Salzbrücken

A Salzbrücke, in electrochemistry often simply called a salt bridge, is a device that connects the two half-cells of a galvanic cell to complete the electrical circuit while preventing the direct mixing of the cell’s different solutions. Its primary function is to maintain electrical neutrality by allowing ions to migrate between the compartments as redox reactions proceed.

A typical Salzbrücke is a U-shaped tube filled with an electrolyte solution, commonly potassium nitrate (KNO3) or potassium chloride (KCl). Some designs use a porous glass frit or a gel containing an electrolyte, sometimes separated by a semi-permeable membrane. As ions move in the cell, the salt bridge supplies counter-ions to balance the charge carried by the electrons flowing through the external circuit, thus preventing charge buildup that would otherwise halt the reaction.

Manufacturing variations include open liquid-filled bridges and gelled or solid-state versions to reduce convection and evaporation. The salts chosen for the bridge are usually inert with respect to the redox couples in the half-cells and are selected to minimize junction potentials, though some potential difference at the interface with the electrode solutions is unavoidable.

Limitations of Salzbrücken include gradual depletion or mixing of bridge electrolytes over time, formation of junction potentials, and possible contamination of the half-cell solutions. In modern practice, alternatives such as solid-state ion conductors or reference-electrode arrangements may be used in place of traditional salt bridges.

See also: galvanic cell, half-cell, junction potential, reference electrode.