The photochemistry of aqueous nitrate ion revisited

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Chemistry., 1996-12, Vol.101 (2), p.89-103
• Main Authors: Mark, Gertraud, Korth, Hans-Gert, Schuchmann, Heinz-Peter, von Sonntag, Clemens
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Nitrate ion; Nitric oxide peroxyl radical; Peroxynitrite; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics)
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/S1010-6030(96)04391-2

Aqueous nitrate solutions were photolysed at 254 nm in the absence of oxidizable additives, in the presence of methanol or propan-2-ol and oxygen and in the presence of cyclopentane under anaerobic conditions. The main nitrogen-containing products are nitrite and peroxynitrite. The quantum yields depend on the pH, nitrate concentration, nature of the additive and the light intensity. The intrinsic nitrite yield in alkaline solutions could not be determined directly because, under the conditions of the nitrite assay, the accompanying peroxynitrite decomposes to form nitrite and nitrate; it is smaller than the apparent nitrite yield. In the acidic (pH 4–7) range, the intrinsic nitrite quantum yield is equal to the apparent nitrite yield because there is no buildup of peroxynitrite under these conditions. The apparent nitrite quantum yield increases from 0.01 (no oxidizable additive) to approximately 0.03 (cyclopentane (millimolar range), oxygen free) to 0.06 (methanol (millimolar range), air saturated). At pH 13 and in the absence of oxidizable additives, the apparent nitrite quantum yield increases to about 0.1, whereas from material balance considerations the intrinsic nitrite quantum yield is estimated to be 0.06, twice the oxygen quantum yield of 0.03. Spectrophotometrically, peroxynitrite is detected in the alkaline range only, because its protonated form is unstable. In the absence of oxidizable additives, the quantum yield of peroxynitrite is about 0.1, i.e. only about two-thirds of the quantum yield in the presence of oxidizable additives. Mechanistic considerations on the basis of the pH dependence of the quantum yields of the products nitrite, peroxynitrite and oxygen, as well as their dependence on the kind of additive, indicate that the decisive factor of photolysis in the absence of additives is the formation of the nitric oxide peroxyl radical, ONOO, formed by reaction of peroxynitrite with the primarily generated OH radical. The decay of ONOO is the source of O 2 in this system. Nitric oxide, NO, the other fragment of this decay reaction, reacts with nitrogen dioxide, which is one of the primarily formed intermediates. The latter reaction is one of the pathways to the product nitrite, particularly in the alkaline range. The formation of NO during photolysis has been verified by electron spin resonance (ESR) spectroscopic detection of the nitroxide 1,1,3,3-tetramethyl-isoindolin-2-oxyl, the NO adduct to 7,7,8,8-tetramethyl- o-quinodimethane. Of