Chromotropic acid

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Chromotropic Acid Test


White crystalline material used as an analytical reagent for the detection of aldehydes. A solution of chromotropic acid dissolved in concentrated Sulfuric acid will turn purple in the presence of aldehydes. A test using chromotropic acid in a closed enviornment to detect the offgassing of volatile aldehydes was recommended in 1994 by Zhang, Thickett, and Greene.

Synonyms and Related Terms

chromotropic acid test; 4,5-dihydroxy-2,7-napthalenedisulfonic acid; 1,8-dihydroxynaphthalene-3,6-disulfonic acid


  • ThermoFisher: SDS

Physical and Chemical Properties

  • Soluble in water.
  • The fluorescence of chromotropic acid changes with pH (colorless below 3.5, and fluorescent blue above 4.5).
Composition C10H8O8S2
CAS 129-96-4 (Anhydrous)
Molecular Weight mol. wt. = 320.29

Resources and Citations

  • J.Zhang, D.Thickett, L.Greene, "Two Tests for the Detection of Volatile Organic Acids and Formaldehyde" JAIC 33(1), 1994, p. 47-53.
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 2298
  • CRC Handbook of Chemistry and Physics, Robert Weast (ed.), CRC Press, Boca Raton, Florida, v. 61, 1980 Comment: The fluorescence of chromotropic acid changes with pH. It is colorless below pH=3.5 and above 4.5, it fluoresces blue.

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