Salt bridge

For the term used in protein chemistry, see Salt bridge (protein)

A salt bridge, in electrochemical cell. Salt bridge usually comes in two types: glass tube and filter paper.

Glass tube bridges

One type of salt bridges consists of U-shaped glass tubes filled with a relatively inert Agar is often used for gelification.

The conductivity of the glass tube bridges depends mostly on the concentration of the electrolyte solution. An increase in concentration below saturation increases conductivity. Beyond-saturation electrolyte content and narrow tube diameter may both lower conductivity.

Filter paper bridges

The other type of salt bridges consists of a filter paper, also soaked with a relatively inert inert. No gelification agent is required as the filter paper provides a solid medium for conduction.

Conductivity of this kind of salt bridges depends on a number of factors: the concentration of the electrolyte solution, the texture of the filter paper and the absorbing ability of the filter paper. Generally smoother texture and higher absorbancy equates to higher conductivity.

A porous disk or other porous barrier between the two half-cells may be used instead of a salt bridge; however, they basically serve the same purpose.

Uses

As electrons leave one half of a galvanic cell and flow to the other, a difference in charge is established. If no salt bridge was used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in electrolytes is allowed, there is no need to make allowance for possible interactions between ionic species.

The technique more specifically allows freedom in the choice of ions in mixture of two different cations in solution might result in the preferential reduction of the wrong one, for the purposes of the experiment. With a salt bridge, the desired cation is isolated in one vessel while the cation in the other vessel may be chosen to make the experiment easier, e.g. using a more soluble, or more stable salt of the anionic species.