890 Balmer s formula was rearranged by Johannes Robert Rydberg (1854-1919) to the form... 

Spectroscope developed Bunsen and Kirchhoff 1869 Mendeleev s first periodic table organizes 63 known elements 1885 Balmer formula for visible H spectrum 1894 First "inert gas" discovered 1895 X rays discovered Roentgen 1896 Radioactivity discovered Becquerel 1874 Tetrahedral carbon atom Le Bel and van t Hoff 1884 Dissociation theory of electrolytes Arrhenius 1869 Chain theory of ammonates Blomstrand 1884 Amendments to chain theory Jorgensen 1892 Werner s dream about coordination compounds... 

In the 1859 the chemist Robert Wilhelm Bunsen and his younger colleague, the physicist Gustav Kirchhoff, discovered a surprising phenomenon of spectroscopy. The emission and absorption spectra of an element are identical. They thus put into place an ideal tool for the discovery and identification of elements. Indeed, they themselves discovered cesium (1860) and rubidium (1861). In total, at least 20 elements were found by using spectroscopic technigues (including X-ray spectroscopy). 

The development of chemistry itself has progressed significantly by analytical findings over several centuries. Fundamental knowledge of general chemistry is based on analytical studies, the laws of simple and multiple proportions as well as the law of mass action. Most of the chemical elements have been discovered by the application of analytical chemistry, at first by means of chemical methods, but in the last 150 years mainly by physical methods. Especially spectacular were the spectroscopic discoveries of rubidium and caesium by Bunsen and Kirchhoff, indium by Reich and Richter, helium by Janssen, Lockyer, and Frankland, and rhenium by Noddack and Tacke. Also, nuclear fission became evident as Hahn and Strassmann carefully analyzed the products of neutron-bombarded uranium. 

The collaboration between Bunsen and Kirchhoff continued. Bunsen wanted to look for new elements while Kirchhoff was more interested in using the spectroscope to determine the chemical composition of the sun. However, this caused no conflict. Both Bunsen and Kirchhoff had ample opportunity to pursue their respective lines of research. 

Kirchhoff s and Bunsen s results created a sensation within the scientific community. Soon scientists throughout Europe were using spectroscopes in their research. Inevitably, some of them found yet more new elements. In 1861 the English chemist and physicist William Crookes discovered thallium, a heavy metal, in a sample of clay. In 1863 two German chemists, Ferdinand Reich and Hieronymous Richter, discovered another new metal, which they called indium. And in 1868 the French astronomer Pierre Jannsen observed the sun with a spectroscope during an eclipse and discovered the spectral lines of a new element, helium, in the sun s atmosphere. This was an especially sensational discovery, because helium had never been observed on Earth. 

Many elements are present in the earth s crust in such minute amounts that they could never have been discovered by ordinary methods of mineral analysis. In 1859, however, Kirchhoff and Bunsen invented the spectroscope, an optical instrument consisting of a collimator, or metal tube fitted at one end with a lens and closed at the other except for a slit, at the focus of the lens, to admit light from the incandescent substance to be examined, a turntable containing a prism mounted to receive and separate the parallel rays from the lens and a telescope to observe the spectrum produced by the prism. With this instrument they soon discovered two new metals, cesium and rubidium, which they classified with sodium and potassium, which had been previously discovered by Davy, and lithium, which was added to the list of elements by Arfwedson. The spectroscopic discovery of thallium by Sir William Crookes and its prompt confirmation by C.-A. Lamy soon followed. In 1863 F. Reich and H. T. Richter of the Freiberg School of Mines discovered a very rare element in zmc blende, and named it indium because of its brilliant line in the indigo region of the spectrum. 

German physicist and physical chemist. Professor of physics at Heidelberg and Berlin. Independent discoverer of the Kirchhoff-Stewart law of radiation and absorption. He explained the Fraunhofer lines of the solar spectrum, and, with Bunsen, founded the science of spectroscopic analysis and discovered the elements cesium and rubidium. 

Left to right) G. Kirchhoff, B. W. Bunsen, and H. E. Boscoe, in 1S62. Kirchhoff and Bunsen invented the spectroscope and founded the science of spectroscopic analysis. Roseoe collaborated with Bunsen in photochemical researches, and was the first to prepare metallic vanadium. 

In 1861 Robert Bunsen and G. R. Kirchhoff separated the alkalies from some lepidolite from Saxony and precipitated the potassium with platmic chloride. After they had washed this precipitate, they examined it with the spectroscope and observed two new hnes which proved to be those of an unknown element, which they named rubidium. The report runs as follows ... 

After the brilliant researches of Bunsen and Kirchhoff had paved the way, other new elements were soon revealed by the spectroscope. Among these may be mentioned thallium, indium, gallium, helium, ytterbium, holmium, thulium, samarium, neodymium, praseodymium, and lutetium. 

Bunsen is remembered chiefly for his invention of die laboratory burner umned after him. He engaged in a wide range of industrial and chemical research, including blast-furnace firing, electrolytic cells, separation of metals by electric current, spectroscopic techniques (with Kirchhoff). and production of light metals by electrical decomposition of their molten chlorides. He also discovered two elements, rubidium and cesium. 

Rubidium is a silver-white, very soft metal tarnishes instantly on exposure to air, soon ignites spontaneously with flame to form oxide best preserved in an atmosphere of hydrogen rather than in naphtha reacts vigorously with H2O forming rubidium hydroxide solution and hydrogen gas, Discovered by Bunsen and Kirchhoff m 1860 by means of the spectroscope. 

Line spectra were first observed by J. von Fraunhofer, D. Brewster, and J. F. W. Herschel in the 1820s.180 In the ensuing decades a considerable amount of work was done on spectral phenomena prior to the demonstration by Bunsen and Kirchhoff in 1859 that line spectra could be used for qualitative chemical analysis. Accounts have appeared of the development of the spectroscope both prior and post Bunsen and Kirchhoff.181-183 Significant observations were undoubtedly made prior to 1860 by Stokes, Stewart, Fox Talbot, and others. The priority claims of Stokes, who recorded his ideas in some private letters to William Thomson, have been examined.184 The work of Bunsen and Kirchhoff did not owe a great deal to that of their predecessors, with the exception of the demonstration by W. Swan in 1856 that the almost omnipresent yellow line that coincided with Fraunhofer s dark solar D line was due to contamination by minute quantities of sodium salts.185 186 Platinum played an important role in the early development of spectroscopy. The metal was widely used to support the material in the flame, since it did not colour the flame itself. Bunsen ensured the purity of all his samples for spectrum analysis by recrystallization (sometimes up to fourteen times) in platinum vessels, thereby preventing contamination by minute quantities of salts that could be leached from glass vessels.187 Sharply contrasting views have been expressed about the failure of chemists prior to Bunsen to exploit spectroscopy.188-190... 

After Bunsen had detected and isolated caesium, spectroscopy was taken up with great enthusiasm by William Crookes, and this led to his detection and isolation of thallium in 1861.191 Crookes letters to Charles Hanson Greville Williams, who was also working with the spectroscope, and who felt he deserved some of the credit for the discovery of thallium, have been published.192 The use of spectrochemistry in the search for hitherto unknown chemical elements in Britain over the period 1860-1869 has been described. It was perceived that, like Crookes, a scientist could make his reputation by discovering a new element. This resulted in several claims for the existence of new elements that later proved to be unfounded.193 Once Kirchhoff had established beyond doubt that the dark Fraunhofer lines were caused by the same element that caused emission lines of identical wavelengths, the way was open for the chemical analysis of the atmosphere of the sun and stars. This was a process which had been declared to be an impossibility by Auguste Comte less than 30 years previously.194... 

The existence of these different practices was not sufficient to create a discipline or subdiscipline of physical chemistry, but it showed the way. One definition of physical chemistry is that it is the application of the techniques and theories of physics to the study of chemical reactions, and the study of the interrelations of chemical and physical properties. That would mean that Faraday was a physical chemist when engaged in electrolytic researches. Other chemists devised other essentially physical instruments and applied them to chemical subjects. Robert Bunsen (1811—99) is best known today for the gas burner that bears his name, the Bunsen burner, a standard laboratory instrument. He also devised improved electrical batteries that enabled him to isolate new metals and to add to the list of elements. Bunsen and the physicist Gustav Kirchhoff (1824—87) invented a spectroscope to examine the colors of flames (see Chapter 13). They used it in chemical analysis, to detect minute quantities of elements. With it they discovered the metal cesium by the characteristic two blue lines in its spectrum and rubidium by its two red lines. We have seen how Van t Hoff and Le Bel used optical activity, the rotation of the plane of polarized light (detected by using a polarimeter) to identify optical or stereoisomers. Clearly there was a connection between physical and chemical properties. 

The righthand illustration shows the simple and elegant construction of Bunsen and Kirchhoff s first form of spectroscope. A prism is supported within a box (A). 

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