Anodic Wave Polarography of Ammonium Diamminetetrakis(isothiocyanato)chromate(III)

Abstract

The anodic polarographic behavior of ammonium diamminetetrakis(isothiocyanato)chromate (III) (Reinecke salt), which forms a slightly soluble salt with mercuric ion, was studied in water and in acetone-water mixture.
In an acidified aqueous solution of sodium perchlorate, Reinecke salt produced a two-step anodic wave, as shown in Fig.1. The half-wave potential of the first wave, which showed the charactaristics of the adsorption prewave, was about +O.21 V vs. SCE. The first wave height increased linearly with increasing concentration of Reinecke salt. However, when the concentration of Reinecke salt exceeded the critical value of 4 x 10-4 mol 'dm^-8, the first wave' height remained constant and the second wave (main wave) appeared at +O.34 V vs. SCE, as shown in Fig.4. The first wave height was proportional to the height of mercury column. The total wave height was proportional to the square root of the mercury column height. The relative temperature coefficients of the first and total wave heights were 1.16x 10-2 K-1 and 1.40 X 10-1 K. - at 25° C, respectively.
On the other hand, Reinecke salt in 1: 1 acetone-water mixed medium showed a single anodic wave, as shown in Fig.2. The limiting current was proportional to the square root of the mercury column height. Though the relative temperature coefficient of the wave height showed a relatively high value of 3.27 x 10-21C-i at 25° C, it was concluded that the wave was diffusion controlled current from the electrocapillary measurement (Fig.8) and that dependence of the current intensity on the mercury column height was present.
From the analysis of the polarographic waves (see Fig.9, 10 and 11), it was concluded that these anodic waves were produced by the formation of Hg [Cr (NCS), (NFI, ), ], at the surface of the dropping mercury electrode.