PURPOSE: To allow the preferential extraction of iron ions and the reverse extraction of the iron ions from an extracting solvent by an aq. inorg. acid soln. of a low concn. by using specific dialkyl phosphate as the extracting solvent.

CONSTITUTION: The aq. metallic salt soln. contg. the iron ions and other metallic ions is brought into contact with the extracting solvent contg. the dialkyl phosphate expressed by general formula I as an extractant. The iron ions, more particularly Fe(III) can preferentially be carried on the above-mentioned extracting solvent by this method. In the formula I, R1 and R2 are the same or different and have the structure of formula II; m is ≥4 integer; m+n is 8 to 20 integer. The reverse extraction of the Fe(III) from the extracting solvent is possible by bringing the extracting solvent carried with the above-mentioned Fe(III) into contact with the aq. inorg. acid soln. of the low concn., for example, ≤7N hydrochloric acid. The corrosion of a reaction device is, therefore, decreased and the intrusion of the iron derived from this corrosion into reaction system is prevented.