In the reaction, potassium ions should move to the electrolyte solution where the positive electrode is located, and anions should move to the electrolyte solution where the negative electrode is located, so as to fill the problem of non-conservation of anion and cation caused by the primary battery reaction and promote the reaction to continue.

Under the action of electric field, different anions and cations have different migration rates. If the migration rates of anions and cations are similar, the impact on the primary battery will be smaller, while the migration rates of potassium ions and aluminum ions are the closest, so KCl saturated solution is often used as the salt bridge (saturation: the maximum ion concentration, the longer the service life of the salt bridge).

When KCl saturated solution can't be used as a salt bridge (such as silver ion in electrolyte solution), there are other substitutes, such as KNO3 (the mobility of potassium ion and nitrate ion is also close).