Electrochemical methods are used to solve many biophisical problems (such as determining the electrical potential of membranes, determination of biopolymer conformation etc.) but also in medical diagnosis.
An electrode 1 is better referred to as an half-cell, because they form one half of a galvanic cell. In these half cells, the origin of the electic voltage resides in their tendency to reach a thermodynamic equilibrium.
Electrodes may be of more types:
- In the First Type of Electrode, ions and electrons are exchanged between the electrode and the solution. The electrode may be cationic (metallic, amalgam, gaseous H2), in which equlibrium betwen neutral atoms and cations is established; or they may be anionic (Cl2), in which an equilibrium between neutral atoms and anions is established. An example electrode is Cu metal immersed in a solution Cu(II) solution.
- The Second Type of Electrode comprises of three parts.
The metal, a covering for the metal – usually a hardly soluble salt or hydroxide, and an electrolyte which has an anion in common with the salt or hydroxide.
- Redox Electrodes consist of a noble metal (Au or Pt) immersed in a solution containing reduced, as well as oxidised forms of the same compound.
- Ion Selective Electrodes are formed by non-porous membranes. Their electric potential depends on the activity of certain ions present in the solution. They can be divided into several group, however it is important to remember the glass electrode specific to hydronium ions (H3O+).
Ion Selective Electrodes:
Enzyme Electrodes are a special group of ion selective electrodes. They contain certain enzymes splitting a substrate whose concentration is to be determined (The products of the reaction should have ionic character).
The Glass Electrode allows the potentiometric measurement of pH values. It is made out of a doped glass membrane that is sensitive to a specific ion. The core is usually a NaCl electrode placed in a medium of known and constant pH. This is again separated from the solution of an unknown pH by a membrane selective for alkaline (Na in this case) ions. The exchange of Na ions for Hydrogen ions is detected, and so the pH is calculated.
Second Kind of Electrodes:
The calomel (Cl-Hg-Hg-Cl) electrode is, together with the AgCl2, one of the most important electrodes of this type. It is used as reference electrode in the determination of potentials of other electrodes.
The Hydrogen Electrode is considered to be the standard electrode in electrochemistry, with a potential conventionally equal to 0 (zero). The potential of an electrode is defined as the measured potential voltage difference in a circuit where one of the half-cells is a hydrogen electrode, and the other one a desired half-cells.
Read about Electrochemical Analytical Methods here.