Salt Bridge Definition

Why a Salt Bridge Is in Chemistry and Why One Is Used

A salt bridge connects the oxidation and reduction half reactions in a galvanic cell, such as a Daniell cell.
A salt bridge connects the oxidation and reduction half reactions in a galvanic cell, such as a Daniell cell. Gringer

A salt bridge is a connection containing a weak electrolyte between the oxidation and reduction half-cells in a galvanic cell (e.g., voltaic cell, Daniell cell). Its purpose is to keep the electrochemical reaction from reaching equilibrium too quickly. If a cell is constructed without a salt bridge, one solution would quickly accumulate positive charge while the other would accumulate negative charge.

This would halt the reaction and thus the generation of electricity.

Types of Salt Bridges

The two main types of salt bridges are a glass tube and a piece of filter paper:

Glass Tube Bridge: This is a U-shaped glass tube filled with an electrolyte, such as sodium chloride, potassium chloride, or potassium nitrate. The electrolyte needs to be relatively unreactive with other chemicals in the cell and have cations and anions with similar migratory speed (comparable ion charge and molecular weight). Because a salt solution could easy diffuse into the cell, the electrolyte is often held in a gel, such as agar-agar. The concentration of the salt solution is the biggest factor in conductivity. The diameter of the tube also has an effect. Lowering the concentration of the electrolyte or narrowing the glass tube lowers conductivity.

Filter Paper Bridge: Another common type of salt bridge consists of filter paper or another porous material soaked in an electrolyte (usually sodium chloride or potassium chloride).

In this bridge, conductivity is affected by electrolyte concentration, porosity of the filter paper, and the roughness of the paper. A smooth, absorbent paper yields higher conductivity than rough paper with low absorbency.

Reference

  • Hogendoorn, Bob (2010). Heinemann Chemistry Enhanced (2). Melbourne, Australia: Pearson Australia. p. 416.
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    Helmenstine, Anne Marie, Ph.D. "Salt Bridge Definition." ThoughtCo, May. 10, 2018, thoughtco.com/definition-of-salt-bridge-605636. Helmenstine, Anne Marie, Ph.D. (2018, May 10). Salt Bridge Definition. Retrieved from https://www.thoughtco.com/definition-of-salt-bridge-605636 Helmenstine, Anne Marie, Ph.D. "Salt Bridge Definition." ThoughtCo. https://www.thoughtco.com/definition-of-salt-bridge-605636 (accessed May 25, 2018).