Chemistry Reference
In-Depth Information
the same procedure as described before, sealed in this tube. Finally, the elec-
trode is polished until the microwire is in contact with the environment. At
the inner side of the tube, electrical contact is made through mercury or Ag
paint.
Role of electrolytes
45
1.6.5
Before ultramicro electrodes became important in voltammetry, a support-
ing electrolyte always needed to be used and was of more importance than
one would think. The role of the supporting electrolyte is summarised here:
•The liquid medium (the solution to be studied) is mainly non- or poor-
conductive (H
2
O, organic solvents), which makes it practically impossi-
ble to pass electrical current between electrodes immersed in that
solution. So, the first role of a supporting electrolyte is to provide the
solution with some conductive properties by adding an electrolyte.
•
Application of a potential difference between electrodes immersed in a
solution causes the presence of an electrical field in solution. Negatively
charged ions in that solution will move towards the anode, and posi-
tively charged ions will move towards the cathode due to this field (this
way of species transport is called migration). This process results in
charge transport or the flow of an electrical current, and the fraction of
this current transported by each species is dependent on its mobility and
concentration. However, in voltammetric experiments, this type of
charge and species transport is complicated and frequently disturbs the
transport properties by diffusion and/or convection so intensively that
interpretation of the data becomes very complicated or even impossi-
ble. To suppress the migration, an electrolyte is added in excess (10-50
times the concentration of the species to be studied). In this case, mainly
the electrolyte will be transported due to the electrical field and not the
species to be studied.
•
Strong acids in pH range of 0-4, strong bases in pH range of 11-14 and
buffer solutions over the entire pH scale can also be used as supporting
electrolyte if the concentration is high enough (see above). Besides the
role of decreasing the solution resistance and suppression of migration
currents, this type of electrolyte can also provide H
+
or OH
-
in a solu-
tion - a species that is frequently involved in the mechanism of elec-
trochemical reactions. This can have a serious impact on the kinetics of
the reaction because less activation energy (read applied overpotential)
is needed owing to H
+
or OH
-
being directly available in large amounts
instead of being delivered from the dissociation reaction of water.
•
Besides being a potential source of H
+
or OH
-
, the ionic form of the
electrolyte can be useful as complexing agent. It is well known in
