The common ion effect is the suppression of the ionisation of a weak electrolyte that occurs when a strong electrolyte sharing a common ion is added to the solution. The added common ion shifts the ionisation equilibrium back towards the un-ionised form, lowering the degree of ionisation. For example, adding sodium acetate to acetic acid reduces the acid's ionisation and raises the pH.

同离子效应是当向溶液中加入含有相同离子的强电解质时,弱电解质的电离受到抑制的现象。所加入的同离子使电离平衡向未电离的形式移动,从而降低电离度。例如,向乙酸中加入乙酸钠会减小该酸的电离并升高 pH。