1859

Flame Spectroscopy

William Hyde Wollaston (1766–1828), Joseph von Fraunhofer (1787–1826), Robert Bunsen (1811–1899), Gustav Kirchhoff (1824–1887), Alan Walsh (1916–1998)

Many non-chemists have heard of the Bunsen burner as a piece of lab equipment (although you don’t see as many of them around these days), but not many people know why Robert Bunsen, a German chemist, invented it. He needed a better way to do flame tests, an old analytical technique that depends on the emission spectra of various elements—i.e., the light given off in the high-energy environment of a flame as electrons jump between energy levels. Sodium, for example, shows a vivid yellow-orange, strontium has bright red, and copper has blue-green. But a hot and colorless flame is needed to see all these colors, and Bunsen invented a better way to mix air and gas to create one.

His colleague, the German physicist Gustav Kirchhoff, then suggested that Bunsen use the new technique of splitting the light with a prism to make even finer distinctions. (Lithium, for example, also gives a red flame, which can be hard to distinguish from strontium with the naked eye.) In 1859, they built the first spectroscope, an extremely powerful tool for identifying elements—and for discovering them, too. A blue line in a sample of mineral water turned out to be the new element cesium, and Bunsen discovered another red-emitting element (rubidium) as well.

There was even more to be found with this technique. English chemist William Hyde Wollaston (followed by the German physicist Joseph von Fraunhofer) had noticed that the prismatic spectrum of sunlight had mysterious dark lines in it. Bunsen and Kirchhoff realized that many of them were in the same places in the spectrum as the bright emission lines they studied, which suggested that they were coming from elements in the sun itself that were soaking up light at these wavelengths. Suddenly it was possible to determine the chemical makeup of the sun (and other stars!) while sitting at a laboratory bench. Modern atomic spectroscopy instruments (invented by British physicist Sir Alan Walsh and others) use the same principles to analyze elements down to parts per billion to trace water contamination, among other uses.

The bright colors of fireworks are flame tests in the sky. Strontium and lithium are used for red, sodium for yellow, and barium for green. A good blue is said to be the hardest to formulate.