salt bridge

Meaning: A salt bridge is a U-tube or porous junction holding an inert electrolyte, such as KNO₃ or Na₂SO₄, that links the two half-cells of an electrochemical cell. In a Zn/Cu galvanic cell, oxidation at the anode adds Zn²⁺ to solution and reduction at the cathode removes Cu²⁺, so charge would build up and halt electron flow. The bridge lets anions move toward the anode compartment and cations toward the cathode compartment, keeping both solutions neutral and closing the circuit. In biochemistry, a salt bridge is instead an ionic interaction between oppositely charged side chains in a protein.

On the MCAT: Expect a cell diagram with a question on the direction of ion flow, or on what happens if the bridge is removed (current quickly drops to zero). The bridge electrolyte must not react with the half-cell ions; KCl would be a poor choice with Ag⁺ because AgCl precipitates.

Common confusion: Students send electrons through the bridge or reverse the ion directions. Electrons travel through the external wire, and ions carry charge through the bridge. Anions go toward the anode because oxidation makes that solution gain positive charge, and this holds in electrolytic cells too, since oxidation still occurs at the anode.