Abstract: When a binary mixture was determined with the proportional equation method according to the difference of reaction rates, the two equations could be set up by selecting two special times. t=0 and t→∞, if the reaction is reversible. Because of the initial concentration and the equilibrium concentration are independent of the reaction rate, the strict control of time in the experiment is unnecessary and a good result could be obtained. Based on this principle, the possibility of determining the binary heavy rare earth mixture by means of the ligand exchange reaction between RE arsenazo Ⅲ and CyDTA is discussed. Experimental results show that the relative error is less than 5% when the mixture of Dy and Er is in the range of 1 : 5～5 : 1 at 10～100 μg/25 mL level.

Key words: Binary heavy rare earth mixture, Proportional equation method, Spectrophotometry