In concentrated chloride media, nickel is
reduced at far more positive potentials than in dilute solutions. The positive
shift in half-wave potential increases as the ionic strength is raised, and is
also greater when the cation of the supporting electrolyte has a high hydration
number. Evidence is presented to show that the reduction in overpotential
is due to the formation of a nickel chloride complex, [Ni(H2O)5Cl]+, which has a stoicheiometric stability constant of 0.094 � 0.009 at an
ionic strength of 10.0. Spectrophotometric results show that this nickel
species is not formed in low ionic strength solutions. In anhydrous methanol
saturated with lithium chloride, nickel is present as the tetrachloro
complex, [NiCl,]2-, which has
similar polarographic behaviour to the monochloro
complex.
Current-potential curves recorded at a
rotated pyrolytic graphite electrode enabled the
behaviour of nickel to be studied in the absence of specific adsorption of the
chloride ion. Nickel is reduced at more positive potentials at a dropping
mercury electrode than at the pyrolytic graphite electrode, and the results
indicate that this difference is due to specific adsorption of chloride on the
mercury electrode.
Electrochemical Investigations of Adenosine-5'-triphosphate (ATP)at
Stationary Pyrolytic Graphite Electrode