Filling solutions:
A variety of filling solutions can be used. The most commonly used are saturated KCl or 3.5M KCl. KCl has the uncanny ability to 'creep' and form a crusty layer of solid KCl where the solution is exposed to the air. This author has seen 'beards' of KCl emanating from a Vycor® frit that were several cm long. If perchlorate electrolytes are to be studied, KClO4 may precipitate in the pores of the frit, and for these electrolytes NaCl (either sat'd or 3M ) is preferred. The author routinely uses NaCl filled electrodes, but has used LiCl in special instances.
Saturated solutions of KCl or NaCl have the advantage that the concentration is reproducible even if the temperature changes (if solid salt is present) and are immune to the effects of water evaporation. However, the solid salts harden into an impenetrable block which may lead to a high impedance electrode. A "nearly saturated" solution (3.5M KCl or 3M NaCl) can change concentration due to evaporation.