Thiocyanic acid

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Thiocyanic acid[1]
Skeletal formula of thiocyanic acid
Skeletal formula of thiocyanic acid with the explicit hydrogen added
Spacefill model of thiocyanic acid
IUPAC name
Other names
  • 463-56-9 ☑Y
3D model (JSmol)
  • Interactive image
  • B00344
  • CHEBI:29200 ☑Y
  • ChEMBL84336 ☑Y
  • 760 ☑Y
ECHA InfoCard 100.006.672
EC Number
  • 207-337-4
  • C01755 ☒N
MeSH thiocyanic+acid
PubChem CID
  • 781
  • DTXSID7047221
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
Harmful Xn
R-phrases (outdated) R20/21/22, R32, R52/53
S-phrases (outdated) (S2), S13
Related compounds
Related alkanenitriles
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is ☑Y☒N ?)
Infobox references

Thiocyanic acid is a chemical compound with the formula HSCN which exists as a tautomer with isothiocyanic acid (HNCS).[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)

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

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.[7]

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.


  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. ^ "thiocyanic acid (CHEBI:29200)". Chemical Entities of Biological Interest. USA: European Bioinformatics Institute. 18 October 2009. Main. Retrieved 5 June 2012.
  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. doi:10.1063/1.1747507.
  6. ^ Martell, A. E.; Smith, R. M.; Motelaitis, R. J. (2001). NIST Database 46. Gaithersburg, MD: National Institute of Standards and Technology.
  7. ^ 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: 227–234. doi:10.1016/S0009-2614(01)01180-0.
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