Voltammetric behavior of selected biologically active organic nitro compounds, namely 2-nitrofluorene, picric acid (2,4,6-trinitrophenol) and metronidazole, has been investigated using direct current voltammetry (DCV) and differential pulse voltammetry (DPV) at a polished silver solid amalgam composite electrode (p-AgSA-CE). The optimum conditions have been found for their determination in a 1 : 1 mixture of methanol and aqueous Britton-Robinson buffer of pH 5.0 for 2-nitrofluorene and in the aqueous Britton-Robinson buffer solutions of pH 2.0 and 4.0 for picric acid and metronidazole, respectively, with the limits of quantification (L Q s) 4, 0.1 and 2 mmol L À1 (DCV at p-AgSA-CE) and 3, 1 and 4 mmol L À1 (DPV at p-AgSA-CE) for 2-nitrofluorene, picric acid and metronidazole, respectively. An attempt to increase the sensitivity using adsorptive stripping voltammetry was not successful for all three test substances. For comparison, the UV-vis spectrophotometric determinations of studied compounds have also been carried out in methanol for 2-nitrofluorene (L Q % 1 mmol L À1) and in deionized water for picric acid). Practical applicability of the newly developed voltammetric methods was verified on direct determination of the studied compounds in drinking and river waters with L Q s around 10 À6 mol L À1 for all the studied compounds.

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Experimental Reagents and chemicalsA silver solid amalgam was prepared by mixing of mercury (99.999%, Polarografie Praha, Czech Republic) and silver powder (2 -3.5 μm, 99.9%, Sigma-Aldrich, Germany) in a weight ratio (60:40, wHg/wAg) by 1 min shaking on a Vortex Genie 2 (Scientific Industries, USA) in a 2-ml Eppendorf tube. The thus-prepared solid amalgam was then immediately pulverized to a rough powder in an agate mortar, and left to harden overnight. Afterwards, the rough amalgam powder was pulverized to a fine amalgam powder in the same agate mortar.