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 YesY
3DMet B00344
ChEBI CHEBI:29200 YesY
ChemSpider 760 YesY
EC Number 207-337-4
Jmol 3D model Interactive image
KEGG C01755 N
MeSH thiocyanic+acid
PubChem 781
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
Vapor pressure {{{value}}}
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 YesYN ?)
Infobox references

Thiocyanic acid is a chemical compound with the formula HSCN which exists as a tautomer with isothiocyanic acid (HNCS).[3] The iso- form tends to dominate with the material being about 95% isothiocyanic acid in the vapor phase.[4]

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.[5]

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.[6]

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. "thiocyanic acid (CHEBI:29200)". Chemical Entities of Biological Interest. USA: European Bioinformatics Institute. 18 October 2009. Main. Retrieved 5 June 2012. 
  3. Holleman, A. F.; Wiberg, E. Inorganic Chemistry Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  4. Beard, C. I.; Dailey, B. P. (1950). "The Structure and Dipole Moment of Isothiocyanic Acid". The Journal of Chemical Physics. 18 (11): 1437. doi:10.1063/1.1747507. 
  5. Martell, A. E.; Smith, R. M.; Motelaitis, R. J. NIST Database 46 National Institute of Standards and Technology: Gaithersburg, MD, 2001.
  6. 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.