Purple acid

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Purple acid
Other names
Blue acid
Nitrogen oxide nitrosyl hydrogen sulfate
Sulphonitronic acid
Sulfonitronic acid
Violet acid
Molar mass 157.08 g/mol
Appearance Purple, dark blue or indigo solution
Melting point Decomposes
Boiling point Decomposes
Solubility Reacts with organic solvents
Safety data sheet None
Flash point Non-flammable
Related compounds
Related compounds
Nitrosylsulfuric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Sulfonitronic acid, blue acid or more commonly purple acid, is an unstable inorganic compound, which yields a purple solution in sulfuric acid, hence its name. It has not been isolated in the pure state and is only known in solution.[1]

The compound has been poorly characterized in literature. Most old sources regard it as a derivative of quadrivalent nitrogen, like nitrosylsulfuric acid (NOHSO4) or peroxylaminic acid (NO(SO2.OH)2). However, its exact formula and even the composition have been debated for a long time. According to more recent investigations it appears that the " acid" is either an oxide of nitrogen intermediate between NO and N2O3, or a compound of sulfuric acid with such an oxide. On account of this uncertainty the compound is frequently referred to merely as "purple acid" (also "blue acid " and "violet acid").[2][3] It should be kept in mind, that because the compound is very unstable and has only been made in solution (which thwarts attempts at simple analytical determination), many of the older theories about its structure were only reasoned from sometimes contradictory evidence and othertimes purely from analogy with known compounds based on their color.

The current (since the 1950s) consensus in the literature is that the purple acid is the salt of the cation N2O2+. The argument is based on the following:[4][5]

  • The idea of a molecular compound like some sulfonic acid in the spirit of Frémys salt is incompatible with the observations that reduction of NOClO4 in 80% HClO4 with Hg produces the same color.
  • The idea of a simple adduct of NO and H2SO4 has been attractive, because pressing NO into H2SO4 can form the purple compound, however, on closer inspection it turns out that NO can be oxidized fairly easily in such a solution, producing NOHSO4. It seems that NOHSO4 is essential for the formation.
  • Similarly adducts of NO with strong Lewis acids are known, such as with HCl, BF3 and SO3.
  • Raman studies of N2O3+ are in agreement with the proposed bonding.
  • Similar compounds have been obtained by pressing NO into NOAlCl4 and NOSbCl6 in liquid SO2 (N2O2AlCl4 can also be made in NOCl as solvent at high pressures), and both show the characteristic thermochromism of purple acid.



Purple solution is unstable and decomposes slowly, with formation of sulfuric acid, sulfur dioxide and nitrogen dioxide. When shaken with air or submitted to oxidation by chlorine, nitric acid or hydrogen peroxide, conversion into nitrosylsulfuric acid is effected, brown fumes being liberated.

Dilution with water also destroys the colored compound. If strongly cooled, the solution changes to an intense red, so that if a solution is too weak to possess a marked color at the ordinary temperature, the presence of the "purple acid" can easily be detected by cooling in a mixture of acetone and dry ice.

Some metal ions cause the solution to have a stronger color than it would have in their absence, and in some cases increase the stability. In the older texts this was interpreted as formation of salts of purple acid. A deep blue solution of the "copper salt" for example may be obtained by the reduction of nitrosylsulfuric acid in sulphuric acid by mercury in the presence of copper. It has been suggested that the color in the "brown ring" test for a nitrate is due to the formation of the ferrous salt of "purple acid," but this is improbable.[6]

The idea of salt formation is incompatible with the modern notion of purple acid as the cationic species N2O2+. The deep blue color of the "copper salt" has instead be interpreted as a complex like CuSO4 · NO or [CuNO]2+.[7][8] The optical spectra of these copper(II)-nitrogen monoxide complexes is very different from the spectrum of free purple acid, and the similarity in color seems to be coincidental. Therefore, when copper(II) is added to purple acid, the solution seems to serve purely as a donor of NO.


Purple acid is a blue or purple liquid, only stable in sulfuric acid, hence all of its properties are derived from said solution.


Purple acid is not sold and has to be made in situ.


Purple acid can be prepared by passing nitrogen dioxide with air into a saturated solution of sulfur dioxide in diluted sulfuric acid (1:1 by volume) at 0° C. Sulfur dioxide can be easily prepared from sodium or potassium metabisulfite, as well as sodium sulfite.[9]

It can also be formed by the addition of sodium bisulfite to a solution of nitrosylsulfuric acid, which can be produced by dissolving sodium nitrite in slightly diluted sulfuric acid. These methods depend on the reduction of the nitrosylsulfuric acid by sulfurous acid or sulfur dioxide. The reduction can also be effected by metals, e.g. mercury.[10]


  • Chemical demonstrations



Purple acid is corrosive and unstable.


Cannot be stored for long. It is only stable in sulfuric acid for a few days under the best conditions.


Purple acid can be safely neutralized by diluting it with water and neutralize the resulting acidic solution with a base.


  1. Gmelins Handbuch der anorganischen Chemie - Schwefel Teil B Lieferung 3, Verlag Chemie GmbH Weinheim Bergstr. 1963, pages 1630-1638
  2. http://sulphur.atomistry.com/sulphonitronic_acid.html
  3. https://web.archive.org/web/20190330040714/http://sulphur.atomistry.com/sulphonitronic_acid.html
  4. F. Seel, B. Ficke, L. Riehl, E. Völkl, Zeitschrift für Naturforschung B 1953, 80, 10, 607-608, https://doi.org/10.1515/znb-1953-1011
  5. Gmelins Handbuch der anorganischen Chemie - Schwefel Teil B Lieferung 3, Verlag Chemie GmbH Weinheim Bergstr. 1963, pages 1630-1638
  6. https://www.degruyter.com/document/doi/10.1515/znb-1953-1011/html
  7. N. Tsumori, Q. Xu, Bulletin of the Chemical Society of Japan 2002, 75(8), 1861–1862. 10.1246/bcsj.75.1861
  8. Gmelins Handbuch der anorganischen Chemie - Kupfer Teil B Lieferung 1, Verlag Chemie GmbH Weinheim Bergstr. 1958, page 579, see this post.
  9. https://www.docdroid.net/xsIwPiK/violet-acid-pdf
  10. Gmelins Handbuch der anorganischen Chemie - Schwefel Teil B Lieferung 3, Verlag Chemie GmbH Weinheim Bergstr. 1963, pages 1630-1638

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