Salt Bridge
In electrochemistry, a salt bridge or ion bridge is an essential laboratory device to minimize the liquid junction potential in an electrochemical cell. The salt bridge was invented over 100 years ago. It contains an electrolyte solution, typically an inert solution, used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell. In short, it functions as an ionically-conducting link connecting the anode and cathode half-cells within an electrochemical cell. It also minimizes and stabilizes the liquid junction potential between the solutions in the half-cells. Additionally, it can be used to minimize cross-contamination between the two half cells.
As shown in the figure on the right, a salt bridge typically consists of tubes filled with an electrolyte solution. These tubes often have diaphragms - such as glass frits - at their ends to help contain the solution within the tubes and prevent excessive mixing with the surrounding environment. When setting up a salt bridge between different solvents of half-cells, it is crucial to ensure that the electrolyte used in the bridge is soluble in both solutions and does not interact with any species present in either solutions.
A salt bridge filled with a solution of a single salt minimizes the liquid junction potential most efficiently when the following conditions are met:
The diffusion coefficients of the salt's anion and cation are almost equal at the selected concentration. K+, NH4+, Rb+, Cl- and NO3- are most often used as anions and cation of the ion bridge salts with different solvents.
The salt concentration in the ion bridge is much higher than in the working solutions, which it connects. This minimizes the effects of the catholyte and anolyte compositions on the measured voltage difference.
Traditionally, concentrated aqueous potassium chloride (KCl) solution has been most often used to minimize the liquid-junction potential. When comparing other salt solutions such as potassium bromide and potassium iodide to potassium chloride, potassium chloride is the most efficient in nullifying the junction potential. Yet, the effectiveness of this salt bridge decreases as the ionic strength of the working solutions (catholyte and anolyte) increases.
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