Adsorptive stripping voltammetric speciation of nickel(II)-histidine in aqueous ammonia

Ni(II)-L-histidine is expected to be the major nickel metabolite although it was not identified in previous atomic absorption analyses of body fluids. In a continuing study of nickel speciation, the adsorptive stripping voltammetric behavior of Ni(II) complexes with ammonia and histidine was examined. Two nickel species, [Ni(H2O)6]2 and [Ni(H2O)5NH3]2, were identified in ammonia buffer, whose peak potentials and currents and also sensitivity to [Ni(II)] varied significantly with the ammonia concentration used. A 1:2 Ni(II)-histidine mixture in 0.05 M ammonia buffer at pH 9.4 gave a new peak at Ep - 1.26 V, which was twice as large for the L-isomer as for the DL-racemate. This peak height was not changed appreciably by the addition of NaCl, NH3 or histidine beyond ca. 10-fold, but increased linearly with [Ni(II)]. Replacing histidine with histamine, a bidentate ligand, gave rise to a peak at Ep - 0.91 V. Hence, the Ni(II)-L-histidine complex is octahedral Ni(L-his)2, involving the histidyl anion as a tridentate. The assignments are consistent with the effects on the peak current of various nickel species from the adsorption potential and time during electrode adsorption.