The study of equilibrium and formation constants of some transition metal complexes with vitamin B-6 in solution by potentiometry

The protonation constants of vitamin B-6 (pyridoxol hydrochloride) and the formation constants of its complexes with some transition metal ions M(II) (M = Cd, Co, Ni, Cu, and Zn) were determined potentiometrically at (25 +/- 0.1)degreesC and I(NaCl) = 0.2 mol dm(-3). Two computer programs MINIQUAD a nd MIQUV were used for calculation, and the results obtained with the two p rograms were in good agreement. There are two complexes species MLH and ML ascertained in the binary system, and the formation constant of complex ML for metal ions, which was found to be for Cd(II) < for Co(II) < for Ni(II) < for Cu(II) > for Zn(II), has an Irving-Williams order.