Bunsen, Robert Wilhelm

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Bunsen, Robert Wilhelm (1811-1899), German chemist, who, with the German physicist Gustav Robert Kirchhoff, invented the spectroscope and discovered spectrum analysis, which led to their joint discovery of the elements cesium and rubidium.

Bunsen was born in Göttingen on March 31, 1811, and was educated at the University of Göttingen. Between 1836 and 1852 he taught successively at the Polytechnic Institute in Kassel and at the universities of Marburg and Breslau; thereafter he was professor at the University of Heidelberg until his retirement in 1889. Considered one of the greatest chemists in the world, Bunsen discovered (1834) the antidote that is still used today for arsenic poisoning: hydrated iron oxide. His research on the double cyanides confirmed the principle in organic chemistry that the nature of a compound depends on the radicals composing it. Contrary to popular belief, he had little to do with the invention of the Bunsen burner, a gas burner used in scientific laboratories. Although Bunsen improved and popularized the device, credit for its design should go to the British chemist and physicist Michael Faraday. Among Bunsen's inventions are the ice calorimeter, a filter pump, and the zinc-carbon electric cell. He used the cell to produce an electric-arc light and invented a photometer to measure its luminosity. The cell was used also in his development of an electrolytic method of producing metallic magnesium. Results of his research on waste gases of blast furnaces were published in the classic Gasometric Methods (1857). Bunsen died in Heidelberg on August 16, 1899.

Kirchhoff, Gustav Robert

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Kirchhoff, Gustav Robert (1824-87), German physicist, born in Königsberg (now Kaliningrad, Russia), and educated at the University of Königsberg. He was professor of physics at the universities of Breslau, Heidelberg, and Berlin. With the German chemist Robert Wilhelm Bunsen, Kirchhoff developed the modern spectroscope for chemical analysis. In 1860 the two scientists discovered the elements cesium and rubidium by means of spectrum analysis. Kirchhoff conducted important investigations of radiation heat transfer and also postulated two rules, now known as Kirchhoff's laws of networks, concerning the distribution of current in electric circuits.

Davy, Sir Humphry

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Davy, Sir Humphry (1778-1829), renowned British chemist, best known for his experiments in electrochemistry and for his invention of a miner's safety lamp.

Davy was born on December 17, 1778, in Penzance, Cornwall, England. In 1798 he began experiments on the medicinal properties of gases, during which he discovered the anesthetic effects of nitrous oxide (laughing gas). Davy was appointed assistant lecturer in chemistry at the newly founded Royal Institution in London in 1801 and the following year became professor of chemistry there.

During his early years at the Royal Institution, Davy started his investigations of the effects of electricity on chemical compounds. In 1807 he received the Napoleon Prize from the Institut de France for the theoretical and practical work begun the year before. He then constructed the largest battery ever built, with over 250 cells, and passed a strong electric current through solutions of various compounds suspected of containing undiscovered elements. Davy quickly isolated the elements potassium and sodium by this electrolytic method. He also prepared calcium by the same method. In later, unrelated experiments, he discovered boron and proved that the diamond is composed of carbon. Davy also showed that the so-called rare earths are oxides of metals rather than elements. His experiments with acids indicated that hydrogen, not oxygen, causes the characteristics of acids. Davy also made notable discoveries in heat.

In the field of applied science, Davy invented a safety lamp for miners in 1815. For this and for related research, he received the gold and the silver Rumford medals from the Royal Society. In 1823 he suggested a method of preventing the corrosion of the copper bottoms of ships by means of zinc and iron sheathing. He was knighted in 1812 and raised to a baronetcy in 1818. In 1820 he became president of the Royal Society. Davy died on May 29, 1829, in Geneva.

Among his writings are Elements of Chemical Philosophy (1812) and Elements of Agricultural Chemistry (1813).

Electrochemistry

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Electrochemistry, that part of the science of chemistry that deals with the interrelationship of electrical currents, or voltages, and chemical reactions, and with the mutual conversion of chemical and electrical energy. In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages.

ELECTRIC CURRENT AND ION MOVEMENT

Most inorganic and some organic chemical compounds, when in a molten state or when dissolved in water or other liquids, become ionized; that is, their molecules become dissociated into positively and negatively charged components, which have the property of conducting an electric current (see Ion; Ionization). If a pair of electrodes is placed in a solution of an electrolyte, or an ionizable compound, and a source of direct current is connected between them, the positive ions in the solution move toward the negative electrode and the negative ions toward the positive. On reaching the electrodes, the ions may gain or lose electrons and be transformed into neutral atoms or molecules, the nature of the electrode reactions depending on the potential difference, or voltage, applied.

The action of a current on an electrolyte can be understood from a simple example. If the salt copper sulfate is dissolved in water, it dissociates into positive copper ions and negative sulfate ions. When a potential difference is applied to the electrodes, the copper ions move to the negative electrode, are discharged, and are deposited on the electrode as metallic copper. The sulfate ions, when discharged at the positive electrode, are unstable and combine with the water of the solution to form sulfuric acid and oxygen. Such decomposition caused by an electric current is called electrolysis.

In all cases, the quantity of material evolved at each electrode when current is passed through an electrolyte follows a law discovered by the British chemist and physicist Michael Faraday. This law states that the quantity of material transformed at each electrode is proportional to the quantity of electricity passed through the electrolyte; and that the weight of the elements transformed is proportional to the equivalent weights of the elements, that is, to the atomic weights of the elements divided by their valences. (See Chemical Reaction; Valence).

All chemical changes involve a regrouping or readjustment of the electrons in the reacting substances; hence all such changes may be said to be electrical in character. To produce an electrical current from a chemical reaction, it is necessary to have a reducible substance, that is, a substance that can gain electrons easily; and an oxidizable substance, one that can give up electrons easily. A reaction of this kind can be understood from the operation of a simple type of electrochemical cell, or battery. If a zinc rod is placed in a dilute solution of sulfuric acid, the zinc, which oxidizes readily, will lose electrons, and positive zinc ions will be liberated into the solution. The free electrons stay in the zinc rod. If the rod is connected through a conductor to an inert-metal electrode placed in the sulfuric acid solution, the electrons will flow around this circuit into the solution, where they will be taken up by the positive hydrogen ions of the dilute acid. The combination of the electrons and the ions produces hydrogen gas, which appears as bubbles on the surface of the electrode. The reaction of the zinc rod and sulfuric acid thus produces a current in the external circuit. An electrochemical cell of this kind is known as a primary cell, or voltaic cell.

In the storage battery or accumulator, commonly known as a secondary cell, electrical energy is fed to the cell from an outside source and stored within in the form of chemical energy. The chemical reaction of a secondary cell is reversible, proceeding in one direction when the cell is being charged, and in the opposite direction when it is discharging. Because the reaction is of this type, a secondary cell can be discharged again and again.

Thermoelectricity

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Thermoelectricity, in physics, electricity generated by the application of heat to the junction of two dissimilar materials. If two wires of different materials are joined at their ends and one end is maintained at a higher temperature than the other, a voltage difference will arise, and an electric current will exist between the hot and the cold junctions. This phenomenon was first observed in 1821 by the German physicist Thomas Seebeck and is known as the Seebeck effect.

For a given combination of materials, the voltage difference varies in direct proportion to the temperature difference. This phenomenon can be utilized for the accurate measurement of temperature by means of a thermocouple in which one wire junction is maintained at a known reference temperature (for example, in an ice bath) and the other at the location where the temperature is to be measured. At moderate temperatures (up to about 260° C/500° F), wire combinations of iron and copper, iron and constantan (a copper-nickel alloy), and copper and constantan are frequently used. At high temperatures (up to 1649° C/3000° F), wires made from platinum and a platinum-rhodium alloy are employed. Because thermocouple wires can be made very small, they also provide a means for the accurate measurement of local spot temperatures. The current can be increased by using semiconductors instead of metals, and a few watts of power can be produced at efficiencies of up to 6 percent (Transistor).

The inverse effect occurs if current is sent through a circuit made of dissimilar materials, the junctions of which are at the same temperature. In this case, heat will be absorbed at one junction and given up at the other. This phenomenon is known as the Peltier effect for the French physicist Jean Peltier, who discovered it in 1834. Semiconductor systems operating on the Peltier effect can be used as low-powered miniaturized refrigerators for special applications.