Nitrous acid measurement

Another important method of following protein hydrolysis is that due to Van Slyke, and consists in estimating the free amino groups liberated by treatment with nitrous acid, whereby gaseous nitrogen is evolved and measured in a special apparatus. 

In an excess of nitric acid, nitrous acid exists essentially as dinitrogen tetroxide which, in anhydrous nitric acid, is almost completely ionised. This is shown by measurements of electrical conductivity, and Raman and infra-red spectroscopy identify the ionic species... 

If we consider the effect of nitrous acid upon zeroth-order nitration in organic solvents we must bear in mind that in these circumstances dinitrogen tetroxide is not much ionised, so the measured concentration of nitrous acid gives to a close approximation the concentration of dinitrogen tetroxide. Further, the negligible self-ionisation of nitric acid ensures that the total concentration of nitrate ions is effectively that formed from dinitrogen tetroxide. Consequently as we can see from the equation for the ionisation of dinitrogen tetroxide ( 4.3.1),... 

In a polluted or urban atmosphere, O formation by the CH oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6] HNO2, (formed from the reaction of OH + NO and other reactions) are also important sources of OH ia polluted environments. An imperfect, but useful, measure of the rate of O formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown ia Table 4 relative to the OH-CH rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and a-piuene. In general, internally bonded olefins are the most reactive, followed ia decreasiag order by terminally bonded olefins, multi alkyl aromatics, monoalkyl aromatics, C and higher paraffins, C2—C paraffins, benzene, acetylene, and ethane. 

If a solution of a nitrite is titrated in the ordinary way with potassium permanganate, poor results are obtained, because the nitrite solution has first to be acidified with dilute sulphuric acid. Nitrous acid is liberated, which being volatile and unstable, is partially lost. If, however, a measured volume of standard potassium permanganate solution, acidified with dilute sulphuric acid, is treated with the nitrite solution, added from a burette, until the permanganate is just decolorised, results accurate to 0.5-1 per cent may be obtained. This is due to the fact that nitrous acid does not react instantaneously with the permanganate. This method may be used to determine the purity of commercial potassium nitrite. 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3, so-called neutral diazotization) or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. Kozlov and Volodarskii (1969) measured the rates of diazotization of l-amino-2-naphthol-4-sulfonic acid in the presence of one equivalent of 13 different sulfates, chlorides, and nitrates of di- and trivalent metal ions (Cu2+, Sn2+, Zn2+, Mg2+, Fe2 +, Fe3+, Al3+, etc.). The rates are first-order with respect to the added salts. The highest rate is that in the presence of Cu2+. The anions also have a catalytic effect (CuCl2 > Cu(N03)2 > CuS04). The mechanistic basis of this metal ion catalysis is not yet clear. 

The kinetics of aromatic nitrosation at ring carbon have received little attention. The first attempt to determine the nature of the electrophile was made by Ingold et a/.117, who measured the rates of the nitrous acid-catalysed nitration of 4-chloroanisole by nitric acid in acetic acid which proceeds via initial nitrosation of the aromatic ring. Assuming that the electrophiles are the nitrosonium ion and... 

No accurate rate measurements have yet been carried out, but qualitative experiments showed that the exchange was acid-catalysed so that it is likely that the protonated form of the nitrosamine (LIII) acts itself as a nitrosating agent. This would account for all the earlier cross-nitrosations that have been observed without necessitating the prior formation of nitrous acid, nitrosyl chloride or any other carrier of NO+. 

The relatively acidic thiol on serum albumin forms a fairly stable S-nitroso adduct with nitric oxide, which may serve to preserve and carry NO throughout the circulatory system [162,163]. Bacterial toxins released in toxic-shock syndrome induce excessive NO-synthase activity in macrophages. The resulting arterial expansion may induce the cardiovascular collapse associated with toxic shock syndrome [164]. Nitrous acid reacts with DNA to form dG-N2-dG interstrand crosslinks at the sequence 5 CG [165]. NO can also deaminate cytidine [166] and deoxyguanosine [167] and so may function as a mutagen. The rate law for NO reacting with O2 has been measured electrochemically as [168] ... 

Chlorocresol is a preservative commonly used in injections and its determination often involves the use of laborious extraction procedures in order to separate it from formulation components, followed by spectrophotometric measurement. A FIA method for chlorocresol was developed by utilising its reaction with nitrous acid to form a coloured nitro compound. The method was accurate to 99.5% of the true value of chlorocresol in a formulation and a precision of 1% was achieved. ... 

Febo, A., and C. Perrino, Measurement of High Concentrations of Nitrous Acid Inside Automobiles, Atmos. Environ., 29, 345-351 (1995). 

Pitts, J. N., Jr., H. W. Biermann, A. M. Winer, and E. C. Tuazon, Spectroscopic Identification and Measurement of Gaseous Nitrous Acid in Direct Auto Exhaust, Atmos. Emiron., 18, 847-854 (1984b). 

Hausmann, M., J. Rudolf, and U. Platt, Spectroscopic Measurement of Bromine Oxide, Ozone and Nitrous Acid in Alert, in The Tropospheric Chemistry of Ozone in the Polar Regions NATO ASI Series I Global Environmental Change (H. Niki and K. H. Becker, Eds.), Vol. 17, pp. 189-203, 1993. 

In developing the reactivity scale, Nielsen first investigated the transformation rates of 25 PAHs and four derivatives of anthracene in water-methanol-dioxane solutions, taken as a model of wet particles, and containing small amounts of dinitrogen tetroxide and nitric and nitrous acids. The measured half-lives and relative rates are shown in Table 10.29. The range of reactivities in solution for PAHs of different structures is remarkable, from 100,000 (arbitrarily set) for an-thanthrene (XXXII) to <0.2 for the least reactive compounds ... 

Febo, R., C. Perrino, and M. Cortiello, A Denuder Technique for the Measurement of Nitrous Acid in Urban Atmospheres, Atmos. Environ., 27A, 1721-1728(1993). 

Vecefa, Z and P. K. Dasgupta, Measurement of Ambient Nitrous Acid and a Reliable Calibration Source for Gaseous Nitrous Acid, Environ. Sci, Technol., 25, 255-260 (1991). 

In the absence of such sources of NO, indoor and outdoor concentrations are quite similar (e.g., Weschler et al., 1994), since removal of NO and N02 indoors, e.g., on surfaces, is relatively slow. However, as discussed shortly, although the surface reaction of N02 is relatively slow, it is still of interest since it generates nitrous acid (HONO). Different surfaces found inside homes have been found to have different removal rates for N02. Figure 15.4, for example, shows measured rates of removal of N02 by a number of common household materials (Spicer et al., 1989). Large variations in removal rate (and hence the formation of products such as NO and HONO see later) are evident, varying from negligible for plastic storm windows to quite large for wallboard. 

The chemistry of nitrite at acidic pH is closely related to that of nitrosonium ion described earlier. The pfC, of nitrous acid is around 3.4-3.6, though precise measurement is difficult because of its rapid secondary reactions. Acidification of nitrite produces nitrous acid, which is in reversible equilibrium with nitrosonium ion and hydroxide ion (Turney and Wright, 1959), although in aqueous solu-... 

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