Liquid-liquid extraction of transition metal ions using an alkylhistidine (histidine-2-ethylhexyl amide, abbreviated as His-EH) as a new extractant was studied. Intermediate metal cations in the Hard and Soft Acids and Bases (HSAB) sense, such as Cu(II), Ni(II), Zn(II) and Co(II), were selectively extracted with His-EH in the presence of other metal cations. The order of extractability for divalent transition metal ions was Cu(II) >> Ni(II) > Zn(II) approximate to Cd(II) > Mn(II), which is approximately the order of the stability constants for complexes of histidine and the metal ions. From the results of slope analysis for Co(II) extraction, the complex stoichiometry was found to be 2:1 for His-EH and Co(II). In addition, the ion-pair extraction reaction of Co(11) with His-EH was in accordance with the equation: Co((aq))(2+) + 2His-EH((org))+ 2ClO(4(aq))(-) = Co(2+)(His-EH)(2)(ClO(4)(-))(2(org)). The extraction equilibrium constant (K(ex)) of Co(II) using His-EH was evaluated as 3.03 x 10(10) [(mol dm(-3))(-4)]. (C) 2011 Elsevier B.V. All rights reserved.