Oleum Fuming Sulfuric Acid · CAS 8014-95-7
Chelora Chemicals is a supplier of Oleum, the fuming grade of sulfuric acid used as a sulfonating and nitrating agent and as a feedstock for sulfonated dye, detergent and agrochemical intermediates. Supplied to your specified free-SO₃ strength, every consignment leaves with a batch-specific COA and the current SDS.
Also known as: Fuming Sulfuric Acid, Disulfuric Acid, Pyrosulfuric Acid
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Price, lead time and a sample COA for Oleum at the free-SO₃ strength you need, usually within one working day. We ship worldwide.
Request a Quote Email the DeskDocumentation
- Batch-specific COA (free SO₃, total SO₃, Fe content)
- Safety Data Sheet (SDS)
- UN 1831 dangerous-goods transport documents
- Third-party inspection on request
- Certificate of origin
What is Oleum?
Oleum, also called fuming sulfuric acid, is a solution of sulfur trioxide (SO₃) dissolved in concentrated sulfuric acid. It is supplied as a colourless to off-white fuming liquid with a strong, pungent odour, and is miscible with water — reacting violently with it, which governs how it must be stored, handled and diluted.
Its value as an industrial intermediate comes from the "available" or free SO₃ it carries over and above 100% sulfuric acid, which makes it a stronger sulfonating and dehydrating agent than concentrated sulfuric acid alone. It is supplied at a range of free-SO₃ strengths, with 20%, 25% and 65% free SO₃ the most widely ordered commercial grades.
Commercially it is produced by absorbing gaseous sulfur trioxide from a contact-process sulfuric acid plant into concentrated sulfuric acid, with the free-SO₃ content controlled by how much SO₃ is absorbed.
Handling follows standard precautions for a strongly corrosive, water-reactive acid — acid-resistant PPE including face shield, ventilation or closed handling, and storage in corrosion-resistant equipment kept strictly isolated from water and organic materials.
Why Oleum specifically
- Stronger than sulfuric acid alone. The free SO₃ gives sulfonation and dehydration chemistry that 98% sulfuric acid cannot drive to completion.
- Graded by free-SO₃ strength. 20%, 25% and 65% free SO₃ grades let you match the reagent strength to your process without over- or under-shooting.
- A core sulfonation reagent. Used across dye, detergent, agrochemical and pharmaceutical intermediate chemistry wherever a sulfonic-acid group has to be introduced onto a ring.
- Also a feedstock, not just a reagent. Used to top up or regenerate sulfuric-acid strength in processes that run short of their own SO₃ balance.
How Oleum is Manufactured
Oleum is produced at sulfuric-acid plants operating the contact process, by absorbing sulfur trioxide gas into concentrated sulfuric acid rather than into water.
- SO₃ generation. Sulfur is burned to SO₂, then catalytically oxidised to SO₃ in the contact process.
- Absorption. SO₃ gas is absorbed into concentrated (98%) sulfuric acid rather than water, forming oleum.
- Strength control. The free-SO₃ content is controlled by the ratio of SO₃ absorbed to acid circulated, giving the 20%, 25% or 65% grades.
- Quality release and loading. Free SO₃, total SO₃ and iron content are confirmed against specification before the batch is released into tanker or drum.
Typical Commercial Specification
Figures below are representative of the two most widely ordered oleum grades. Tell us your downstream process and the strength you need, and we will advise on the grade to order against.
| Parameter | Oleum 25% | Oleum 65% |
|---|---|---|
| Appearance | Colourless to off-white fuming liquid | Colourless to off-white fuming liquid |
| Free SO₃ | 25 – 26.5% w/w min | 65% w/w min |
| Total SO₃ | 86.2% w/w min | 93.5% w/w min |
| Residue on ignition | 0.2% w/w max | 0.2% w/w max |
| Fe content | 0.05% w/w max | 0.05% w/w max |
Physical & Chemical Properties
| Property | Oleum 25% | Oleum 65% |
|---|---|---|
| Melting point | About 14 °C | About 0.5 °C |
| Boiling point | About 130 °C | About 60 °C |
| Specific gravity (20 °C) | About 1.935 | About 1.992 |
| Solubility | Miscible with water; reacts violently, generating heat and acid mist | |
| Incompatibilities | Water, organic materials, strong bases, most metals | |
Behaviour in the process
Oleum's reactivity is what sulfonation and dehydration process chemists are buying:
- Powerful sulfonating agent. The free SO₃ introduces sulfonic-acid groups onto aromatic rings more completely than concentrated sulfuric acid.
- Strong dehydrating agent. Drives condensation and dehydration steps that need an acid stronger than 98% H₂SO₄.
- Highly exothermic on dilution. Always add oleum slowly to the receiving medium with cooling and agitation — never the reverse, and never add water to oleum directly.
Oleum and Related Sulfonating Acids
Choosing between sulfonating and acidulating reagents comes down to how aggressive the chemistry needs to be and how the free-SO₃ strength matches your reaction.
| Material | Strength | When it is the right choice |
|---|---|---|
| Oleum 20% / 25% | Moderate free SO₃ | General sulfonation where a controlled excess of SO₃ is needed |
| Oleum 65% | High free SO₃ | Harder sulfonations and processes needing a concentrated SO₃ source |
| Sulfuric Acid (98%) | No free SO₃ | Acidulation, catalysis and processes that do not need excess SO₃ |
| Chlorosulfonic Acid | SO₃·HCl equivalent | Routes needing a sulfonyl chloride rather than a sulfonic acid directly |
Stated plainly: a process calling for excess SO₃ needs oleum at the matching strength — plain sulfuric acid is not a substitute where the reaction depends on that free SO₃.
Industries We Supply Oleum To
The same fuming sulfuric acid feeds a dye-intermediate manufacturer, a detergent producer and a fertiliser or agrochemical plant. What changes is the free-SO₃ strength ordered and the documentation each requires.
Where Oleum is Used
In downstream processing
Oleum's free SO₃ is consumed in the sulfonation or dehydration step itself, so the strength ordered should match the stoichiometry of your process rather than simply "more is safer" — over-strength oleum can over-sulfonate or char sensitive substrates.
Need a specific free-SO₃ strength?
Tell us the free-SO₃ percentage your process needs, 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 reported alongside total SO₃ because it is the free-SO₃ figure that determines how the acid behaves in your process.
- Free SO₃ content — by acidimetric titration, the primary grade measure.
- Total SO₃ content — reported alongside free SO₃ for a full picture of acid strength.
- Iron content and residue on ignition — reported because trace metals and residue can affect colour-sensitive downstream chemistry.
Common Process Problems & Likely Causes
| Symptom | Likely cause | What to check |
|---|---|---|
| Incomplete sulfonation | Free-SO₃ strength lower than the process is designed for | Free SO₃ figure on the COA against your specified grade |
| Charring or over-sulfonation | Free-SO₃ strength too high, or local overheating on addition | Grade ordered versus grade needed; addition rate and cooling |
| Discoloured product | Iron or residue carryover | Fe content and residue-on-ignition figures on the COA |
Packaging, Storage & Shipping
- Standard packing. Bulk road or rail tankers for volume orders, with mild-steel or lead-lined drums and IBCs available for smaller quantities.
- Material compatibility. Store and transport only in equipment rated for fuming sulfuric acid; most common metals, plastics and elastomers are not suitable.
- Moisture exclusion. Keep containers sealed and vented correctly; oleum reacts violently with atmospheric or liquid moisture.
- Temperature control. Store in a cool, dry, well-ventilated area; lower free-SO₃ grades have a higher freezing point and can solidify in cold storage or transit.
- Transport classification. Commonly shipped under UN 1831, Class 8, packing group I — confirm the correct entry and packing group against your own current SDS before shipping.
Hazards & Safe Handling
Oleum causes severe skin burns and eye damage, may be corrosive to metals, and reacts violently with water. It is a strongly corrosive, water-reactive material, and handling it safely means never improvising with equipment or PPE rated only for ordinary sulfuric acid.
- Skin and eye contact. Full acid-resistant PPE including face shield and apron is standard practice for fuming sulfuric acid; flush immediately with large volumes of water and seek medical attention.
- Inhalation. Acid mist and fumes are released continuously; use closed handling or adequate local exhaust ventilation.
- Dilution rule. Always add oleum slowly to the receiving medium with cooling and agitation; never add water to oleum.
- PPE. Acid-resistant gloves, chemical goggles with a face shield, and acid-resistant protective clothing; respiratory protection where mist exposure is a concern.
- Environment. Corrosive and harmful to aquatic life; contain spills rather than flush them, and follow local effluent and neutralisation rules.
Regulatory & Compliance Notes
Oleum is a long-registered, widely regulated industrial acid. Because obligations differ by territory, free-SO₃ strength and use, the notes below are orientation rather than legal advice.
- GHS classification. Classified as causing severe skin burns and eye damage, and as potentially corrosive to metals (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 1831 (Sulfuric acid, fuming), 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 sulfuric acid / oleum heading; confirm the exact tariff line with your customs broker for your jurisdiction.
We Supply Oleum Worldwide
Chelora Chemicals is a supplier and export partner, not a single-plant trader. We ship Oleum from India and from partner sources to customers across Asia, Europe, the Middle East, Africa, and North and South America, arranging DG-compliant tanker or drum freight and the export documentation 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 and tanker availability allow.
- Documentation. Commercial invoice, packing list, certificate of origin, COA and SDS issued as a complete set, alongside the full dangerous-goods transport paperwork.
- Flexible volumes. From drummed trial quantities up to full tanker loads on a supply contract.
Buying Oleum from Chelora
We are an independent sourcing and supply partner. For this product that means settling three things at enquiry: the free-SO₃ strength your process needs, the packing format, and the quantity you are buying.
- Documentation on every lot. Batch-specific COA with free SO₃, total SO₃, iron and residue figures, plus the current SDS.
- Analysis before commitment. Representative analysis shared ahead of a commercial order so your lab can qualify the material.
- Quantities. Drummed trial quantities through to full tanker loads on contract supply.
- Inspection. Independent third-party inspection available at loading when the contract requires it.
- Related acids. Sulfuric Acid, Sulfur Trioxide and Chlorosulfonic Acid alongside this product from one supplier.
How ordering works
- Send the enquiry. Free-SO₃ strength, quantity, packing format, 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, 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. Loaded with the full export and dangerous-goods document set, and tracked through to arrival.
Oleum — Common Questions
The questions our technical desk is asked most often about Oleum. If yours isn't here, call or email and you will reach someone who knows the chemistry.
What is the CAS number of Oleum?
The CAS number commonly used for Oleum (fuming sulfuric acid) is 8014-95-7, with EC number 231-639-5. Oleum is a solution of sulfur trioxide in sulfuric acid, generally represented as H₂SO₄·xSO₃.
What is Oleum used for?
It is used primarily as a sulfonating agent in dye, detergent, agrochemical and pharmaceutical intermediate manufacture, and as a strong dehydrating and acidulating agent in other industrial processes.
Do you supply Oleum worldwide?
Yes. Chelora Chemicals supplies and exports Oleum worldwide by bulk tanker and drummed shipments — across Asia Pacific, Europe, the Middle East, Africa and the Americas, as well as throughout India. Every consignment carries the COA and SDS.
What free-SO₃ grades are available?
20%, 25% and 65% free SO₃ are the most widely ordered commercial grades. Tell us your downstream process and we will advise on, and supply to, the strength you need.
How is Oleum manufactured?
By absorbing sulfur trioxide gas from a contact-process sulfuric acid plant into concentrated sulfuric acid, with the free-SO₃ content controlled by the ratio of SO₃ absorbed.
Is Oleum hazardous?
Yes. It causes severe skin burns and eye damage, may be corrosive to metals, 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?
Oleum is commonly shipped under UN 1831 (Sulfuric acid, fuming), Class 8, packing group I. Confirm the correct entry and packing group against your own current SDS before relying on it for shipping documentation.
Can water be added to Oleum to dilute it?
No — water must never be added to oleum. The reaction is violently exothermic. Where dilution is required, oleum is added slowly to the receiving medium with cooling and agitation, never the reverse, and only by personnel trained for the procedure.
How should Oleum be stored?
In sealed, corrosion-rated equipment designed for fuming sulfuric acid, kept strictly away from water and moisture, in a cool, dry, well-ventilated area. Lower free-SO₃ grades have a higher freezing point and can solidify in cold conditions.
What packing sizes and order quantities are available?
Bulk road or rail tankers for volume orders, with lined drums and IBCs available for smaller quantities. Orders run from drummed trial quantities up to full tanker loads on contract supply.
Ready to source Oleum?
Send us your free-SO₃ 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.