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Abstract:The nitrate ions ) (NO 2 − , products of disproportionation of NO in aqueous solution, were detected by an Orion Nitrite Electrode 97-46. Calibrations by means of standard NaNO2 solutions within the range 0.001÷100 ppm indicated linear dependence of EMF on ppm within 1÷100 ppm. Measurements justified the usefulness of this detection method of NO in solutions of OH − concentration lower than 10 -2 mol dm -3 since at higher concentrations the EMF values exceeded the measurement range of the electrode. Occurrence of nitrate ions produced in the disproportionation reaction was additionally confirmed in dependence of OH − concentration by near UV and fluorescence spectra. The calibrated ion-selective nitrate electrode has also been shown, on the basis of Co(II)-dipeptide-OH -systems, as a useful tool in studying reversible NO uptake by Co(II) chelates in aqueous solution. Such a reaction may be regarded as simulating the harmful binding of NO by hemoglobin, where it substitutes the isoelectronic dioxygen.Keywords: nitrogen(II) oxide, disproportionation of NO, nitrate ) (NO 2 − ion-selective electrode, UV spectrophotometry, spectrofluorimetry, harmful uptake of NO Nitric(II) oxide is recently one of the most intensively studied signaling particles in living systems. This small, versatile molecule takes part in a number of activities of the living system, eg blood-vascular system control, nerve signal conductance, organism immune response to pathogens as well as cell partitioning [1,2]. The studies on biochemistry of NO started in the 70-ties of the last century when it was stated that nitric(II) oxide is an active metabolite of nitrate medicines used in the therapy of cardiac ischemia [3,4]. Soon, it was also found that nitric(II) oxide is produced endogenously in the living

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