Nitric reaction + metal atoms

Excess magnesium ribbon is added to dilute nitric acid. During this addition an effervescence is observed due to the production of hydrogen gas. In this reaction the hydrogen ions from the nitric acid gain electrons from the metal atoms as the reaction proceeds. 

Kolomnikow, I. S. Lysyak, T.V. Carbon-Dioxide in Coordination Chemistry and Catalysis, Russ. Chem. Rev. 1990,59, 589. (f) Behr, A. Carbon Dioxide Activation by Metal Complexes, VCH, Weinheim, 1988. (g) Darensbourg, D. J. Kudaroski, R. A. The activation of carbon dioxide by metal complexes, Adv. Organomet. Chem. 1983, 22, 129. (h) Eisenberg, R. Hendricksen, D. E. The binding and activation of carbon monooxide, carbon dioxide, and nitric oxide and their homogeneously catalyzed reactions, Adv. Catal. 1979, 28, 79. (i) Souter, P. F. Andrews, L. P. F. A Spectroscopic and Theoretical-Study of the Reactions of Group-6 Metal Atoms with Carbon-Dioxide,/. Am. Chem. Soc. 1991,119, 7350. 

For example, the industrial preparation of mineral acids, such as sulfuric, hydrochloric and nitric, inevitably leads to them containing small concentrations of metals as impurities. If the acid is to be used purely as an acid in a simple reaction, the presence of small amounts of metals is probably unimportant. If, however, the acid is to be used to digest a sample for the determination of trace metals by atomic absorption spectrometry, then clearly the presence of metallic impurities in the acid may have a significant effect on the results. For this latter application, high-purity acids that are essentially metal-free are required. 

Nitrogen oxides. Nitric oxide (NO) itself, has been shown to be a poor nitrosating agent (28), probably because it is unable to abstract an amino-H atom to generate the dialkyl-amino radical, which might then combine with further NO. However, the presence of even a small amount of air results in complete conversion, presumably via oxidation of NO to NO2. Nitrosation by NO is catalyzed by metal salts, such as Znl2, CuCl, and CUSO4. The metal catalyzed reaction is inhibited in acid or aqueous media (29). 

Protons present in aqueous acid also act as reasonably efficient electron acceptors. If the reduced hydrogen atoms are formed on metallized suspensions, catalytic hydrogenation can result. For example, in contrast to the oxidative chemistry reported earlier for cyclohexene-4,5-bis-dicarboxylic acid (Eq. 28), if the reaction is conducted in the absence of oxygen in aqueous nitric acid, catalytic hydrogenation of the double bond becomes a major pathway, Eq. (34). ... 

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