Microcrystalline forms (size range 0.1-10 rm) of cis-Cr(CO)z(dpe)2 (cifl), trans-Cr(C0)2(dpe)Z ( t r a d ) , and trans-[Cr(C0)2(dpe)z]+ (trans+) (dpe = PhzPCH2CH2PPhz) may be mechanically attached to carbon electrodes. The voltammetry of these water-insoluble materials produces exceedingly well defined processes over a wide scan rate range when the electrode is placed into aqueous media containing 0.1 M NaC104 or 0.1 M KC104 as the electrolyte. Electron probe microanalysis demonstrates that C104-partially covers the edges and the surface of the solid after oxidative electrolysis. This suggests that oxidative voltammetry of the uncharged complex occurs at the crystalelectrode-solution interface to form a perchlorate complex. The redox processes observed for the arrays of microcrystalline carbonyl compounds attached to the electrode may be summarized by the following reaction schemes: cis-Cr(C0)z-(dpe)z + cis-Cr(CO)Z(dpe)z + e-and trans-Cr(CO)z(dpe)z + trans-[Cr(C0)2(dpe)z] + + e-+ trans-[Cr(C0)2(dpe)z]+ + e-with cis-[Cr(CO)z(dpe)2]+ slowly isomerizing to trans-[Cr(C0)2(dpe)z]+. Interestingly, the tranfl complex may be reversibly oxidized to trans+ and t r a d + under most conditions, but not as readily reduced from trans+ back to t r a d if the potential is held for short periods of time at potentials intermediate between the trans+/O and trans2+/+ processes. This indicates that the presence of a pure trans+ phase hinders reduction; however stepping the potential to a value more negative than the reduction potential of the trans+/transO couple and then scanning in the positive potential direction restores the current to its original value. The experimental results are in accord with an electrochemical process that takes place at the solid-solution interface to form a layer of oxidized material. Electron transfer is postulated to occur by electron hopping via self exchange and cross redox reactions with the rate (apparent diffusion coefficient) being dependent on the state of the electrodeampound-solution interface and the surface charge.

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A very sensitive and simple method is presented for the determination of Se(IV) by Osteryang square-wave cathodic stripping voltammery (OSWCSV). The method is based on the reduction of Se(IV) with Bi(III) onto an edge-plane type of pyrolytic graphite substrate, followed by a cathodic potential scan. OSWCSV studies indicate that the reduced selenium produced a distinct catalytic hydrogen wave at -1150 mV vs. Ag/AgCl. The peak height of the catalytic hydrogen wave was directly proportional to the initial Se(IV) concentration in the ranges of 0.1 - 1.0 and 1.0 - 20.0 microg L(-1) (correlation coefficients 0.9800 and 0.9901, respectively) when the optimized parameters were used. A 3sigma detection limit of 0.025 microg L(-1)0 Se(IV) was obtained at 30 s deposition time. The relative standard deviation was 4.0% on replicate runs (n = 12) for the determinations of 0.10 microg L(-1) Se(IV). Analytical results of natural water samples demonstrate that the proposed method is applicable to speciation analysis of Se(IV) and Se(VI).

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A sensitive and selective method has been developed for the simultaneous determination of cadmium, zinc, nickel and cobalt. The method is based on the chelation of metal ions with 2-(8-quinolylazo)-4,5-diphenylimidazole (QAI) and the subsequent reversed-phase (RP) high-performance liquid chromatographic separation and spectrophotometric detection of the metal chelates. The chelates were separated on an RP column with acetonitrile-water containing ethylenediamine tetraacetic acid and sodium acetate (pH 7.5). Though Zn(II) and Cd(II) chelates with azo compounds were generally labile in the RP column, these chelates with QAI were successfully detected. When analyses were carried out at 575 nm and at 0.001 absorbance unit full scale, the peak height calibration curves were linear up to 2.0 ng for Cd(II), 2.4 ng for Zn(II), 0.14 ng for Ni(II) and 0.72 ng for Co(II) in 100-µL injections, respectively; the detection limits (3σ, three times of the standard deviation for the blank signal) for Cd(II), Zn(II), Ni(II) and Co(II) were 4.8, 24, 2.4 and 7.2 pg in 100 µL of injected solution, respectively. The proposed method was successfully applied to the analysis of tobacco without any preliminary concentration or separation.