The use of non-aqueous solvents is a modern trend in titrimetric analysis. Acetonitrile especially seems to be a very promising medium for acid-bane titrations as well as for redox titrations. In connection with tile latter category, investigations with cerium(IV) 1, iron(III) 2 and copper(II)3-5 should be mentioned. However, the literature does not appear to provide information on tile kinetics of redox reactions in anhydrous acetonitrile, hence the present investigation was started.
The method consisted in observing the time-dependence of the limiting current of the cobalt(II)-bipyridyl complex, [Co(bipy)a] °-+, after addition of the oxidant. A similar method has been reported for investigations in aqueous solutions 6-s.
First, the polarographic behaviour of the cobalt(II)-and cobalt(llI)-bipyridyl complexes and of the perchlorates of copper(II) and iron(III) was studied at the rotating platinum electrode. The experimental conditions for the kinetic measuremerits followed from the polarographic results.
XPER I l~I E N TAL
Reagents
Acetonitrite. Reagent-grade acetonitrile (Merck) was used after purification by the method proposed by FORClER AND OLVER ~ (three distillations: first from sodium hydride, then from phosphorus pentoxide). The solvent was kept in the dark and protected against humidity by P2Os.
Sodium j~erchlorate.
This was prepared from reagent-grade sodium carbonate by adding reagent-grade perchloric acid xo. It was dried at 15 o° for 24 h. A o.I M solution in acetonitrile was used as supporting electrolyte. Silver nitrate. The reagent-grade salt was dried at 12o °. Copper(II)tetra(cyanomethane) perchlorate [Cu( II) (C HaC N) a(CIOa)z]. Tiffs was prepared from copper(II) perchlorate as described in the literature a. The, copper content was checked by titration with EDTA with murexide as indicator. 1o-2 and 5 "Io-2 M solutions in acetonitrile were used. Cobalt(III)lris(bijbyridyl) ~bemhlorate trihydrate. This was prepared by tile method of BURSTA.LI, AND INIYHOLM 12. The cobalt content was deterrnined after potentiostatic reduction, by titration with iron(III) sulfate in the presence of an excess