The extraction equilibrium of Fe(I), Be( I), Mn( I), Co( I), Ni( I), Cu( I)
, Zn( I), Pb( Il ) and Cd( I) from nitrate medium with di-(2-ethylhexyl) ph
osphoric acid (HDEHP) in kerosene were studied at (298+/-1) K, The correlat
ion between the extraction equilibrium constants of metal ions and the HSAB
rule was discussed. The experimental results indicate that their K-cx and
Delta G(Theta) are increasing with the increase of hardness of Lewis acid.