1. Electrochemical Analysis

Electrochemical Analysis is the group of chemical analytical methods in which a signal is generated by an electrochemical reaction and is used for quantitative determination of a substance.

These methods depend on electrode reactions. Two of these methods : Potentiometry and Potentiometric Titrations use Ion-Selective Electrodes (ISEs) as a sensor.

The possibilities of modern electronics and data processing make these methods powerful and comfortable analytical tools for a wide range of applications.

An Electrode is an Electronic Conductor in contact with an Ionic Conductor, the Electrolyte. Passive (inert) electrodes act as Electron donors or Electron acceptors, active (participating) electrodes act as Ion donors or Ion acceptors.

The Electrode Reaction is an electrochemical process in which charge transfer takes place at the interface between the electrode and the electrolyte.

An Indicator Electrode shows by means of its potential the Activity of an ion in a solution. The relationship between the potential and the activity is given by the Nernst Equation.

The Potential between an electrode and a solution cannot be directly measured, a Reference Electrode is needed. The reference electrode should have a known, or at least, a constant Potential value under the prevailing experimental conditions.

An electrode in contact with an electrolyte represents a Half-Cell. Two combined half-cells form an Electrochemical Cell - i.e. one solution in contact with an ISE and a reference electrode. The Electrochemical Potential (E) can be measured across the two terminals of the cell.

However, in many cases, both electrodes cannot be placed directly in the same solution because the electrolyte in the reference electrode may oxidise, complex or otherwise react with the solution, or introduce ions which may interfere with the ion-selective electrode. In these cases it is necessary to interpose a second solution (known as a Bridge Solution or Salt Bridge) between the sample solution and the reference electrode to prevent any unwanted interactions. The bridge solution is normally included within the body of the reference electrode to form a Double Junction Reference Electrode.

The two solutions in a double junction reference electrode are connected by a so called Liquid Junction. A liquid junction is a semi-permeable membrane separating two solutions, which prevents wholesale mixing but allows the passage of ions by diffusion.