Gustav Kirchhoff – Colored Flames, Prisms, and Element Discovery

Gustav Robert Kirchhoff was born on March 12, 1824, in Königsberg, Prussia (today Kaliningrad in Russia) [1–3]. He is well-known for his contributions to the understanding of electrical circuits, spectroscopy, black-body radiation, and thermochemistry. Together with Robert Bunsen, Kirchhoff invented the spectroscope, and using this device, the two researchers discovered the alkali metal elements cesium and rubidium [3,4].

Career and Honors

Gustav Kirchhoff studied mathematics and physics in Königsberg and graduated with a Dr. phil. degree in 1847. He then moved to Berlin, where he completed his habilitation, and in 1850, he became Associate Professor in Breslau (today Wrocław in Poland). There, he met Robert Bunsen, who moved to Heidelberg, Germany, in 1952, and lobbied for Kirchhoff to join him, which he did in 1854. There, Kirchhoff taught experimental physics and mathematical physics, until an accident in 1868 confined him to a wheelchair and put an end to his experimental work. Kirchhoff focused on theoretical work, and moved to the University of Berlin, Germany, in 1875 as Professor of Theoretical Physics. Gustav Kirchhoff retired in 1886 and passed away on October 17, 1887, in Berlin.

Kirchhoff was a Member of the American Philosophical Society, the American Academy of Arts and Sciences, the Prussian Academy of Sciences, the U.S. National Academy of Sciences, and the Royal Netherlands Academy of Arts and Sciences, a Foreign Member of the Göttingen Academy of Sciences and the Royal Society, UK, a Corresponding Member of the Russian Academy of Sciences and the French Academy of Sciences, an Associate Member of the Royal Academy of Science, Letters and Fine Arts of Belgium, and an Honorary Fellow of the Royal Society of Edinburgh. He received the Rumford Medal in 1862 and the Davy Medal in 1877, both from the Royal Society, as well as the Cothenius Medal from the German National Academy of Sciences Leopoldina in 1876. The Bunsen–Kirchhoff Award for Spectroscopy of the German Working Group for Applied Spectroscopy is named after Kirchhoff and Bunsen.

Research

Kirchhoff’s Circuit Laws

In 1845, while still at university, Kirchhoff devised laws that describe the behaviors of currents and voltages in electrical circuits [5]. They are widely used in electrical engineering.

Kirchhoff’s first law, or Kirchhoff’s junction rule, states that, for any node in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node. Kirchhoff’s second law, also called Kirchhoff’s voltage law or Kirchhoff’s loop rule, states that the directed sum of the potential differences (i.e., voltages) around any closed loop is zero.

Kirchhoff’s Law of Thermochemistry

Kirchhoff showed that the variation of the enthalpy of a reaction with the temperature is given by the difference in heat capacity between products and reactants (at constant pressure and if the heat capacities do not vary with temperature) [6], i.e.:

dΔH / dT = ΔC_p
(H = enthalpy, T = temperature, and C_p = heat capacity)

Integrating this equation allows for the evaluation of the enthalpy of a reaction at one temperature from measurements at another temperature.

Spectral Analysis and the Discovery of Cesium and Rubidium

In Heidelberg, Robert Bunsen worked on identifying different elements by flame tests. For this, a sample is heated in a flame and the resulting color change can provide information about some of the elements in the sample. Bunsen used colored glass as a filter to be able to distinguish similar flame colors—lithium and strontium, for example, both give red flames. Bunsen created the burner named after him for use in such flame tests. It provides a non-luminous flame that does not interfere with the flame color induced by the sample.

Kirchhoff suggested that a prism could help to distinguish elements with similar flame colors by separating the colors of light, which led the two researchers to develop the spectroscope [3,7]. This instrument constituted a breakthrough in the elemental analysis of minerals—and led to the discovery of new elements.

Kirchhoff and Bunsen used their new spectroscope to analyze mineral waters from Bad Dürkheim, a spa town about 40 km from Heidelberg. They observed blue lines in the spectrum that corresponded to a previously unknown element, which they called cesium, from the Latin caesius for a type of blue. Cesium was the first element to be discovered with a spectroscope. To obtain an actual sample of a cesium salt, they had to evaporate over 40 tons of mineral water and isolate the cesium compound from the resulting concentrated salt solution. About 7 g of cesium chloride were obtained [8]—which demonstrates how sensitive the spectroscopic detection of elements can be.

There also were fainter red spectral lines that belonged to a second new element, which they named rubidium after the Latin rubidus for a deep red. The concentrated salt solution obtained from the Bad Dürkheim mineral water contained about 9 g of rubidium chloride. Another source for this element is lepidolite, a mineral in the mica group. Bunsen and Kirchhoff processed 150 kg of lepidolite to isolate a sample of rubidium chloride [8].

Overall, the systematic attribution of spectra to chemical elements developed by the two researchers was a breakthrough in chemical analysis. It can also be used in astronomy because absorption lines in the spectra of stars can be attributed to the presence of certain elements.

Kirchhoff’s Three Laws of Spectroscopy

Based on his work on spectroscopy and during his time in Heidelberg [2], Kirchhoff devised three laws that explain the different types of spectra that can be observed:

An incandescent solid, liquid, or dense gas emits a continuous spectrum.
A hot gas at low density emits light at specific wavelengths, giving a spectrum composed of emission lines.
Light from a continuous-spectrum source viewed through a cool, low-density gas produces a spectrum with absorption lines.

Kirchhoff’s Law of Thermal Radiation

All objects with a non-zero temperature radiate electromagnetic energy. A perfect “black body” in thermodynamic equilibrium absorbs all light that strikes it and radiates energy depending on its temperature and surface area. The ratio of the energy radiated by an object that is not a perfect black body to the energy radiated by a black body at the same temperature is called the emissivity. The fraction of incident light that is absorbed by the object is called absorptivity.

Kirchhoff’s law of thermal radiation states that for an arbitrary body emitting and absorbing thermal radiation in thermal equilibrium, the emissivity is equal to the absorptivity. He formalized this law in 1860 [9].

Gustav Robert Kirchhoff is the answer to Guess the Chemist (147).