It is shown theoretically and experimentally that the first catalytic pre-wave in the Ni*+-NCS-system (the reduction of the [Ni(NC%] complex) at COOS-< (l*O-1.5) X IO-* M (cNC#-> twin+) and within a wide range of ionic strength I can be described by a simplified kinetic equation based on the assumption of the proximity of the first of the two consecutive chemical stages of the Ni(NCS), formation on the electrode surface to the equilibrium state. The process is controlled by the second stage (Ni(NCS)+ + NCS-kPL Ni(NCS),). This conclusion conforms to the variation of ka with the variation of I. Both of these stages are of a bulk nature, as indicated by the proximity of k, to the rate constant of Ha0 exchange in Ni(H,O) ra+, the absence of any effect of the supporting electrolyte cation, or of gelatine, on the rate of the process; the thickness of the reaction layer is ca lo-& cm.
The second (total) catalytic pre-wave results from the reduction of Ni(NCS& formed from Ni'+ and NCS&, which occurs simultaneous with the first pre-wave process. Therefore, this reaction has a surface nature as confirmed by the rate increase in the (CsH&N+ < Cs+ < Na+ < Li+ sequence and with I decrease (the yl-effect was confirmed quantitatively) and also by the inhibition caused by gelatin.
The polarographlcally active Ni(NCS&, adsorbed from the bulk of the solution:
complex is not identical to the corresponding complex this latter would not be reduced at the pre-wave potentials.
R&m&-11 est rendu wmpte thtoriquement et exp@mentalement que la premi&e p&vague du syst&me Ni+a-NCS-@duction du complexe [Ni(NCS),I pour CNOS-4 (13-13 X lo-' M (cxes-> eNi*+) et dans un large! domaine de force ionique I, peut Ctre d&rite au moyen d'une 6quation cin& tique simplif& bas6e sur l'hypoth&se de la proximitb par rapport a 1'Cquilibre de la premi&e des deux &apes chimiques wns&utives qu'int&gre la formation de Ni(NCS), 21 la surface de 1'6lectrode. Le processus serait contrblb par la sewnde &ape (Ni(NCS)+ + NCS--% Ni(NCS),), en conformit de la variation de k, avec I. Les deux &apes sont cependant de m2me nature wmme I'idiquent le voisinage de k, et de la wnstante de vitesse d'&hange de HP0 dans Ni(H,0),2+, ainsi que l'absence de toute inffuence du cation de l%lectroIyte-support ou de la @atine sur la vitesse du processus. L'kpaisseur de la wuche r&actionnelle est d'environ lOA6 cm.
La seconde p&-vague catalytique provient de la rCduction de Ni(NCS)& form5 5 partir de Ni'+ et NCS& d'une man&e wnwmmittante au premier processus de p&-vague. En condquence,