Nitrosylsulfuric acid

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Nitrosylsulfuric acid
IUPAC name
Nitrosylsulfuric acid
Other names
Chamber crystals
Nitrosyl hydrogen sulfate
Nitrosyl sulphuric acid
Nitrosonium bisulfate
Nitrosulfonic acid
Nitrososulfuric acid
Nitrous sulfuric anhydride
Molar mass 127.08 g/mol
Appearance Pale yellow crystals
Odor Acidic
Density 1.612 g/cm3 (20 °C) (40% H2SO4 sol.)
Melting point 70 °C (158 °F; 343 K)
Boiling point Decomposes
Solubility Soluble in fuming nitric acid, conc. sulfuric acid
Safety data sheet Sigma-Aldrich (40% sol. H2SO4)
Flash point Non-flammable
Related compounds
Related compounds
Nitrating mixture
Purple acid
Sulfuric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Nitrosylsulfuric acid is a chemical compound with the chemical formula NOHSO4. The compound is the mixed anhydride of sulfuric acid and nitrous acid.



Nitrosylsulfuric acid is used in organic chemistry to prepare diazonium salts from amines, for example in the Sandmeyer reaction. Related NO-delivery reagents include nitrosonium tetrafluoroborate and nitrosyl chloride.

Nitrosylsulfuric acid hydrolyses in contact with water to form sulfuric acid and nitrous acid, the latter decomposing rapidly, giving off nitric oxide which oxidizes in air to form nitrogen dioxide.

Reduction of nitrosylsulfuric acid in sulfuric acid with metabisulfite, sulfite, nitrogen monoxide or various other reducing agents will yield a blue-purple, unstable solution called purple acid. If copper is present, a blue-purple copper(II)-nitrogen monoxide complex is formed instead.[1]


Nitrosylsulfuric acid is a pale yellowish solid, that reacts with water and alcohols. It is very soluble in conc. sulfuric acid and fuming nitric acid. The compound can precipitate out as solid crystals from a solution of 30% conc. sulfuric acid, indicating that the hydrolysis is an equilibrium reaction.[2]


Nitrosylsulfuric acid is sold by chemical suppliers. It is more commonly available as solution in sulfuric acid (usually 40-50%).


A simple route involves dissolving sodium nitrite in cold conc. sulfuric acid.[3]

HNO2 + H2SO4 → NOHSO4 + H2O

A nitrite-free route involves bubbling cold sulfur dioxide into cold fuming nitric acid. As the reaction is exothermic, the temperature should be kept below 5 °C.[4][5]


The nitrosylsulfuric acid prepared this way, has a concentration of 45-50% in fuming nitric acid.

Nitrosylsulfuric acid is a side product of the lead chamber process, hence its name "chamber crystal".


  • Synthesis of caprolactam
  • Preparation of diazonium salts



Nitrosylsulfuric acid is corrosive and will react with water, releasing hazardous fumes. Proper protection must be worn when handling the compound.


Can be kept in concentrated sulfuric acid as solution.


Should be added to ice cold water or just crushed ice, followed by neutralization with a base.


  4. Coleman, G. H., Lillis, G. A., Goheen, G. E., Herrmann, C. V., & Booth, H. S. (2007). Nitrosyl Chloride. Inorganic Syntheses, 55–59
  5. J. Am. Chem. Soc. 1925, 47, 1, 143–147

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