2 50 OXIDATION AND REDUCTION ELEMENTS, chap.
})ecomes used up. Nevertheless, these elements are largely used in practice, for with them a fair yield of current can be obtained at an almost constant electromotive force com- paratively cheaply. Before the introduction of accumulators the commonest element in use was the Bunsen : Zn | HsSOf I HNO3 I C. The great disadvantage possessed by this ele- ment is that it gives off unpleasant nitrous fumes.
Leclanche's element differs from these others, inasmuch as the electrolyte is not sulphuric acid, but a concentrated solution of ammonium chloride. In common with acids, this substance possesses the power of dissolving metal oxides (ZnO) which are formed during the passage of the current ; water and ammonia are formed, and this latter combines partially with the metal chloride simultaneously produced. Ammonium chloride is also used in other elements, such as PoUak's regenerative element, which consists of porous (air-absorbing) carbon coated on the under side with galvanically deposited copper, ammonium chloride solution, and zinc. The copper is first oxidised by the absorbed oxygen, and then dis- solved by the ammonium chloride with formation of cupric chloride. Zinc then dissolves with production of zinc chloride, and an equivalent quantity of copper is deposited at the positive pole (the carbon), thus giving rise to the current. When the element is at rest the copper is again oxidised.
Alkalis can also dissolve certain metallic oxides, and therefore may replace acids in a Volta pile. This is made use of in the element of Lalande and Chaperon (copper element) (5), which consists of a metal (iron or copper) coated with copper oxide, 40 per cent, potassium hydroxide solution, and zinc. In order to prevent absorption of carbon dioxide by the alkali this must be covered tightly, or protected from the access of air by a film of petroleum. The chemical process which takes place is that zinc is oxidised by the copper oxide, and the zinc oxide dissolves in the alkali, with formation of potassium zincate. Copper is deposited at the negative pole, and by roasting this in the air it can again be oxidised.
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