J. Electroanal. Chem., 230 (1987) 77-84 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands DETERMINATION OF ACTIVITY COEFFICIENTS IN AQUEOUS SOLUTIONS OF TRICHLOROACETIC ACID FROM EMF MEASUREMENTS AT 25°C MIGUEL A. ESTESO, LUIS FERNANDEZ-MERIDA and FELIPE F. HERNANDEZLUIS Dept. Quimica Fisica, Univ. La Luguna, Tenerife (Spain) (Received 13th June 1986; in fiial forrn 16th March 1987) ABSTRACT Activity coefficients were obtained for the hydrogen ion in aqueous solutions of HTCA at 25 o C from the measurement of the emf of the cell: glass 1HTCA (m) /KC1 (3 M), AgCl 1Ag in which m varied between 0.006 and 0.1 mol kg-‘. In order to eliminate the asymmetry potential of the glass electrode and to be able to neglect the liquid junction potential, this cell was combined with an auxiliary reference one. The equivalent cell resulting from this coupling is: glass 1HTCA (m) 1 KC1 (3 M) I HTCA (ma) lglass; EJI 42 which presents a small liquid junction potential, AEj = E,l - I?,,. Therefore, the values obtained here are apparent values, vi, which will, in every case, lie between those of y + and y _, approaching the mean ionic value, y+. The concentration of the acid, ma, was constantly kept equal to 0.09549 mol kg-‘. The Guggenheim equation was used to calculate y> with C’ = - 0.26,. The very negative value found for this constant indicates the presence of a strong ionic association of the acid, as reported previously by other authors. INTRODUCTION To study an electrochemical process it is necessary to know the properties of the medium in which it takes place. Many such processes occur in solution and include the presence of dissolved electrolytes. It is well known that electrolytic solutions present a very different behaviour from that displayed by an ideal solution, particularly if their concentration is not very low. Thus, knowledge of the activity coefficients of ionic species - those that measure the deviation from the above- mentioned ideal behaviour - is absolutely necessary when calculating as precisely as possible any magnitude influenced by the medium. In the past few years much attention has been devoted to studies on electrodic kinetics. Numerical values for the thermodynamic magnitudes related to the elec- trode-solution double layer can be obtained only on the basis of experimental 0022-0728/87/$03.50 0 1987 Elsevier Sequoia S.A.