BC-535 Preliminary, Rev. 060126
32
Chloriding electrodes
Silver-silver chloride electrodes act as signal transducers by converting ionic currents in solution
to an electric current within a wire. This is achieved by utilizing a reversible oxidation/reduction
reaction between the electrode and Cl
-
ions in solution. The chemical reaction is:
Cl
-
+ Ag
⇔
AgCl + e
-
The potential developed by one electrode is proportional to the standard electrochemical potential
for Ag/AgCl plus the Cl
-
concentration at the solution/electrode interface. Since this potential is
dependent on [Cl
-
], a voltage bias will be introduced by changing the solution Cl
-
concentration.
Therefore, we recommend that Ag/AgCl electrodes be connected to the bath through agar salt bridges
to maintain a constant Cl
-
concentration near the electrode. In addition, the isolation provided by the
agar bridge will prevent Ag
+
ions from contaminating the baths.
The
BC-535
is shipped with two silver wires which must be chlorided prior to use. Over time, the
AgCl coating on the wires will deteriorate. This will be most apparent as a gradual increase in the
value of the junction potential seen at the beginning of each experiment. In addition, the electrodes
may lose their purple-brown color. Once it has been determined that the electrodes require cleaning,
the oxide should be removed and re-applied.
Techniques for chloriding silver wires
Before using Ag
+
wire as a current or voltage electrode, it must first be chlorided. New
(previously unused) wire should be cleaned with ETOH before chloriding. Previously chlorided wire
should be cleaned before re-chloriding.
Two methods for chloriding most commonly used are the plating techniques described below.
These are soaking in household bleach or electroplating using a voltage source. As with a new wire,
clean the wire with ETOH before proceeding to remove finger oils.
A)
Soaking in bleach - This technique places a very useable, but relatively thin coating on the
wire. Simply immerse the clean wire in full strength common household bleach (Clorox) for 5-
15 minutes or until a purple-gray color is observed. Rinse and use.
B)
Electroplating – While this technique requires more effort, it places a thicker and more
uniform coating on the silver wire. Electroplating a silver wire with chloride is achieved by
making the wire positive with respect to a solution containing NaCl (0.9%) or KCl (3M) and
passing a current through the electrode at a rate of ~1 mA/cm
2
of surface area for 10-15
seconds or until adequately plated. A 1 cm length of 1 mm diameter wire will require
approximately 0.3 mA. The color of a well plated wire will also be purple-gray. Periodic
reversal of the polarity while plating the electrode tends to yield a more stable electrode.
When electroplating a previously plated wire, you may find that it does not plate evenly.
Complete removal of the residual silver chloride is often necessary to effect a uniform coat.
Before making the wire positive to the chloriding solution, reverse the polarity for 5-10
seconds to remove any remaining chloride that might be left in pits on the wire. Then proceed
as described above.
Warner
Instruments
A Harvard Apparatus Company .