A new simple and reliable procedure was developed for the spectrophotometric determination of nitrogen(II) oxide. The procedure is based on the determination of excess oxygen after its reaction with NO. Alkaline solutions of thiourea dioxide were used for the determination of oxygen. It was found that the decomposition of an alkaline solution of thiourea dioxide under aerobic conditions is accompanied by the formation of dithionite, and its concentration is proportional to the concentration of oxygen in the solution. The absorbance of the resulting dithionite was measured at 315 nm. The solutions obeyed Beer's law at oxygen concentrations of 1 × 10 -5 -1.5 × 10 -3 M. The analytical range for NO was 1 × 10 -5 -1.5 × 10 -3 M. The proposed procedure was also used for the determination of nitrogen(II) oxide in aqueous-ethanolic solutions.

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Complex formation of NO and the NO 2 -ion with cobalt porphyrins bearing various substituents in the porphyrin macrocycle, viz., tetraphenylporphine (1), β octabromo meso tetraphenylporphyrin (2), protoporphyrin IX (3), and 5,10,15,20 tetra(4N carboxymethylene pyridyl)porphyrin tetrabromide (4), was studied. The stability constants of the nitrosyl and nitrite extracomplexes in water and in ethanol were determined. Porphyrin 4 forms the most stable extracomplexes. Nitrogen monoxide is produced by enzymatic systems in mammals and acts as a versatile regulator of metabolic processes. 1-3 Nitrogen monoxide is found 4,5 to be an endogenic vasodilator and neurotransmitter and oppresses thrombocyte aggregation; the superoxide radical and NO during oxidative stress act in conjugation.Nitrogen monoxide is produced in biological systems by NO synthases from L arginine, which is classified as an enzyme forming the NO • free radical and citrulline as a by product. 6 In the organism nitrogen monoxide is rapidly oxidized to stable end products, NO 2 -and NO 3 -ions. The nitrite ion in vivo oxidizes oxyhemoglobin and cata lyzes reactions involving NO. The mechanism of toxic effect of nitrites is mainly attributed to the oxidation of hemoglobin to methemoglobin accompanied by the vio lation of oxygen transport to tissues due to which hemic hypoxia is developed. Nitrogen monoxide can coordinate to thiols, sulfur-iron proteins of plasma, secondary amines, and hemoglobin and be consumed in reactions with oxygen, superoxide radicals, and hemoglobin. 5-8 Therefore, non heme models of iron containing proteins are being developed and their nitrosyl complexes are un der study. 9 However, the single compensatory adjustment form of NO depositing and utilization is complex forma tion with hemoglobin that occurs via axial coordination on the protoheme. 10 Therefore, investigation of binding (axial coordination) processes of nitrogen monoxide and its metabolite (nitrite ion) by metal porphyrins in solu tions is key for understanding of the essence of biological processes involving NO.In the present work, we studied the equilibrium of formation of the axial NO and NO 2 -complexes with Co II porphyrins of different structural types: tetraphe nylporphine (1), β octabromo meso tetraphenylpor phyrin (2), protoporphyrin IX (3), and 5,10,15,20 tetra(4N carboxymethylenepiridyl)porphyrin tetrabro R = H (1), Br (2)

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