Thiocyanic acid[1]
Skeletal formula of thiocyanic acid with the explicit hydrogen added
Spacefill model of thiocyanic acid
Preferred IUPAC name
Thiocyanic acid[3]
Other names
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
Appearance colorless, oily liquid
Odor pungent
Density 2.04 g/cm3
Melting point 5 °C (41 °F; 278 K)
Solubility soluble in ethanol, diethyl ether
log P 0.429
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 alkanenitriles
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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).[4] The iso- form tends to dominate with the material being about 95% isothiocyanic acid in the vapor phase.[5]

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

It is a moderately strong acid,[6] with a pKa of 1.1 at 20 °C and extrapolated to zero ionic strength.[7]

HSCN is predicted to have a triple bond between carbon and nitrogen. It has been observed spectroscopically but has not been isolated as a pure substance.[8]

The salts and esters of thiocyanic acid are known as thiocyanates. The salts are composed of the thiocyanate ion (SCN) and a suitable metal cation (e.g., potassium thiocyanate, KSCN). The esters of thiocyanic acid have the general structure R–SCN.

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.[9] HNCS acceptor properties are discussed in the ECW model.


  1. ^ Merck Index, 11th Edition, 9257.
  2. ^ Richter, Victor von; Spielmann, Percy E., trans. (1922). Organic Chemistry or Chemistry of the Carbon Compounds. Vol. 1. Philadelphia, Pennsylvania, U.S.A.: P. Blakiston's Son & Co. p. 466.
  3. ^ International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. p. 784. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
  4. ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
  5. ^ 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.
  6. ^ 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.
  7. ^ Martell, A. E.; Smith, R. M.; Motelaitis, R. J. (2001). NIST Database 46. Gaithersburg, MD: National Institute of Standards and Technology.
  8. ^ 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.
  9. ^ . 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