Quick Search
Thiocyanic acid is a reactive sulfur-containing pseudohalogen acid used primarily in analytical chemistry, coordination chemistry, specialized metal-separation processes, and controlled organic synthesis.
Because the free acid readily undergoes tautomerization, polymerization, and thermal decomposition, it is normally handled as a freshly prepared solution or generated in situ from a thiocyanate salt.
CHEMICAL IDENTITY
Product Name: Thiocyanic Acid
CAS Number: 463-56-9
EC Number: 207-337-4
Molecular Formula: HSCN
Elemental Formula: CHNS
Molar Mass: 59.09 g/mol
Structural Formula: H–S–C≡N
Conjugate Base: Thiocyanate ion, SCN−
SMILES: SC#N
InChIKey: ZMZDMBWJUHKJPS-UHFFFAOYSA-N
Chemical Family: Inorganic pseudohalogen acid
COMMON NAMES AND SYNONYMS
Synonyms:
Hydrogen thiocyanate, hydrogen thiocyanic acid, hydrogen rhodanide, rhodanic acid, sulfocyanic acid, sulphocyanic acid, HSCN, HS–C≡N, nitridosulfanidocarbon, nitridosulphanidocarbon, thiocyansäure, rhodanwasserstoff, and acide thiocyanique.
TAUTOMERISM AND PRODUCT DISTINCTIONS
Thiocyanic acid has the connectivity H–S–C≡N and exists in equilibrium with isothiocyanic acid, H–N=C=S.
The vapor-phase equilibrium strongly favors the isothiocyanic acid tautomer.
Isothiocyanic acid has CAS number 3129-90-6 and should not be treated as a simple synonym for CAS 463-56-9 in specifications or regulatory records.
Both tautomers produce the thiocyanate ion after deprotonation, but their structures and reaction pathways are not identical.
Sodium thiocyanate, potassium thiocyanate, ammonium thiocyanate, and metal thiocyanates are separate salts rather than alternative names for thiocyanic acid.
Organic thiocyanates have the general structure R–S–C≡N, while organic isothiocyanates have the general structure R–N=C=S.
Thiocyanogen, hypothiocyanous acid, thiocyanuric acid, and hydrocyanic acid are also chemically distinct substances.
PHYSICAL AND CHEMICAL PROPERTIES
Appearance:
Colorless liquid in the pure state and colorless solution when freshly prepared without colored impurities.
Melting Point:
Approximately 5°C.
Water Solubility:
Readily soluble in water.
Relative Vapor Density:
Approximately 2.0, with air equal to 1.
Acid Dissociation Constant:
The aqueous pKa is approximately 0.9 to 1.1 at 20°C.
Partition Coefficient:
Estimated log Pow is approximately 0.58.
Flammability:
Pure thiocyanic acid is noncombustible.
Solvent-based preparations can inherit the flammability characteristics of their carrier solvent.
Thermal Behavior:
Heating promotes tautomerization, polymerization, and decomposition with formation of toxic fumes.
Boiling Behavior:
A dependable normal boiling point is not assigned because decomposition and polymerization occur before conventional boiling behavior can be used as a practical specification.
Vapor Behavior:
Vapor is heavier than air and can accumulate in low or poorly ventilated areas.
FUNCTIONAL CHARACTERISTICS
Thiocyanic acid dissociates substantially in water to produce hydrogen ions and thiocyanate ions.
The resulting thiocyanate ion is an ambidentate ligand that can coordinate to metals through either sulfur or nitrogen.
This dual coordination behavior supports the formation of metal-thiocyanate complexes with different structures, colors, solubilities, and extraction characteristics.
Iron(III) forms intensely red thiocyanato complexes that are widely used in colorimetric and spectrophotometric chemistry.
Cobalt and numerous other transition metals also form characteristic thiocyanate complexes under controlled conditions.
The neutral acid form can partition into selected organic media more readily than an ionic thiocyanate salt.
This behavior is important in solvent extraction, metal separation, and phase-transfer processes.
Thiocyanic acid also provides a reactive source of thiocyanate functionality for controlled synthetic reactions.
PRODUCTION AND COMMERCIAL FORM
Thiocyanic acid is produced by controlled acidification of a thiocyanate salt with a strong, non-oxidizing acid.
A classical preparation uses barium thiocyanate and sulfuric acid.
Ba(SCN)2 + H2SO4 → 2 HSCN + BaSO4↓
Precipitation of barium sulfate separates the sulfate component and leaves thiocyanic acid in the liquid phase.
Sodium, potassium, or ammonium thiocyanate can also serve as starting materials when the accompanying ions and mineral-acid residues are compatible with the intended process.
Low-temperature operation, controlled addition, efficient mixing, and prompt use reduce unwanted polymerization and decomposition.
The free acid may be retained in an aqueous phase or transferred into a compatible alcohol, ketone, ether, or other validated organic medium.
Commercial procurement is normally based on a declared solution concentration and solvent system rather than prolonged storage of neat thiocyanic acid.
Fresh preparation and in-situ generation are often preferred for processes requiring maximum reactivity and minimum degradation products.
APPLICATIONS
Analytical Chemistry
Thiocyanic acid provides thiocyanate in strongly acidic analytical media.
The intense color produced by iron(III)-thiocyanate complex formation supports photometric determination of iron and studies of complex-formation equilibria.
Low iron content is especially important when the material is used for trace analysis because iron contamination can create a visible red background.
Acidic thiocyanate media are also used in the extraction, identification, and spectrophotometric determination of selected transition and high-valent metals.
Coordination Chemistry
The SCN− ion functions as a versatile ligand in metal-complex synthesis.
Coordination through sulfur or nitrogen depends on the metal center, oxidation state, solvent, counterions, concentration, and reaction conditions.
Thiocyanic acid is useful when a protonated thiocyanate source is required without introducing large quantities of alkali-metal or ammonium ions.
Metal Separation and Solvent Extraction
Acidic thiocyanate systems are used in specialized extraction and separation chemistry.
Zirconium and hafnium separation is an established example in which metal-thiocyanate complexes distribute differently between aqueous and ketone-based phases.
Related systems are used or investigated for selected high-valent and transition metals when differences in complex stability and phase distribution provide useful selectivity.
Acidic thiocyanate media have also been investigated in hydrometallurgical systems for precious-metal dissolution and recovery.
Such processes require close control of redox conditions because uncontrolled oxidation can consume thiocyanate and generate undesirable sulfur, nitrogen, or cyanide-containing decomposition products.
Organic Synthesis
Thiocyanic acid can serve as an acidic source of thiocyanate functionality in the preparation of selected organic thiocyanates and related sulfur-nitrogen intermediates.
It can also be used to prepare amine thiocyanate salts and other compounds requiring both protonation and thiocyanate introduction.
The instability of isolated HSCN makes in-situ formation from a thiocyanate salt the preferred approach for many synthetic operations.
Reaction selectivity must distinguish between thiocyanate products containing R–S–C≡N and isothiocyanate products containing R–N=C=S.
Research and Spectroscopy
Thiocyanic acid is used in studies of proton transfer, tautomerism, acid dissociation, hydrogen bonding, and pseudohalogen chemistry.
Low-temperature and gas-phase investigations examine the equilibrium and structural differences between HSCN and HNCS.
The molecule is also relevant to atmospheric, combustion, and interstellar chemical research involving small sulfur- and nitrogen-containing species.
APPLICATION BOUNDARY
Many high-volume industrial applications attributed broadly to “thiocyanate” belong specifically to sodium thiocyanate, potassium thiocyanate, ammonium thiocyanate, or metal thiocyanates.
Uses of those stable salts in textiles, photographic processing, construction formulations, oilfield chemistry, electroplating, and other industries should not automatically be assigned to free thiocyanic acid.
The free acid is a specialized reactive intermediate rather than a direct substitute for every thiocyanate salt.
GRADE SELECTION
Aqueous Process Solution:
Selected for acidic metal-complexation, extraction, and process-development work.
Important parameters include HSCN concentration, total thiocyanate, free acidity, residual mineral acid, metal content, clarity, color, and preparation date.
Organic Process Solution:
Selected for solvent extraction or synthesis requiring the acid in a compatible organic carrier.
Important parameters include solvent identity, HSCN concentration, water content, acidity, nonvolatile residue, color, and degradation products.
Analytical Grade:
Selected for spectrophotometric analysis and trace-metal work.
Important parameters include very low iron and transition-metal content, low blank absorbance, controlled chloride and sulfate, known concentration, and documented solution age.
Synthesis Grade:
Selected for thiocyanation and intermediate preparation.
Important parameters include total thiocyanate, free acid, water, oxidizing contaminants, residual starting ions, and polymerization products.
Thiocyanic acid is not conventionally specified as a food, cosmetic, consumer, or pharmacopeial ingredient.
FORMULATION AND PROCESS CONSIDERATIONS
Temperature control is central to successful use because warming accelerates decomposition and polymerization.
Concentration, acidity, solvent composition, and residence time should be designed together rather than treated as independent variables.
The process should minimize unnecessary storage between generation and consumption.
Strong oxidizing conditions must be excluded unless decomposition chemistry is an intentional and fully engineered part of the process.
Strong bases can produce rapid neutralization, heat release, and hazardous secondary reactions.
Excess mineral acid can change phase distribution, reaction selectivity, corrosion behavior, and decomposition pathways.
Total thiocyanate and free acidity should be monitored separately because an acidity measurement alone does not establish the HSCN concentration.
Trace iron can create red coloration and interfere with optical measurements.
Equipment materials, seals, pumps, transfer lines, and sampling components must be compatible with the complete formulation rather than only with dilute aqueous acid.
Closed transfer and local exhaust ventilation reduce occupational exposure and help contain decomposition vapors.
QUALITY CONTROL
Identity:
Identity can be evaluated using infrared or Raman spectroscopy and confirmed through determination of thiocyanate after controlled dilution.
Total Thiocyanate:
Ion chromatography, argentometric titration, or a validated spectrophotometric method can be used.
Free Acidity:
Standardized acid-base titration can determine total titratable acidity.
Paired free-acidity and total-thiocyanate results help distinguish HSCN from residual mineral acid.
Concentration:
Concentration should be reported using an unambiguous basis such as weight percent, molarity, or grams per liter.
Metal Impurities:
Iron and other relevant metals can be measured by plasma-based elemental analysis.
Anion Impurities:
Sulfate, chloride, nitrate, and other process-related anions can be measured by ion chromatography.
Water:
Water content in organic solutions can be determined by a validated moisture method.
Appearance:
Fresh material should meet the agreed requirements for color, clarity, and absence of suspended polymeric material.
Nonvolatile Residue:
Organic solutions can be evaluated for residual salts, polymerization products, and other nonvolatile matter.
DOCUMENTATION
A procurement specification should identify the product as thiocyanic acid, CAS 463-56-9, and state the supplied solvent and concentration basis.
The certificate of analysis should include lot identification, preparation or production date, HSCN or total-thiocyanate concentration, free acidity, solvent identity, appearance, and agreed impurity limits.
The safety data sheet should correspond to the exact concentration and carrier solvent.
Technical documentation should define storage conditions, handling controls, compatibility considerations, and the assigned shelf-life or retest period.
SAFETY AND REGULATORY CONSIDERATIONS
Thiocyanic acid is harmful by ingestion and can present hazards through inhalation and skin exposure.
Concentrated solutions can cause serious irritation or acid injury to the eyes, skin, respiratory tract, and gastrointestinal tract.
Repeated or prolonged systemic exposure to thiocyanate can affect thyroid function and may contribute to hypothyroidism.
Heating causes decomposition and formation of toxic, irritating fumes.
Violent reactions with strong oxidants or strong bases can generate hazardous gases, including hydrogen cyanide.
Pure thiocyanic acid is noncombustible, but fire exposure can accelerate decomposition.
Preparations containing combustible organic solvents require the fire controls assigned to the carrier solvent.
Vapors are heavier than air, making low-level extraction important in enclosed areas.
Releases must be kept away from drains, surface water, and soil because thiocyanate-containing material can harm aquatic organisms and produce persistent environmental effects.
The applicable transport description and packaging group are determined by the declared concentration, solvent, acidity, and complete hazard profile of the supplied formulation.
PERSONAL PROTECTIVE EQUIPMENT
Eye Protection:
Use chemical splash goggles and a face shield where splashing or pressurized transfer is possible.
Hand Protection:
Use chemical-resistant gloves selected from documented compatibility data for the complete formulation.
Body Protection:
Use acid-resistant protective clothing and closed footwear appropriate to the transfer scale.
Respiratory Protection:
Use closed handling and local exhaust as the primary exposure controls.
Respiratory protection must be selected within a formal respiratory-protection program.
Unknown concentrations, decomposition incidents, and major spills require positive-pressure supplied-air protection.
FIRST AID
Inhalation:
Move the affected person to fresh air, keep the person at rest, and obtain urgent medical attention.
Skin Contact:
Remove contaminated clothing and rinse the skin continuously with water for at least 15 minutes.
Eye Contact:
Rinse cautiously with clean water for at least 15 minutes, remove contact lenses when easy to do, and obtain immediate medical attention.
Ingestion:
Rinse the mouth, do not induce vomiting, and contact emergency medical services or a poison center immediately.
Medical Evaluation:
Assessment should address acid injury, respiratory effects, neurological and cardiovascular symptoms, acid-base status, and possible hydrogen cyanide exposure following decomposition or incompatible reactions.
Thyroid evaluation may be appropriate after significant or repeated thiocyanate exposure.
HANDLING AND STORAGE
Handle thiocyanic acid only in controlled industrial or laboratory environments with trained personnel.
Use a closed system or effective local exhaust ventilation, including extraction near floor level.
Avoid heating, aerosol formation, unnecessary headspace, and prolonged residence in transfer equipment.
Store in a cool, dark, temperature-controlled, and well-ventilated location.
Keep containers tightly closed and protect them from physical damage and external heat.
Segregate the product from strong oxidants, strong bases, reactive mineral acids, food, and animal feed.
Use small, application-sized packages when practical to reduce ageing and repeated container opening.
Do not retain visibly discolored, pressurized, precipitated, or polymerized material in routine service.
SPILL RESPONSE
Restrict access, ventilate the area at low level, and eliminate all incompatible materials.
Do not use oxidizing absorbents or alkaline neutralization methods without an engineered procedure.
Collect liquid with compatible inert absorbent and transfer it into suitable covered waste containers.
Prevent the material from entering drains, waterways, confined spaces, or soil.
Unknown vapor concentrations or signs of decomposition require evacuation and specialist hazardous-material response.
PACKAGING AND PROCUREMENT
Packaging materials must be selected for compatibility with the specified HSCN concentration, carrier solvent, temperature, and intended storage period.
Containers should incorporate secure closures, secondary containment, appropriate headspace management, and protection from heat and light.
The purchasing description should state the required solution concentration, solvent, free-acidity range, total-thiocyanate content, impurity limits, analytical grade, package size, intended application, and delivery destination.
For analytical work, iron background and optical blank requirements should be included.
For extraction processes, solvent identity, water content, mineral-acid residue, and metal-impurity limits should be defined.
For organic synthesis, water, oxidizing contaminants, residual salts, and polymerization products should be controlled.
CONTACT ATAMAN KIMYA
Ataman Kimya supports technical evaluation, specification development, sourcing, documentation, and packaging selection for thiocyanic acid and related thiocyanate products.
Phone:
+90 216 577 10 10
Email:
info@atamankimya.com
