Diamond paste-based electrodes for determination of Cr(III) in pharmaceutical compounds

A new class of monocrystalline diamond paste-based electrodes is proposed for the determination of chromium(III) at trace levels in vitamins. Three types of monocrystalline diamond-natural diamond 1mu (natural diamond), synthetic diamond 50mu (synthetic-1), and synthetic diamond 1mu (synthetic-2)-were used for electrode construction. The linear concentration ranges are between 10(-10) and 10(-8); 10(-9) and 10(-7), and 10(-10) to 10(-8) mol L(-1), with limits of detection of 10(-12), 10(-12), and 10(-11) mol L(-1), when natural diamond, synthetic-1, and synthetic-2, respectively, are used as electrode materials. For electrodes based on natural diamond and synthetic-1 it was found that Cr(III) yields a peak at about +0.275+/-0.015 V (vs. Ag/AgCl) within a predetermined positive potential range situated between +0.4 and +0.2 V, while for the electrode based on synthetic-2 the peaks are found at +0.300+/-0.015 V (vs. Ag/AgCl). The proposed method is reliable for the determination of chromium(III) at trace levels in two vitamin tablets (RSD<0.2%).