Abstract: A sensitive Bi/MWCNTs (multi-walled carbon nanotubes) composite film modified electrode was fabricated by two-step electrodeposition. First, the MWCNTs were electrodeposited instead of dropping coated on the glass carbon electrode(GCE), and the bismuth film was then deposited on the MWCNTs/GCE. A protocol used for the determination of total chromium and hexavalent chromium in water by square wave cathodic adsorptive voltammetry in the presence of cupferron (N-nitroso-N-phenylhydroxylamine ammonium salt) was proposed. In the presence of cupferron, Cr(Ⅲ) can form the complex with cupferron and be accumulated on the surface of the modified electrode by adsorption at a potential of -035 V, and then is reduced with the cathodic scan from -035 V to -110 V. An accumulation time of 2 min results in a detection limit of 005 μg/L Cr(Ⅵ) and a relative standard deviation of 52% (n=75) for 10 μg/L. Application to river water samples is demonstrated. The total chromium can be directly determined by standard addition of Cr(Ⅲ) or Cr(Ⅵ); the Cr(Ⅵ) can be separated with Cr(Ⅲ) by using an anion trapping column and then determined by standard addition of Cr(Ⅵ). The results are consistent with those detected by ICP-MS (inductive coupled plasma mass spectrometry). The attractive behavior of the new “mercury-free” chromium sensor holds great promise for on-site environmental and industrial monitoring of chromium.

Key words: Bi/MWCNTs (multi-walled carbon nanotubes) modified electrode, square wave cathodic adsorptive voltammetry, chromium, speciation analysis, cupferron