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.
Now that the general features of electrode processes have been mentioned, the details of the passage of the electric current through the body of the solution can be investigated. Since the flow of either the positive or the negative ions, or both, might be responsible for conduction processes, our first goal is the determination fo the fraction of the curent carried by each type of ion of a given electrolyte. For this type of purpose the transference numbers t+ and t- are introduced according to the definitions:
t+ = fraction of current carried by cation.
t- = fraction of current caried by anion.
The fractional character of these transference numbers implies:
t+ + t- = 1
in metal conductors, e.g. a copper wire, all the current is carried by the electrons, and for such conductors one could write t- = 1 and t+ = 0. For solutions of electrolytes it is clearly difficult to anticipate what fraction of current is carried past some position in the electrolyte by the cations and what fraction by the anions.
One method, known as the Hittorf method, for measuring transference numbers is now illustrated by two examples. A schematic diagram of a cell marked off this three compartments. In practice, a cell of the type can be used, and the three compartments that can be drained off correspond to those marked off by the cross-section lines. The following treatment will show that transference numbers can be deduced from the analysis for the amount of electrolyte is the speparate compartments following passage of a mesured amount of current through the cell.
HCl example: consider an experiment in which a cell is filled with an HCl solution and charge is completely passed through the cell. The electrode processes, inidcated those processes. The current is carried across cross sections by the flow of ions. In view of the definitions of t+ and t-, the passage of charge across these sections is accompalished the flow of t+ mol of H+ to the right and t- mol of Cl- to the left. The net flow across these sections is then t+ + t- = 1 mol of ions, which corresponds to charge. It is clear from the number of moles of HCl in the central compartmetn should not be changed by the passage of current. Consider now the changes thath occur in the cathode portion. The change in the number of moles of H+ and Cl- due to ion migrations is shown by the transfers across the cross section line. In addition to migration, there is a removal of 1 mol of H+ at the electrode by the reaction:
H+ + e- ? ½ H2
The net cathode compartment changes for the passage are calculated as:
Change in H+ = electrode reaction + migration effect
= -1 + t+ = t+ - 1 = t- mol