Speciation and stability of methylene blue-metal-thiocyanate ion-association complexes

With thiocyanate, zinc(II), cobalt(II) and copper(II) ions form stable anion, [M(SCN) 4]2-. This anion in turn with methylene blue (MB+) forms ion-pair of the type MB2[M(SCN)4] and turns methylene blue to violet. The stoichiometry and stability constants of the ion-association complexes were determined using Benesi-Hildebrand method and the stoichiometry was confirmed by Job’s continuous variation method. The kinetics of formation of ternary complexes was studied using stopped-flow technique. The equilibrium studies showed that the stability of ternary complexes increased in the order Co(II) > Cu(II) > Zn(II) and the formation rate constants increased in the order, Cu(II) > Zn(II) > Co(II). The relative stabilities indicate that cobalt is preferred to other two metals in the speciation of ternary complexes comparable with similar complexes in biosystems. This study also provides a method for the spectrophotometric determination of Co(II) and Zn(II) ions at nanogram levels at 25 oC and an ionic strength of 0.15 M.