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7782-77-6 Usage


Nitrous acid (molecular formula HNO2) is a weak and monobasic acid that is only known in solution and in the form of nitrite salts. It rapidly decomposes into nitrogen oxide, nitric oxide, and water when in solution, and can also decompose into nitric acid, nitrous oxide, and water.


Used in Chemical Industry:
Nitrous acid is used as a diazotizing agent for the production of azo dyes. It reacts with primary aromatic amines to form diazo derivatives, which are then used to create azo dyes.
Used in Medical Applications:
Nitrous acid, in the form of sodium nitrite, is used as part of an intravenous mixture with sodium thiosulfate to treat cyanide poisoning. There is ongoing research to explore its potential applications in treating heart attacks, brain aneurysms, pulmonary hypertension in infants, and Pseudomonas aeruginosa infections.
Used in Formation of Diazotizing Compounds:
Nitrous acid is used to form diazotizing compounds by reacting with primary aromatic amines, which serve as a source of nitric oxide.


Nitrous acid is unstable. It decomposes to form nitric acid and nitric oxide: 3HNO2 → NO3ˉ + H3O+ + 2NO Strong oxidizing agents, such as permanganate, readily oxidize nitrous acid to nitric acid. Nitrous acid is an effective oxidizing agent. It oxidizes hydrogen sulfide to sulfur forming either nitric oxide or ammonia, depending on the acidity of the solution: 2HNO2 + H2S → S + 2NO + 2H2O HNO2 + 3H2S → 3S + NH3 + 2H2O In acid medium it oxidizes iodide ion to iodine: HNO2 + Iˉ + 6H+ → 3I2 + NH3 + 2H2O


Nitrous acid may be obtained in solution by adding a strong acid to nitrite; e.g., adding hydrochloric acid to sodium nitrite solution: H+ + NO2 ˉ → HNO2.


Rapidly forms nitric oxide and nitric acid in water; [Merck Index] A strong oxidizer; Causes burns; Highly toxic by ingestion and inhalation.

Safety Profile

Mutation data reported. Flammable by chemical reaction; a powerful oxidizer. Explodes on contact with phosphorus trichloride. Reacts violently with PH3 and Pcb. Reactions with l-amino- 5-nitrophenol, ammonium decahydroborate(2-), hydrazine (product is hydrogen azide) may give explosive products. Incompatible with anilines (e.g., 4- bromoahe , 2-chloroaniline, 3- chloroaniline, 2-nitroadine, 3-nitroaniline, 4-nitroaniline, aniline ), semicarbazone, silver nitrate. When heated to decomposition it emits hghly toxic fumes of NOx. See also NITRIC OXIDE.

Check Digit Verification of cas no

The CAS Registry Mumber 7782-77-6 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 7,7,8 and 2 respectively; the second part has 2 digits, 7 and 7 respectively.
Calculate Digit Verification of CAS Registry Number 7782-77:
136 % 10 = 6
So 7782-77-6 is a valid CAS Registry Number.



According to Globally Harmonized System of Classification and Labelling of Chemicals (GHS) - Sixth revised edition

Version: 1.0

Creation Date: Aug 12, 2017

Revision Date: Aug 12, 2017


1.1 GHS Product identifier

Product name nitrous acid

1.2 Other means of identification

Product number -
Other names hydrogen nitryl

1.3 Recommended use of the chemical and restrictions on use

Identified uses For industry use only.
Uses advised against no data available

1.4 Supplier's details

1.5 Emergency phone number

Emergency phone number -
Service hours Monday to Friday, 9am-5pm (Standard time zone: UTC/GMT +8 hours).

More Details:7782-77-6 SDS

7782-77-6Relevant articles and documents

Shan, J. H.,Wategaonkar, S. J.,Vasudev, R.

, p. 317 - 320 (1989)

A convenient preparation of pentaaquanitrosylchromium(2+) sulfate: The crystal structure revisited

D?ssing, Anders,Frey, Anne Mette

, p. 1681 - 1684 (2006)

The complex pentaaquanitrolsylchromium(2+) sulfate, [Cr(OH 2)5(NO)]SO4 has been prepared in a high yield by the hydrolysis of [Cr(NCCH3)5(NO)](BF4) 2 in dilute sulfuric acid. Crystals of [Cr(OH2) 5(NO)]SO4·H2O have been grown and characterized by X-ray crystallography. Continuous photolysis of [Cr(NCCH 3)5(NO)]2+ in acetonitrile solution with 404 nm light results in a release of NO with the quantum yield Φ = 0.55 mol einstein-1 at 298 K with the resulting solvated Cr2+ ion being trapped by molecular dioxygen present in the solution.

Production processes of H(D) atoms in the reactions of NO(A2Σ+) with C2H2, C2H4, H2O, and their isotopic variants

Umemoto, Hironobu,Terada, Naoki,Tanaka, Kunikazu,Takayanagi, Toshiyuki,Kurosaki, Yuzuru,Yokoyama, Keiichi

, p. 39 - 47 (2000)

The production of H(D) atoms was identified in the reactions of NO(A2Σ+) with C2H2, C2H4, H2O, and their isotopic variants. By measuring the Doppler profiles, the translational energies of H(D) atoms were determined. As for C2H2 and H2O, around 1/4 of the NO(A2Σ+) energy is partitioned into the translational mode, while the ratio is around 1/7 for C2H4. Such a large partitioning of the available energy into the translational mode cannot be explained by a C-H(O-H) bond rupture after an energy transfer. Nitrogen compounds, such as ONC2H and HONO, must be produced as pair products. The measured translational energies are much larger than those statistically expected, suggesting that the intermediate species are short-lived. No large isotope effect was observed in the average translational energies or in the relative yields. Quantum effects, such as tunneling, do not play important roles. Ab initio molecular orbital calculations were carried out to characterize these reactive processes. (C) 2000 Elsevier Science B.V.

Fine tuning the reactivity of corrole-based catalytic antioxidants

Okun, Zoya,Gross, Zeev

, p. 8083 - 8090 (2012)

In order to determine the electronic factors that may affect the catalytic antioxidant activity of water-soluble metallocorroles a series of 10-aryl-5,15-pyridinium manganese(III) corroles was prepared. These complexes were examined regarding the effect of the C10 substituent on the MnIV/MnIII redox potentials, catalytic rate constants for decomposition of HOONO, prevention of tyrosine nitration, and superoxide dismutase activity. This structure-activity relationship investigation provides new insight regarding the mechanism by which manganese(III) corroles act as catalytic antioxidants. It also discloses the superiority of the C 10-anysil-substituted complex in all examined aspects.

Crystal structure, optical, magnetic, and photochemical properties of the complex pentakis(dimethyl sulfoxide)nitrosylchromium(2+) hexafluorophosphate

Dethlefsen, Johannes W.,D?ssing, Anders,Kadziola, Anders

, p. 1585 - 1590 (2009)

The nitrosyl complex [Cr(dmso)5(NO)](PF6)2 (1) (dmso = dimethyl sulfoxide) has been prepared by the solvolysis of [Cr(NCCH3)5(NO)](PF6)2 in neat dmso. The optical absorption spec

Near-UV Absorption Cross Sections and Trans/Cis Equilibrium of Nitrous Acid

Bongartz, A.,Kames, J.,Welter, F.,Schurath, U.

, p. 1076 - 1082 (1991)

The A 1A'' X 1A' absorption spectrum of gaseous nitrous acid has been measured in the 300-400-nm range.Absolute cross sections were determined by a combination of gas-phase and wet chemical analysis.The cross sections of prominent bands are 25percent larger than the recommended values of Stockwell and Calvert.The influence of spectral resolution on absolute and differential absorption cross sections was also investigated.The integrated band area of the n?* transition yields an oscillator strength f = (8.90 +/- 0.36) x 10-4, less than the reported liquid phase value of 2 x 10-3.The equilibrium constant K = ptrans/pcis, base d on the assumption that the oscillator strength of the n?* transition is the same for both rotamers, was found to be 3.25 +/- 0.30 at 277 K.This yields an energy difference ΔE between trans- and cis-HONO of -2700 J mol-1 in the electronic ground state, and -6000 J mol-1 in the excited state.

Experimental and theoretical study of the reaction of HO- with NO

Doren, Jane M. Van,Viggiano, A. A.,Morris, Robert A.,Miller, Amy E. Stevens,Miller, Thomas M.,et al.

, p. 7940 - 7950 (1993)

Hydroxide ion (HO-) reacts with nitric oxide by slow reactive electron detachment with a rate coefficient ca. 4 x 10-12 cm3 s-1 at 298 K.The detachment process is presumably associative detachment forming nitrous acid and an electron.Observations, data analysis, and alternative explanations for these observations are discussed.The associative detachment reaction was also investigated theoretically through calculations of the geometries, relative energies, and normal-mode vibrational frequencies of the relevant species HO-, HO, NO, cis- and trans-HONO, and cis- and trans-HONO-.These calculations indicate that in the ion HONO-, the cis conformer is more stable, while in the neutral HONO, the trans conformer is more stable.The HO-NO bond in HONO, which is formed in this reaction, is much stronger than the HO--NO bond in HONO- with an energy of 198.7+/-1.8 kJ mol-1 for cis HONO and 52.2+/-5 kJ mol-1 for cis-HONO- at 0 K.HONO- is bound with respect to HONO.The adiabatic electron detachment energy resulting from detachment from cis-HONO- forming the same conformer of the neutral molecule cis-HONO is 0.29+/-0.05 eV.The HO-NO equilibrium bond distance in HONO- is considerably longer than that in HONO, with values of 1.750 and 1.640 Angstroem for trans- and cis- HONO-, respectively, and 1.429 and 1.392 Angstroem for trans- and cis-HONO, respectively.These geometric and energetic characteristics of HONO- and HONO are combined with calculations of relative energies of these species at nonequilibrium/distorted HO-NO bond lengths to give a qualitative picture of the potential energy curves for these species along the reaction coordinate.While no significant energy barrier to autodetachment of HONO- is present, the Franck-Condon wave function overlap for aitodetachment is small and is likely the reason for the observed inefficiency.The maximum calculated rate constant for associative detachment is 4 x 10-12 cm3 s-1, in good agreement with the observed value.

Heterogeneous chemistry of HONO on liquid sulfuric acid: A new mechanism of chlorine activation on stratospheric sulfate aerosols

Zhang, Renyi,Leu, Ming-Taun,Keyser, Leon F.

, p. 339 - 345 (1996)

Heterogeneous chemistry of nitrous acid (HONO) on liquid sulfuric acid (H2SO4) was investigated at conditions that prevail in the stratosphere. The measured uptake coefficient (γ) of HONO on H2SO4 increased with

Heterogeneous reaction of NO2 on hexane soot: A Knudsen cell and FT-IR study

Al-Abadieh, Hind A.,Grassian

, p. 11926 - 11933 (2000)

NO2 can react with soot particles to produce HONO. The heterogeneous reaction of NO2 on freshly prepared hexane soot was investigated using a Knudsen cell reactor and FTIR spectroscopy. Initial uptake coefficients were determined using gas-diffusion models that take into consideration the surface area of the top layer of soot particles as well as the accessible underlying layers of soot particles. Under dry conditions, the initial uptake coefficient was near 5 x 10-5 at a gas concentration of 2.5 x 1011 molecules/cc and 295 K. The adsorbed products remained on the surface in agreement with previous results. HONO was a gas-phase reaction product, accounting for 36% of the NO2 reacted. Knudsen cell data were in better agreement with the values of the uptake coefficient determined from other experimental methods when the BET areas of the soot samples were taken into account and average values of the uptake coefficient were compared.

Dhanya, S.,Saini, R. D.,Bhattacharyya, P. K.

, p. 93 - 96 (1986)

Measurement of absolute absorption cross sections for nitrous acid (HONO) in the near-infrared region by the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique coupled to laser photolysis

Jain, Chaithanya,Morajkar, Pranay,Schoemaecker, Coralie,Viskolcz, Bela,Fittschen, Christa

, p. 10720 - 10728 (2011)

Absolute absorption cross sections for selected lines of the OH stretch overtone 2ν1 of the cis-isomer of nitrous acid HONO have been measured in the range 6623.6-6645.6 cm-1 using the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique. HONO has been generated by two different, complementary methods: in the first method, HONO has been produced by pulsed photolysis of H2O2/NO mixture at 248 nm, and in the second method HONO has been produced in a continuous manner by flowing humidified N2 over 5.2 M HCl and 0.5 M NaNO2 solutions. Laser photolysis synchronized with the cw-CRDS technique has been used to measure the absorption spectrum of HONO produced in the first method, and a simple cw-CRDS technique has been used in the second method. The first method, very time-consuming, allows for an absolute calibration of the absorption spectrum by comparison with the well-known HO2 absorption cross section, while the second method is much faster and leads to a better signal-to-noise ratio. The strongest line in this wavelength range has been found at 6642.51 cm-1 with σ = (5.8 ± 2.2) × 10-21 cm2.

Barnhard, Katherine I.,Santiago, Alejandro,He, Min,Asmar, Federico,Weiner, Brad R.

, p. 150 - 156 (1991)

Zabarnick, Steven

, p. 265 - 274 (1993)

Suter, H. U.,Huber, J. Robert

, p. 203 - 209 (1989)

Radical-Molecule Kinetics in Pulsed Uniform Supersonic Flows: Termolecular Association of OH + NO between 90 and 220 K

Atkinson, Dean B.,Smith, Mark A.

, p. 5797 - 5800 (1994)

The low-temperature dependence of the temolecular association reaction of OH with NO employing N2 as the third body has been investigated using a new pulsed uniform supersonic expansion flow reactor.The absolute low-pressure reaction rate coefficient is reported for the temperature range 90-220 K.The temperature dependence of the rate coefficient for this reaction is found to be well fit by k=(7.0 +/-2.0)E-31(T/300)-2.6 +/-0.3 cm6s-1.The results agree with those obtained in the higher temperature regime and with RRKM predictions of the rate coefficient both validating the new technique and providing valuable information on the extended temperature dependence of this atmospherically relevant reaction.

Infrared absorption cross-section measurements for nitrous acid (HONO) at room temperature

Barney, William S.,Wingen, Lisa M.,Lakin, Matthew J.,Brauers, Theo,Stutz, Jochen,Finlayson-Pitts, Barbara J.

, p. 1692 - 1699 (2000)

Infrared absorption cross sections for nitrous acid (HONO) were measured using HONO spectra recorded simultaneously by UV/visible and FTIR spectroscopy. HONO was prepared by the reaction of HCl(g) and NaNO2(s) and was introduced into a 561 L environmental chamber equipped with parallel sets of White optics with total path 52.5 m for UV/visible and FTIR spectroscopy. Alternatively, HONO was prepared in situ by reaction of ClNO(g) with water vapor. Absolute concentrations of HONO were determined independently using the UV spectrum and published UV absorption cross sections. All experiments were carried out at 750 Torr total pressure in N2 at 294-297 K. We report both Q-branch intensities and integrated absorbances for the HONO modes trans-v3 (1263 cm-1), cis-v4 (852 cm-1), and trans-v4 (790 cm-1). For trans-v3 and cis-v4 we also include synthetic reference spectra composed of Gaussian functions which give an accurate reproduction of our experimental references, and can easily be generated by computer for ease of use in other laboratories.

Gamma radiolysis of Cu(II) complex of metronidazole


, p. 2975 - 2980 (1990)


New experimental and theoretical approach to the heterogeneous hydrolysis of NO2: Key role of molecular nitric acid and its complexes


, p. 6886 - 6897 (2006)

Although heterogeneous chemistry on surfaces in the troposphere is known to be important, there are currently only a few techniques available for studying the nature of surface-adsorbed species as well as their chemistry and photochemistry under atmospheric conditions of 1 atm pressure and in the presence of water vapor. We report here a new laboratory approach using a combination of long path Fourier transform infrared spectroscopy (FTIR) and attenuated total reflectance (ATR) FTIR that allows the simultaneous observation and measurement of gases and surface species. Theory is used to identify the surface-adsorbed intermediates and products, and to estimate their relative concentrations. At intermediate relative humidities typical of the tropospheric boundary layer, the nitric acid formed during NO2 heterogeneous hydrolysis is shown to exist both as nitrate ions from the dissociation of nitric acid formed on the surface and as molecular nitric acid. In both cases, the ions and HNO3 are complexed to water molecules. Upon pumping, water is selectively removed, shifting the NO3--HNO 3(H2O)y, equilibria toward more dehydrated forms of HNO3 and ultimately to nitric acid dimers. Irradiation of the nitric acid-water film using 300-400 nm radiation generates gaseous NO, while irradiation at 254 nm generates both NO and HONO, resulting in conversion of surface-adsorbed nitrogen oxides into photochemically active NOx, These studies suggest that the assumption that deposition or formation of nitric acid provides a permanent removal mechanism from the atmosphere may not be correct. Furthermore, a potential role of surface-adsorbed nitric acid and other species formed during the heterogeneous hydrolysis of NO2 in the oxidation of organics on surfaces, and in the generation of gas-phase HONO on local to global scales, should be considered.

Probing the Reaction Mechanisms Involved in the Decomposition of Solid 1,3,5-Trinitro-1,3,5-triazinane by Energetic Electrons

Singh, Santosh K.,Zhu, Cheng,Vuppuluri, Vasant,Son, Steven F.,Kaiser, Ralf I.

, p. 9479 - 9497 (2019/11/11)

The decomposition mechanisms of 1,3,5-trinitro-1,3,5-triazinane (RDX) have been explored over the past decades, but as of now, a complete picture on these pathways has not yet emerged, as evident from the discrepancies in proposed reaction mechanisms and the critical lack of products and intermediates observed experimentally. This study exploited a surface science machine to investigate the decomposition of solid-phase RDX by energetic electrons at a temperature of 5 K. The products formed during irradiation were monitored online and in situ via infrared and UV-vis spectroscopy, and products subliming in the temperature programmed desorption phase were probed with a reflectron time-of-flight mass spectrometer coupled with soft photoionization at 10.49 eV (ReTOF-MS-PI). Infrared spectroscopy revealed the formation of water (H2O), carbon dioxide (CO2), dinitrogen oxide (N2O), nitrogen monoxide (NO), formaldehyde (H2CO), nitrous acid (HONO), and nitrogen dioxide (NO2). ReTOF-MS-PI identified 38 cyclic and acyclic products arranged into, for example, dinitro, mononitro, mononitroso, nitro-nitroso, and amines species. Among these molecules, 21 products such as N-methylnitrous amide (CH4N2O), 1,3,5-triazinane (C3H9N3), and N-(aminomethyl)methanediamine (C2H9N3) were detected for the first time in laboratory experiments; mechanisms based on the gas phase and condensed phase calculations were exploited to rationalize the formation of the observed products. The present studies reveal a rich, unprecedented chemistry in the condensed phase decomposition of RDX, which is significantly more complex than the unimolecular gas phase decomposition of RDX, thus leading us closer to an understanding of the decomposition chemistry of nitramine-based explosives.

Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces

Laufs, Sebastian,Kleffmann, J?rg

, p. 9616 - 9625 (2016/04/19)

During the last few decades, nitrous acid (HONO) has attracted significant attention as a major source of the OH radical, the detergent of the atmosphere. However, the different daytime sources identified in the laboratory are still the subject of controversial discussion. In the present study, one of these postulated HONO sources, the heterogeneous photolysis of nitric acid (HNO3), was studied on quartz glass surfaces in a photo flow-reactor under atmospherically relevant conditions. In contrast to other investigations, a very low HNO3 photolysis frequency for HONO formation of J(HNO3 → HONO) = 2.4 × 10-7 s-1 (0° SZA, 50% r.h.) was determined. If these results can be translated to atmospheric surfaces, HNO3 photolysis cannot explain the significant HONO levels in the daytime atmosphere. In addition, it is demonstrated that even the small measured yields of HONO did not result from the direct photolysis of HNO3 but rather from the consecutive heterogeneous conversion of the primary photolysis product NO2 on the humid surfaces. The secondary NO2 conversion was not photo-enhanced on pure quartz glass surfaces in good agreement with former studies. A photolysis frequency for the primary reaction product NO2 of J(HNO3 → NO2) = 1.1 × 10-6 s-1 has been calculated (0° SZA, 50% r.h.), which indicates that renoxification by photolysis of adsorbed HNO3 on non-reactive surfaces is also a minor process in the atmosphere.

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