Thiocyanic acid[1]
Skeletal formula of thiocyanic acid with the explicit hydrogen added
Spacefill model of thiocyanic acid
  Carbon, C
  Sulfur, S
  Nitrogen, N
  Hydrogen, H
IUPAC name
Thiocyanic acid[4]
Other names
  • Hydrogen thiocyanate[2]
  • Sulfocyanic acid[3]
3D model (JSmol)
ECHA InfoCard 100.006.672 Edit this at Wikidata
EC Number
  • 207-337-4
MeSH thiocyanic+acid
  • InChI=1S/CHNS/c2-1-3/h3H checkY
  • SC#N
Molar mass 59.09 g·mol−1
  • Colourless liquid[5]
  • Colourless gas, autopolymerizing to white solid[2]
Odor Pungent
Density 2.04 g/cm3
Melting point
  • 5 °C (oligomers?)[5]
  • -110 °C (monomer?)[6]
Solubility Soluble in ethanol, diethyl ether
log P 0.429
Vapor pressure 4.73 mmHg (631 Pa)[7]
Acidity (pKa) 0.926
Basicity (pKb) 13.071
GHS labelling:
GHS07: Exclamation mark
H302, H312, H332, H412
P261, P264, P270, P271, P273, P280, P301+P312, P302+P352, P304+P312, P304+P340, P312, P322, P330, P363, P501
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Thiocyanic acid is a chemical compound with the formula HSCN and structure H−S−C≡N, which exists as a tautomer with isothiocyanic acid (H−N=C=S).[8] The isothiocyanic acid tautomer tends to dominate with the compound being about 95% isothiocyanic acid in the vapor phase.[9]

Tautomerism between thiocyanic acid (left) and isothiocyanic acid (right)

It is a moderately strong acid,[10] with a pKa of 1.1 at 20 °C and extrapolated to zero ionic strength.[11]

One of the thiocyanic acid tautomers, HSCN, is predicted to have a triple bond between carbon and nitrogen. Thiocyanic acid has been observed spectroscopically.[12]

The salts and esters of thiocyanic acid are known as thiocyanates. The salts are composed of the thiocyanate ion ([SCN]) and a suitable cation (e.g., potassium thiocyanate, KSCN). The esters of thiocyanic acid have the general structure R−S−C≡N, where R stands for an organyl group.

Isothiocyanic acid, HNCS, is a Lewis acid whose free energy, enthalpy and entropy changes for its 1:1 association with a variety of Lewis bases in carbon tetrachloride solution at 25 °C have been reported.[13] HNCS acceptor properties are discussed in the ECW model. The salts are composed of the thiocyanate ion ([SCN]) and a suitable cation (e.g., ammonium thiocyanate, [NH4]+[SCN]). Isothiocyanic acid forms isothiocyanates R−N=C=S, where R stands for an organyl group.


  1. ^ Merck Index, 11th Edition, 9257.
  2. ^ a b "Thiocyanic acid". The Merck Index. Royal Society of Chemistry.
  3. ^ von Richter, Victor (1922). Organic Chemistry or Chemistry of the Carbon Compounds. Vol. 1. Translated by Spielmann, Percy E. Philadelphia: P. Blakiston's Son & Co. p. 466.
  4. ^ "Thiocyanic acid" entry in PubChem (database).
  5. ^ a b ILO and WHO staff. "Thiocyanic acid" safety card. European Commission
  6. ^ Birckenbach, Lothar (1942). Forschungen und Fortschritte. 18: 232–3 ((cite journal)): Missing or empty |title= (help). As cited in CAS Common Chemistry.
  7. ^ Brown, Jay A. (ed.; 2024), "Thiocyanic Acid" in Haz-Map (database). Engineered IT.
  8. ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
  9. ^ Beard, C. I.; Dailey, B. P. (1950). "The Structure and Dipole Moment of Isothiocyanic Acid" (PDF). The Journal of Chemical Physics. 18 (11): 1437. Bibcode:1950JChPh..18.1437B. doi:10.1063/1.1747507. hdl:1721.1/4934.
  10. ^ Munegumi, Toratane (23 January 2013). "Where is the Border Line between Strong Acids and Weak Acids?". World Journal of Chemical Education. 1 (1): 12–16.
  11. ^ Martell, A. E.; Smith, R. M.; Motelaitis, R. J. (2001). NIST Database 46. Gaithersburg, MD: National Institute of Standards and Technology.
  12. ^ Wierzejewska, M.; Mielke, Z. (2001). "Photolysis of Isothiocyanic Acid HNCS in Low-Temperature Matrices. Infrared Detection of HSCN and HSNC Isomers". Chemical Physics Letters. 349 (3–4): 227–234. Bibcode:2001CPL...349..227W. doi:10.1016/S0009-2614(01)01180-0.
  13. ^ . Barakat, T. M.; Nelson, J.; Nelson, S. M.; Pullin, A. D. E. (1969.) “Spectra and hydrogen-bonding of characteristics of thiocyanic acid. Part 4.—Association with weak proton acceptors”. Trans. Faraday Soc., 1969,65, 41-51