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Some Basics
There are two different types of Electrical Conductivity:
1) In Metals the electric current is carried by Electrons.
2) In Liquids the electric current is carried by Ions
Every Electrochemical Process (Galvanic Cell, Electrolysis, Electro-Analysis) involves both these types of conductivity. The junctions where they meet and transfer the electrical charge are referred to as Metal-Liquid Interfaces. These interfaces were originally called Electrodes, but now this term is also used for various other devices such as welding electrodes or electro-cardiogram electrodes.
At the Metal-Liquid interface there is an exchange of Electrons in one or other direction (details can be found in standard chemistry text books, in sections on Galvanic or Electrolytic Cells.
(NB: Galvanic [Voltaic] Cells generate electricity; Electrolytic Cells consume electricity).
For example, in a Copper-Silver Galvanic Cell, on one electrode an Oxidation reaction takes place:
Cu (metallic) à Cu 2+ (ionic, in solution) +2 e-
on the other electrode a Reduction reaction takes place:
Ag+ (ionic, from solution) + e- à Ag (metallic - deposited on electrode surface)
This explains how the electric current in the wire (Electrons) becomes a current in the liquid (Ions)
The Electrochemical Circuit for an Ion Selective Electrode measurement.
An ISE (with its own internal reference electrode - more details later) is immersed in an aqueous solution containing the ions to be measured, together with a separate, external reference electrode. (NB: this external reference can be completely separate or incorporated in the body of the ISE to form a Combination Electrode.) The electrochemical circuit is completed by connecting the electrodes to a sensitive milli-volt meter using special low-noise cables and connectors. A potential difference is developed across the ISE membrane when the target ions diffuse through from the high concentration side to the lower concentration side (a detailed description follows later).
