Potassium was first isolated in 1807 by Humphry Davy (1778-1829), who electrolyzed “potash” with a newly invented battery designed to contain a series of voltaic cells, with electrodes made out of zinc and copper plates dipped in a solution of nitrous acid and alum. In Davy’s time, the term “potash” referred to any number of different compounds, including “vitriol of potash” (potassium sulfate), “caustic potash” (potassium hydroxide), and “muriate of potash” (potassium chloride as well as potassium carbonate), the last of which was formed by leaching ashes from a wood fire and evaporating the solution to near dryness in an iron pot. Today, potash is usually potassium carbonate, although potassium chloride is still called potash by fertilizer manufacturers (Kent 1983: 262). The potash Davy used was potassium hydroxide that he had dried and melted. He wrote of his experiment:
A small piece of pure potash, which had been exposed for a few seconds to the atmosphere, so as to give conducting power to the surface, was placed upon an insulated disc of platina, connected with the negative side of a battery of the power 250 of 6 and 4, in a state of intense activity; and a platina wire communicating with the positive side, was brought in contact with the upper surface of the alkali. The whole apparatus was in the open atmosphere. Under these circumstances a vivid action was soon observed to take place. The potash began to fuse at both its points of electrization. There was a violent effervescence at the upper surface; at the lower, or negative surface, there was no liberation of elastic fluid; but small globules having a high metallic lustre, and being precisely similar in visible characters to quicksilver formed, and others remained and were merely tarnished, and finally covered with a white film which formed on their surfaces. These globules, numerous experiments soon shewed to be the substance I was in search of, and a peculiar inflammable principle the basis of potash. (Davy 1839-40,5:60)
The next day, Davy isolated sodium metal by electrolyzing soda ash (sodium hydroxide) in much the same way. In the history of chemistry, isolating potassium and sodium was no mean accomplishment. It had been suspected by several people, but especially by Antoine Lavoisier (1743-94), that potash was a compound and that the “basis of potash” was, indeed, a metal (Partington 1962, 3: 485). Davy’s experiments confirmed this suspicion. Several years earlier, in 1801, Carl Friedrich Kielmeyer, also suspecting that potash was an oxide of some metal, had attempted to electrolyze potash using a voltaic pile but was unsuccessful (Partington 1964, 4: 45). Thus, the credit for discovering the two most important alkali metals clearly goes to Humphry Davy.
Between 1808 and 1809, the French chemists Louis Thenard (1777-1857) and Joseph Gay-Lussac (1778-1850) found that only small quantities of potassium and sodium could be derived by the electrolysis of fused alkali hydroxides and went on to develop a much improved method for producing larger quantities of both (Partington 1964,4: 94).Thenard and Gay-Lussac reacted the fused alkali with red-hot iron turnings in an iron gun barrel lined with clay and sand and collected the condensed metal vapor in a receiver attached to the gun barrel. An explosion using this dangerous device nearly blinded Gay-Lussac.
A variation that further improved the method for producing potassium used potassium carbonate as the source of potassium and carbon instead of iron as the reducing agent to produce elemental potassium and carbon dioxide as the reaction products. In 1827,
Frederich Wohler (1800-82) first employed potassium produced by this technique to isolate metallic aluminum in more or less pure form. He reacted anhydrous aluminum chloride with potassium metal as the reducing agent and obtained enough aluminum metal to measure its properties (Ihde 1964: 467). Today, metallic potassium is usually produced by reacting molten sodium with molten potassium chloride and condensing the gaseous potassium formed by this reaction. There are very few industrial uses for elemental potassium, although many of its compounds are widely utilized throughout industry and agriculture. For example, potassium nitrate is commonly employed as a fertilizer in the tobacco industry where chloride-containing fertilizers are undesirable.
World History
