Nitric oxide, nitrite/nitrate addition

The bath components for a nitrite—nitrate accelerated bath basic to this conversion coating process are (/) 2inc metal or 2inc oxide dissolved in acid (2) phosphate ions added as phosphoric acid (J) addition of an oxidant such as sodium nitrite and (4) addition of nitric acid. Other oxidants such as peroxide, chlorate, chlorate in combination with nitrate, or an organic nitro compound may also be used. 

At room temperature, Htde reaction occurs between carbon dioxide and sodium, but burning sodium reacts vigorously. Under controUed conditions, sodium formate or oxalate may be obtained (8,16). On impact, sodium is reported to react explosively with soHd carbon dioxide. In addition to the carbide-forrning reaction, carbon monoxide reacts with sodium at 250—340°C to yield sodium carbonyl, (NaCO) (39,40). Above 1100°C, the temperature of the DeviHe process, carbon monoxide and sodium do not react. Sodium reacts with nitrous oxide to form sodium oxide and bums in nitric oxide to form a mixture of nitrite and hyponitrite. At low temperature, Hquid nitrogen pentoxide reacts with sodium to produce nitrogen dioxide and sodium nitrate. 

Sodium nitrite has been synthesized by a number of chemical reactions involving the reduction of sodium nitrate [7631-99-4] NaNO. These include exposure to heat, light, and ionizing radiation (2), addition of lead metal to fused sodium nitrate at 400—450°C (2), reaction of the nitrate in the presence of sodium ferrate and nitric oxide at - 400° C (2), contacting molten sodium nitrate with hydrogen (7), and electrolytic reduction of sodium nitrate in a cell having a cation-exchange membrane, rhodium-plated titanium anode, and lead cathode (8). 

Nitrite reductases (NiRs)—enzymes found in several strains of denitrifying bacteria— catalyze the one-electron reduction of nitrite anion to nitric oxide (Equation 1). - In addition to the importance of this process in the global nitrogen cycle (Figure 1), further incentive for the study of the denitrification process is provided by its environmental impact, ranging from the production of NO as a pollutant and NjO as a potent greenhouse gas, to lake eutrophication due to farm runoff that contains high concentrations of nitrates and nitrites. 

In 1987, Hibbs published a landmark paper in the nitric oxide field (Hibbs et al., 1987a), by demonstrating that the L-arginine that is required for CAM cytostatic activity is metabolized to L-citrulline (similar to L-arginine but less one nitrogen atom from the guanidinium group) and nitrite plus nitrate. In addition,... 

In this particular case, it is not easy to follow the reaction by FTIR spectroscopy as only a significant shift can be observed for weak bands of isolated groups (OOHj 3550 OHj 3620 cm 1). However, an additional treatment by nitric oxide for both POOH and POH allows to follow the reaction since nitrates and nitrites have specific absorption (nitrates 853, 1276 nitrites 780 cm 1) see fig. 5. 

Nitrates and nitrites in water are frequently estimated together, e.g. by reduction to ammonia,1 which can be determined m the manner described below alternative processes are based on the reduction of these salts to nitric oxide which may be measured volumetrically, and on reduction of the nitrate to nitrite when the total nitrite may be estimated eolonmetncaUy by the addition of sulphanihc acid and a-naphthylamine.3 For the estimation of nitrites and mtrates separately, organic colorimetric methods are usually applied.4... 

Ferrous salts reduce nitrites to nitric oxide. On addition, for example, of barium nitrite to ferrous sulphate, barium sulphate is precipitated, and the liquid turns brown. Ferric hydroxide and a basic ferric nitrate are next precipitated, nitric oxide being evolved. Apparently the first product of the reaction is ferrous nitrite, which then spontaneously decomposes in accordance with the equation 2... 

Catalases are mainly present in microorganisms such as Kocuria and Staphylococcus and are responsible for peroxide reduction. This reaction contributes to eolor and flavor stabilization. Nitrate reductase, also present in those microorganisms, is a very important enzyme for the reduction of nitrate to nitrite in slow ripened sausages where there is initial addition of nitrate. Recently, two strains of Lactobacillus fermentum have proved to be able to generate nitric oxide and give an acceptable color in sausages without nitrate/nitrite. This may be important in the case of cured meats with no addition of either nitrate or nitrite (Moller et al. 2003). 

In addition to dietary sources, a significant amount of nitrate is formed endogenously by the metabolism of nitric oxide - 1 mg per kg of body weight per day (about the same as the average dietary intake), increasing 20-fold in response to inflammation and immune stimulation. There is considerable secretion of nitrate in saliva, and up to 20% of this may be reduced to nitrite by oral bacteria. Under the acidic conditions of the stomach, nitrite can react with amines in foods to form carcinogenic N-nitrosamines, although it is not known to what extent this occurs in vivo. 

In testing for a nitrate advantage is taken of the fact that a very small amount of nitric oxide will form a dark brown compound with ferrous sulphate, FeS04,N0. As has already been shown nitric acid is reduced to nitric oxide by certain reducing agents, of which ferrous sulphate is one. In other words, a dark brown compound is formed when nitric acid is added to ferrous sulphate, because the latter first reduces the nitric acid and then unites with the nitric oxide produced. Thus the formation of this compound may serve as a test for nitric acid. The test may be made applicable not only to nitric acid but also to any nitrate by the addition of concentrated sulphuric acid, since the sulphuric acid Avill convert the nitrate, if present, to nitric acid, which will in turn react with the ferrous sulphate as just described. In actual practice the sulphuric acid is added last and in such a way that it will form a layer in the bottom of the test tube, because it is heavier than the other liquids. The reactions will then take place where the layers meet and the brown compound will appear as a ring, if a nitrate is present. It must be remembered, however, that nitrous acid and the nitrites also produce nitric oxide, and consequently the brown compound, under these circumstances. The test is of value, therefore, only when nitrites are known to be absent. 

The reaction between ONOO" and nitric oxide in phosphate buffer at physiological pH does not consume or produce oxygen, nor is nitrous oxide (N2O) produced (Crow and Beckman, unpublished observations). The relatively high concentrations of nitric oxide used for product analysis experiments resulted in the production of both nitrite and nitrate, as did decomposition of ONOO" alone NO2 and NOj production were merely additive... 

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