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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).
مقاله How Ion Selective Electrodes Work