Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueoussolutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hy drochloric acid over an electrode potential range of -0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen e volution and chromium(II) formation. At electrode potentials more negative than -0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occu rred simultaneously. At electrode potentials -0.8 to -1.2 V vs SCE, the ele ctrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous ca rbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl3, allowed the exchange current density and Tafel slope for hydrogen evolutio n, and for the reduction of Cr(III) to Cr(II), to be determined. The chromi um(III) reduction on vitreous carbon and graphite electrodes can be predict ed by the extended high field approximation of the Butler-Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).