Reduction of Hexamminechromium(III) Ions at the Dropping Mercury Electrode

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Abstract
The reduction of hexamminechromium(III) ion at the dropping mercury electrode has been studied in neutral and acid aqueous solutions. Hexamminechromium(III) ion gives no well-defined d. c. polarographic wave in an unbuffered neutral solution, whereas in an acid solution it gives a well-defined two-step wave, the first wave corresponding to the reduction of chromium(III) to chromium(II) and the second, to that of chromium(II) to chromium-(0). The first wave of the d. c. polarogram obtained in an acid solution is diffusion-controlled and gives a slope of the log-plot close to that expected for a reversible wave. The a. c. and the Kalousek polarogram, however, are of the irreversible type. No chemical reaction involving hydrogen ions, ammonium ions or ammonia takes place preceding the electron transfer. Considering these results, the following mechanism is temporarily suggested for the reduction of hexamminechromium(III) ions at the dropping mercury electrode: [Cr(NH3)6]3++e\ightleftarrows[Cr(NH3)6]2+[Cr(NH3)6]2++nH2O→[Cr(H2O)n(NH3)6−n]2++nNH3NH3+H+\ightleftarrowsNH4+