Page:Text-book of Electrochemistry.djvu/260

 hydrogen ions to the oxidising agent, and negative ions to the reducing agent.

In a similar way, the chemical energy which is obtained when solutions of sodium chloride and silver nitrate are brought into contact (silver chloride being precipitated, and sodium nitrate remaining in solution) may be transformed. This can be done in the element —

Ag I NaCl I NaNOs | AgNOs | Ag.

This may be viewed as a concentration element. The osmotic pressure of the silver ions in the sodium chloride solution is very small, therefore silver ions pass into solu- tion there, and silver is separated from the silver nitrate solution. A layer of silver chloride is formed evidently at the expense of the silver nitrate and sodium chloride solu- tions, with simultaneous production of sodium nitrate, as shown by the scheme —

(1) AgAg I ClNa I NOaNa | NOsAg | Ag,

(2) Ag I AgCl I NaNOs + NaNOa | AgAg.

It is a characteristic of all galvanic elements that at the poles two (or more) substances are present which, when brought into contact, react with one another chemically, but which are so separated in the element by one (or more) electrolyte from each other that no chemical action takes place between them except that due to unavoidable diffusion. By means of the current, ions are transported from the separating electrolytes, and so the chemical action becomes possible. In a Daniell element, for instance, the reacting substances are zinc and copper sulphate, which are at the poles, but are separated from each other by sulphuric acid, zinc sulphate, or some other sulphate — magnesium sulphate in Meidinger's modification (3) of the element. By putting the poles in metallic connection, a current is spon- taneously produced, which transports the ions according to the scheme —

Cu| CuSOitxSOi I ZnZn,

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