The ground in a vibrating probe measurement chamber needs to be designed
to add minimal noise and artifactual voltages. The usual arrangement is
a Ag/AgCl pellet or wire attached to a 3M KCl agar bridge. The KCL bridge is
used to avoid high junction potentials associated with other ion pairs
(K+ and Cl- have approximately identical diffusion
coeficients and unit charges and as a result neither ion gets ahead of the
other in a diffusion situation).
In the event that the measured ion is K+ or Cl- and those ions are low concentration in the bathing medium, another ion pair should be used in the ground salt bridge in order to minimise the addition of the ion of interest during the course of an experiment.
(1) Craft your ground capillaries to be of convenient shape for your measurement chamber and electrode holders. Examples:
(2) Heat a 3M KCl solution in distilled water to a boil in an erlenmeyer flask of sufficient volume to contain substantial boiling and foam. Make the solution 2% in agarose and continue heating until all signs of undisolved agarose disappear.
(3) Preheat batches of capillary tubes on a hot plate to discourage gelling of the agar on contact with cool tubing.
(4) Place the capillary ground tubes individually into the molten agar held in a tall container (such as a histological Coplin jar) making sure that the air is completely displaced in the capillary as it is inserted. [air in ground bridge makes it useless!]
(5) Store the batches of ground tubes in the solidified agar until they are to be used. Cover the container and provide extra 3M KCl to maintain moisture.