The principles of electrochemistry

Abstract

The Hittorf method of determining transference numbers is the one most commonly used in elementary physical chemistry laboratories to demonstrate the migration of ions.1 There are some points in both the theory of the method and in the experimental procedure which many students have difficulty understanding. The purpose of this paper is to show how these difficulties can be cleared up in a fairly rigorous manner. The method requires the determination of the change in concentration of the electrolyte in either the anode or cathode compartment after the passage of an amount of electricity which is measured by a coulometer. The initial concentration of the solution is known, and the final concentration is measured by withdrawing the solution from one electrode compartment and analyzing it. However, since volume concentrations such as normality and molarity can change without there being any transport of matter involved, the concentration of the electrolyte in the compartment is reported in terms of equivalents per grams of water in the compartment , and this is compared with the equivalents of electrolyte that were with these same grams of water before electrolysis. In order to make sure that all of the solution which has changed in concentration is obtained, the compartment is washed with the original solution. These washings are combined with the liquid withdrawn, the entire amount is weighed, and then its concentration is determined. Invariably the student asks the following questions: (1) Why is it necessary to draw out all the liquid and weigh it, since only a part of it is used for the volumetric analysis in-volved? (2) Why can't the calculations be made on the basis of equivalents per one gram of water rather than equivalents per total grams of water in the compartment? (3) Doesn't washing the compartment with the original solution and combining it with the compartment liquid change the latter's concentration and consequently the transference number? (4) Shouldn't the amount of wash solution used be known and correction made for it? (5) Why can't the compartment be washed with water rather than with the original solution? Furthermore, in their calculations students often obtain values for transference numbers which are reversed , i. e., the value for the cation is really the trans-ference number of the anion. In order to clear up these difficulties in our laboratory the theory of the method is developed in such a general way as to apply to all possible systems, whether or not the electrodes used are inert with respect to the solution.2 If v is the faradays of electricity passed through the apparatus, then v equivalents of element will be formed in the coulometer. In the Hittorf cell v equivalents of ions will be involved in the electrode reaction, if there is one; t+v equivalents of cations will migrate out of the anode compartment and into the cathode compartment; and tv equivalents of anions will migrate out of the cathode compartment into the anode compartment. The transference numbers of positive and negative ions are t+ and t-respectively.