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electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an out ...
, a salt bridge or ion bridge is a laboratory device used to connect the oxidation and reduction half-cells of a
galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
(voltaic cell), a type of
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
. It maintains electrical neutrality within the internal circuit. If no salt bridge were present, the solution in one-half cell would accumulate a negative charge and the solution in the other half cell would accumulate a positive charge as the reaction proceeded, quickly preventing further reaction, and hence the production of electricity. Salt bridges usually come in two types: glass tubes and filter paper.


Glass tube bridges

One type of salt bridge consists of a U-shaped glass tube filled with a relatively inert
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
. It is usually a combination of potassium or ammonium cations and chloride or nitrate anions, which have similar mobility in solution. The combination is chosen which does not react with any of the chemicals used in the cell. The electrolyte is often gelified with
agar-agar Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from ogonori (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar i ...
to help prevent the intermixing of fluids that might otherwise occur. The conductivity of a glass tube bridge depends mostly on the concentration of the electrolyte solution. At concentrations below
saturation Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds ** Degree of unsaturation **Saturated fat or fatty aci ...
, an increase in concentration increases conductivity. Beyond-saturation electrolyte content and narrow tube diameter may both lower conductivity.


Filter paper bridges

Porous paper such as filter paper may be used as a salt bridge if soaked in an appropriate electrolyte such as the electrolytes used in glass tube bridges. No gelification agent is required as the filter paper provides a solid medium for conduction. The conductivity of this kind of salt bridge depends on a number of factors: the
concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', ''number concentration'', ...
of the electrolyte solution, the texture of the paper, and the absorbing ability of the paper. Generally, smoother texture and higher absorbency equate to higher conductivity. A porous disk or other porous barriers between the two half-cells may be used instead of a salt bridge; these allow ions to pass between the two solutions while preventing bulk mixing of the solutions.


See also

*
Liquid junction potential Liquid junction potential (shortly LJP) occurs when two solutions of electrolytes of different concentrations are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated on ...
*
Ion transport number In chemistry, ion transport number, also called the transference number, is the fraction of the total electric current carried in an electrolyte by a given ionic species : :t_i = \frac Differences in transport number arise from differences in ...


References

Electrochemical concepts Laboratory equipment {{electrochem-stub