The Cat,4lytic Polarogrxphic Current of a Rietal Complex

The polarographic reduction of Ni(II), in aqueous solutions containing small quantities of pyridinel. 2 as well as other organic amines”. and in acetonitrile containing chloride ion 4, has previously been reported to exhibit a catalytic wave before the main free-metal ion wave. This pre-wave is thought to represent the reduction of Ni(I1) which is complexed with the organic aminer or the chloride ion”. Because of the great sensitivity of a catalytic polarographic wave, the Ni(II)-complex catalytic pre-wave has been used successfully for the determination of small quantities. of pyridine”, ethylenediamines, and o-phenylenediamine3. Although it was postulated -that the mechanism of this catalytic wave probably involved (i) the ability of the ligand to form a complex, (ii) adsorption of the ligand on the mercury slurface,.-(iii) the ability of the ligand to act as a bridge which facilitates the electron transfer, arid (iv) various competitive effects of protonation on certain of these processesl~‘, no conclusive experimental evidence has been previously found to substantiate such a mechanism. An -extensive -investigation of the properties of the catalytic pre-wave observed when Ni(II) was reduced in the presence of o-phenylenediamine was undertaken in an eifort to determine the details of the mechanism involved_ This aromatic diamine was chosenbecause-it gave the most well-defined pre-wave of any other substance studiedz~a. The E* of -this wave is approximately 0.30 V more positive than. the Ni(Hz0) sZ+ background wave andthe catalytic enhancement of the wave is quite large”. This paper_ describes the experimental results of this study which indicate that the p&-wave_ current is limited by t&o factors: (i) the surface concentration of adsorbed o-phenylenediamine, and (ii) the rate of complexation of the free Ni(I1) with the adsorbed diamine.