Passage of a current through a solution can produce an electrolysis reaction.
Much additional information on the properties of the ions in an aqueous solution can be obtained from studies of the passage of a direct current (dc) through a cell containing a solution of an electrolyte. Such depression dc experiments involve chemical reactions at the electrodes, a feature that is avoided in conductivity studies by the use of an alternating current (ac). It is first necessary, therefore, to describe and classify these electrodes processes.
When electrodes are inserted in a solution of electrolyte and a suficient potential, of the other several voids, is applied, chemical reactions are observed at the electrodes. Electrolysis is said to be occuring. The electrode that is charged postively charged, i.e. having a deficit of electrons, by the applied potential is called the anode, and that charged negatively is called cathode. The electrodes consist of conductors that introduce the source and sink of electrons into the solution.
In classfying the reactions that occurs as a result of the charged electrodes, it is convinient to distinguish inert electrodes, ususally a platinum wire, that serve only to transfer electrons to and from the solutoin, from reacting electrodes that enter chemically into the electrode reactoin. Most simply, the reacting electrode is a metal that contributes metal ions to the solutoin.
The two major categories of electrode reactons that occur in the electrolysis cells can be constructed that incolve various combinations of these reactoin types, consistent with the requirement that at the cathode electrons are introduced by the external circuit and reducton occurs, whereas at the cathode electrons are introduced and oxidatoin occurs.
More complicated electrode reactions do occur, but the solutions studied can be dealt with terms of electrode reactions of these major types.
Electrolysis of the type illustrated here were extensively and quantitively studied as early as 1820 by Michael Faraday. He recognised that the amount of charge passed through an electrolyte was quantitatively related to the amounts of products formed at the electrodes. These quantities are conviniently related by introducing the formula of faraday unit of charge, with the symbol f, defined as the charge of 1 mol, or an Avogadro's number, of electrons.
The transference number gives the function of the current carried throughout solution by an ion of a particular type.
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