Chlorosulfonic Acid CSA · Chlorosulfuric Acid · CAS 7790-94-5
Chelora Chemicals is a supplier of Chlorosulfonic Acid, a powerful sulfonating and chlorosulfonating agent used across dye-intermediate, pharmaceutical, surfactant and agrochemical manufacturing. Every consignment leaves with a batch-specific COA and the current SDS.
Also known as: CSA, Chlorosulfuric Acid, Sulfuryl Chloride Hydroxide, Chlorosulphonic Acid
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Price, lead time and a sample COA for Chlorosulfonic Acid, usually within one working day. We ship worldwide.
Request a Quote Email the DeskDocumentation
- Batch-specific COA (assay, colour, free SO₃/HCl)
- Safety Data Sheet (SDS)
- UN 1754 dangerous-goods transport documents
- Third-party inspection on request
- Certificate of origin
What is Chlorosulfonic Acid?
Chlorosulfonic Acid (CSA) is a mixed acid chloride-acid of sulfuric acid, formula HSO₃Cl, combining the reactivity of a sulfonic acid with that of an acid chloride. It is supplied as a colourless to pale brown liquid that fumes strongly in moist air due to its violent reaction with atmospheric moisture.
Its value as a process chemical comes from this dual reactivity: it sulfonates aromatic rings directly and, where the chlorosulfonyl group is retained, delivers a sulfonyl chloride functionality in one step — a combination that makes it central to dye-intermediate, surfactant and pharmaceutical sulfonyl chemistry.
Commercially it is produced by reacting hydrogen chloride with sulfur trioxide, typically carried out in sulfuric acid solution under controlled conditions.
Handling requires rigorous moisture exclusion and specialised corrosion-resistant equipment — CSA reacts violently with water to release sulfuric acid and hydrogen chloride fumes, and standard precautions for a severely corrosive, fuming liquid apply throughout: sealed transfer systems, acid-resistant PPE, and strictly moisture-free storage and packaging.
Why Chlorosulfonic Acid specifically
- Dual sulfonation and chlorination reactivity. CSA can sulfonate, chlorosulfonate, or dehydrate/chlorinate substrates depending on stoichiometry and conditions.
- Direct sulfonyl chloride formation. A one-step route to aryl and alkyl sulfonyl chlorides, intermediates for dyes, pharmaceuticals and agrochemicals.
- Surfactant manufacture. Used to produce alkylbenzene sulfonates and related sulfonated surfactants for detergent formulation.
- Distinct from oleum and sulfuric acid. CSA's chlorosulfonyl reactivity is not matched by oleum or concentrated sulfuric acid, which is why it is specified where a sulfonyl chloride or chlorosulfonation step is required.
How Chlorosulfonic Acid is Manufactured
Chlorosulfonic Acid is produced industrially by reacting hydrogen chloride gas with sulfur trioxide, most commonly with the SO₃ dissolved or generated in sulfuric acid.
- Gas-phase reaction. HCl and SO₃ are combined under controlled temperature and pressure: HCl + SO₃ → ClSO₃H.
- Condensation and collection. The product is condensed from the reaction stream in moisture-free, corrosion-resistant equipment.
- Purification. The crude acid is purified to the required assay, with free SO₃ and HCl content controlled to specification.
- Quality release and packing. Assay, colour and free-acid content are confirmed before the batch is packed under strictly anhydrous, nitrogen-blanketed conditions.
Typical Commercial Specification
Figures below are representative of commercial-grade Chlorosulfonic Acid. Tell us your downstream route and the grade you need, and we will advise on the specification to order against.
| Parameter | Typical Value |
|---|---|
| Appearance | Colourless to pale brown, fuming liquid |
| ClSO₃H content | 99% min by assay |
| Free SO₃ | Low, to grade |
| Free HCl/H₂SO₄ | Low, to grade |
Physical & Chemical Properties
| Property | Value |
|---|---|
| Chemical formula | HSO₃Cl (ClSO₃H) |
| Molecular weight | 116.52 g/mol |
| Appearance | Colourless to pale brown fuming liquid |
| Melting point | About -80 °C |
| Boiling point | About 151 – 158 °C (with decomposition) |
| Density | About 1.75 g/cm³ |
| Reactivity with water | Violent; forms H₂SO₄ and HCl fumes |
| Incompatibilities | Water, moisture, bases, most organic materials |
Behaviour in the process
CSA's dual reactivity is what dye, pharma and surfactant producers are buying:
- Electrophilic sulfonation. Reacts with aromatic substrates to install sulfonic acid or sulfonyl chloride groups directly.
- Moisture sensitivity. Reacts violently and exothermically with water; all equipment and packaging must be rigorously dry.
- Fuming and corrosivity. Fumes in moist air and is severely corrosive to metals, skin and tissue on contact.
Chlorosulfonic Acid and Related Sulfonating Agents
Choosing between sulfonating and chlorosulfonating agents comes down to whether the target synthesis needs a sulfonic acid, a sulfonyl chloride, or simple acid strength.
| Material | Key reactivity | When it is the right choice |
|---|---|---|
| Chlorosulfonic Acid | Sulfonation + chlorosulfonation | Routes needing a sulfonyl chloride group directly, or a strong sulfonating/dehydrating agent |
| Oleum (Fuming Sulfuric Acid) | Sulfonation only | Routes needing a sulfonic acid without the chlorosulfonyl functionality |
| Sulfuric Acid (Concentrated) | Sulfonation, weaker | Milder sulfonation or general acid-catalysed process steps |
| Thionyl Chloride | Chlorination of -OH/-COOH | Routes needing acid or alcohol chlorination rather than ring sulfonation |
Stated plainly: a synthesis that needs the chlorosulfonyl group installed directly needs this specific reagent — oleum and sulfuric acid cannot deliver that functionality on their own.
Industries We Supply Chlorosulfonic Acid To
The same reagent feeds a dye-intermediate manufacturer, a pharmaceutical sulfonamide producer and a surfactant formulator. What changes is the downstream chemistry and the documentation each requires.
Where Chlorosulfonic Acid is Used
In downstream functionalisation
CSA's chlorosulfonyl group is carried through to the target sulfonamide, sulfonate ester or sulfonic acid, depending on the nucleophile used in the next step. This single-reagent route to sulfonyl chlorides is often exactly why a process is designed around CSA rather than a two-step sulfonation/chlorination sequence.
Need a specific purity grade?
Tell us the purity grade you need, your downstream chemistry, the packing format you can receive and the annual quantity. We will come back with a price, a lead time and a sample COA.
Quality Control & Testing
Every lot is released against a batch-specific Certificate of Analysis, with free SO₃ and free HCl/H₂SO₄ content reported alongside the main assay because both affect downstream sulfonation selectivity and yield.
- ClSO₃H content — by assay, the primary grade measure.
- Free SO₃ — reported because excess SO₃ affects reaction stoichiometry and by-product formation.
- Colour — reported because discolouration can indicate moisture ingress or degradation.
Common Process Problems & Likely Causes
| Symptom | Likely cause | What to check |
|---|---|---|
| Heavy fuming or discolouration on receipt | Moisture ingress during storage or transit | Container seal integrity; colour figure on the COA |
| Lower sulfonation/chlorosulfonation yield | Free SO₃/HCl content out of range, or hydrolysed material | Assay and free-acid figures on the COA |
| Equipment corrosion faster than expected | Material-of-construction mismatch or moisture contamination | Storage/transfer system compatibility; moisture control procedures |
Packaging, Storage & Shipping
- Standard packing. Glass-lined or specially lined steel drums/ISO tanks, typically 180 – 250 kg net in drums, with ISO tank bulk formats on contract supply.
- Moisture exclusion. Packed and sealed under strictly anhydrous, nitrogen-blanketed conditions; any moisture ingress triggers a violent reaction.
- Temperature control. Store in a cool, dry, well-ventilated area away from direct sun and heat.
- Material compatibility. Requires corrosion-resistant equipment; avoid contact with water, bases and most organic materials.
- Transport classification. Commonly shipped under UN 1754, Class 8, Packing Group I — confirm the correct entry and packing group against your own current SDS before shipping.
Hazards & Safe Handling
Chlorosulfonic Acid causes severe skin burns and eye damage, is corrosive to the respiratory tract, and reacts violently with water. It is a severe inhalation and contact hazard and must be handled only by trained personnel with the appropriate engineering controls.
- Skin and eye contact. Acid-resistant gloves, face shield and full protective clothing are mandatory; flush immediately with large volumes of water and seek urgent medical attention.
- Inhalation. Fumes are corrosive to the respiratory tract; use closed-system transfer and local exhaust ventilation, never open handling.
- PPE. Acid-resistant suit, face shield, chemical-resistant gloves and respiratory protection appropriate to the exposure scenario.
- Spill response. Do not use water directly on a spill without appropriate controls; follow your SDS spill-response procedure and have qualified responders handle any release.
Regulatory & Compliance Notes
Chlorosulfonic Acid is a long-established, widely registered industrial process chemical. Because obligations differ by territory and use, the notes below are orientation rather than legal advice.
- GHS classification. Classified as causing severe skin burns and eye damage, corrosive to the respiratory tract, and reacting violently with water (Signal word: Danger); the SDS carries the full hazard statements for your grade.
- REACH and chemical inventories. Registered under EU REACH and listed on major chemical inventories.
- Dangerous goods transport. Commonly shipped under UN 1754, Class 8, Packing Group I. Confirm the correct entry and packing group with your logistics/EHS team against your current SDS before shipping.
- HS code. Commonly declared under the inorganic acid/acid-chloride heading; confirm the exact tariff line with your customs broker for your jurisdiction.
We Supply Chlorosulfonic Acid Worldwide
Chelora Chemicals is a supplier and export partner, not a single-plant trader. We ship Chlorosulfonic Acid from India and from partner sources to customers across Asia, Europe, the Middle East, Africa, and North and South America, handling the export documentation and freight so the material arrives ready to use.
- Incoterms. EXW, FOB, CIF, CFR and DAP quoted to suit your purchasing terms and freight arrangements.
- Ports. Regular shipments through Nhava Sheva (JNPT) and Mundra, with other ports arranged as routing allows.
- Documentation. Commercial invoice, packing list, certificate of origin, COA and SDS issued as a complete set.
- Flexible volumes. From drum quantities up to ISO-tank bulk shipments on a supply contract.
Buying Chlorosulfonic Acid from Chelora
We are an independent sourcing and supply partner. For this product that means settling three things at enquiry: the purity grade your application needs, the packing format, and the quantity you are buying.
- Documentation on every lot. Batch-specific COA with assay and free-acid breakdown, plus the current SDS.
- Analysis before commitment. Representative analysis shared ahead of a commercial order so your lab can qualify the material.
- Quantities. Drum quantities through to ISO-tank bulk shipments on contract supply.
- Inspection. Independent third-party inspection available at loading when the contract requires it.
- Related process chemicals. Oleum, concentrated sulfuric acid and related sulfonating agents alongside this product from one supplier.
How ordering works
- Send the enquiry. Purity, quantity, destination port and the Incoterm you buy on.
- We quote. Price, lead time, packing format and a sample COA, normally within one working day.
- Qualify the supply. For first orders or dye/pharma-grade applications, we share the analysis so your technical team can sign off before you commit.
- Confirm the order. Proforma invoice issued; payment terms agreed against the volume and the relationship.
- Ship and document. Packed, labelled and loaded with the full export document set, and tracked through to arrival.
Chlorosulfonic Acid — Common Questions
The questions our technical desk is asked most often about Chlorosulfonic Acid. If yours isn't here, call or email and you will reach someone who knows the chemistry.
What is the CAS number of Chlorosulfonic Acid?
The CAS number for Chlorosulfonic Acid (Chlorosulfuric Acid) is 7790-94-5, and its EC number is 232-234-6. Its molecular formula is HSO₃Cl with a molecular weight of 116.52 g/mol.
What is Chlorosulfonic Acid used for?
It is used as a sulfonating and chlorosulfonating agent to produce sulfonyl chlorides, sulfonic acids and sulfonamide intermediates for dyes, pharmaceuticals, surfactants and agrochemicals.
Do you supply Chlorosulfonic Acid worldwide?
Yes. Chelora Chemicals supplies and exports Chlorosulfonic Acid worldwide — across Asia Pacific, Europe, the Middle East, Africa and the Americas, as well as throughout India. Every consignment carries the COA and SDS.
How does it differ from oleum or sulfuric acid?
Chlorosulfonic Acid combines sulfonating reactivity with a chlorosulfonyl group, letting it install a sulfonyl chloride directly. Oleum and sulfuric acid can sulfonate but cannot deliver that chlorosulfonyl functionality on their own.
How is Chlorosulfonic Acid manufactured?
By reacting hydrogen chloride gas with sulfur trioxide, typically with the SO₃ dissolved or generated in sulfuric acid, under controlled, moisture-free conditions.
Is Chlorosulfonic Acid hazardous?
Yes — it causes severe skin burns and eye damage, is corrosive to the respiratory tract, and reacts violently with water. Refer to the current SDS for your grade for the complete hazard classification.
What is the UN number and transport classification?
Chlorosulfonic Acid is commonly shipped under UN 1754, Class 8 (Corrosive), Packing Group I. Confirm the correct entry and packing group against your own current SDS before relying on it for shipping documentation.
What are the melting point, boiling point and density?
Typical figures are a melting point around -80 °C, a boiling point around 151–158 °C (with decomposition), and a density around 1.75 g/cm³. Confirm against your own supplier's COA for batch-specific figures.
How should Chlorosulfonic Acid be stored?
In sealed, corrosion-resistant containers under strictly anhydrous, nitrogen-blanketed conditions, cool, dry and away from any source of moisture. It reacts violently with water, so container integrity is critical.
What purity grades do you offer?
Standard commercial grade is 99% minimum by assay, with free SO₃ and free HCl/H₂SO₄ content reported on the COA.
What packing sizes and order quantities are available?
Specially lined drums, typically 180–250 kg net, with ISO tank bulk formats available on contract supply. Orders run from drum quantities up to repeat bulk loads.
Ready to source Chlorosulfonic Acid?
Send us your purity requirement and annual volume. You will get a price, a realistic lead time and a sample COA from someone who understands what the material has to do in your process — delivered anywhere in the world.