Abstract: An universally applicable formula of element dilution is derived according to the approximate reactivity of two metal ions with identical reagent. Based on measurably extended physical quantity of reaction product,numerous reactions are chosen to realize element dilution. The formula of isotope dilution which represents isotope effect is derived from the formula mentioned above. The formula of element dilution is verified by the extraction of mixture of Cu2+ and Pb2+ by DDTC/CCl4. The experiment in di lution is tested to imitate isotope effect,and the result accords with theoretical equation.