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.

The compound has been poorly characterized in literature. Most 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 is still debated. 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").[1][2]



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.

Certain of the salts in solution have a stronger color than the free acid, and in some cases are more stable. A deep blue solution of the copper salt 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.[3]


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.[4]

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.[5]


  • 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. http://sulphur.atomistry.com/sulphonitronic_acid.html
  2. https://web.archive.org/web/20190330040714/http://sulphur.atomistry.com/sulphonitronic_acid.html
  3. https://www.degruyter.com/document/doi/10.1515/znb-1953-1011/html
  4. https://www.docdroid.net/xsIwPiK/violet-acid-pdf
  5. Gmelins Handbuch der anorganischen Chemie - Schwefel Teil B Lieferung 3, Verlag Chemie GmbH Weinheim Bergstr. 1963, pages 1630-1638

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