The electromotive force of the galvanic cell SCE/satd KCl/solution (2)
/cationic membrane/solution (1)/anionic membrane/solution (2)/satd KCl
/SCE has been measured for various electrolytes, using cellulosic cati
on- and anion-exchange membranes which behave as ideal or nearly ideal
reversible electrodes. This method affords a simple means of determin
ing the mean activities of numerous salts, including many for which ot
her electrometric methods are not available, without the need to make
extra-thermodynamic assumptions. The agreement between the values of t
he activities found and those tabulated in the literature is excellent
in the range of concentrations studied less than or equal to 0.1 mol/
kg, even for salts containing di- and tri-valent ions. This method is
also suitable for measuring the mean activity of salts in solutions of
polyelectrolytes as we report for the variation of 0.1 KCI activity w
ith different amounts of the protein beta-lactoglobulin. The main reas
on for the high performance of this system as a reversible electrode f
or a variety of salts, lies in the fortunate mutual compensation of th
e water transport effect for the two cellulosic membranes, together wi
th a negligible effect on the cell emf of the liquid film adhering to
the membranes, circumstances which have been thoroughly discussed. The
measurements were carried out at 25 +/- 0.05 degrees C.