Oleum
Fuming sulfuric acid supplied in 23% and 65% free SO3 grades for sulfonation, mixed-acid nitration, and dehydration duty.
What is Oleum?
Oleum, also called fuming sulfuric acid, is sulfuric acid holding dissolved sulfur trioxide. It is a dense, oily liquid that fumes visibly in air as SO3 escapes and meets atmospheric moisture.
The grade number is the free SO3 content by weight, and it is the only thing that distinguishes one oleum from another. Both grades share CAS 8014-95-7 because they are the same substance at different compositions: 23% oleum equates to about 105.2% sulfuric acid, and 65% oleum to about 114.6%, using the standard conversion of 100 plus 0.225 times the free SO3 percentage.
At Chelora Chemicals, oleum is supplied in both 23% and 65% grades to a controlled free SO3 specification with iron held low, since trace metals carry directly into product colour in sulfonation and dye service.
- Appearance
- Colourless to dark brown oily fuming liquid
- Odour
- Sharp, choking (SO3 fumes)
- Solubility
- Reacts violently with water
- Function
- Sulfonating agent / dehydrating acid
- Grades
- 23% and 65% free SO3
Formula diagram
Sulfur trioxide dissolved in sulfuric acid
- CompositionH2SO4 with free SO3
- Grade 2323% free SO3 — approx. 105.2% H2SO4 equivalent
- Grade 6565% free SO3 — approx. 114.6% H2SO4 equivalent
- ConversionH2SO4 equiv. = 100 + 0.225 × % free SO3
- CAS number8014-95-7
Technical product information
Standard specification for Chelora Chemicals' 23% and 65% free SO3 Oleum. Custom-grade and bulk specifications are available on request.
| Chemical name | Oleum (fuming sulfuric acid) |
|---|---|
| Synonyms | OL23, OL65, Fuming sulphuric acid, Disulfuric acid solution |
| CAS number | 8014-95-7 (both grades) |
| Composition | H2SO4 with dissolved free SO3 |
| Free SO3 — Grade 23 | 23.0–24.5% |
| Free SO3 — Grade 65 | 64.0–66.0% |
| Total acidity — Grade 23 | approx. 105.2% as H2SO4 |
| Total acidity — Grade 65 | approx. 114.6% as H2SO4 |
| Density — Grade 23 (20°C) | approx. 1.90 g/cm³ |
| Density — Grade 65 (20°C) | approx. 1.98 g/cm³ |
| Appearance | Colourless to dark brown oily fuming liquid |
| Iron (Fe) | ≤ 50 ppm |
| Packaging | 250 kg drums, IBC, dedicated road tanker |
| Storage | Heated and insulated where the grade requires it |
Applications of Oleum
Sulfonation
Introduces sulfonic acid groups in dye, detergent, and surfactant intermediate manufacture.
Mixed-acid nitration
Supplies the dehydrating power that drives nitration to completion in nitroaromatic production.
Dye & pigment intermediates
Used where strong, water-free sulfonation is required for colour chemistry.
Petroleum refining
Applied in alkylation and refinery stream treatment.
Sulfuric acid strengthening
Added to spent or dilute acid to restore working concentration.
Specialty chemical synthesis
Provides the dehydrating and sulfonating duty that concentrated sulfuric acid alone cannot deliver.
Why choose Chelora Chemicals
Every batch of Oleum is manufactured, tested, and documented to a standard that formulators and process chemists can build on with confidence — from first sample to full production volume.
Consistent purity
Free SO3 controlled to the ordered grade with iron and appearance verified before dispatch.
Full documentation
COA, SDS, and regulatory data supplied with every shipment.
Flexible volumes
From 250 kg drums to IBC and dedicated road tanker supply in both grades.
Technical support
Process, handling, and compliance guidance from our chemistry team.
Storage and guidelines
- 01Store in dedicated, clearly grade-labelled, bunded tanks or containers — never mix grades or transfer between them without checking.
- 02Exclude moisture absolutely; the product reacts violently with water and fumes on contact with humid air.
- 03Manage temperature to the grade: several oleum compositions solidify near or above ambient, and heated, insulated storage is normal for those.
- 04Segregate rigorously from water, alkalis, organic materials, oxidising agents, and metal powders.
- 05Use appropriate materials of construction and keep the system closed and vented through a suitable scrubber.
- 06Ensure spill containment, dry neutralising material, emergency showers, and eyewash are available at every handling point.
Handling precautions
Oleum causes severe skin burns and eye damage and releases sulfur trioxide fumes that cause serious respiratory injury. It is for industrial use by trained personnel only.
It reacts violently with water, generating heat, acid mist, and dense fume. Never add water to oleum and never use a water jet on a spill or fire involving it.
Fumes are visible and dense, and a leak produces a rapidly spreading acid mist cloud rather than a contained puddle. Emergency plans must account for that behaviour and local emergency services should know the product is on site.
Freezing behaviour varies sharply with grade, and a line or vessel that solidifies then expands can rupture. Temperature control is a mechanical integrity issue, not just a handling convenience.
Full acid suit, face shield, chemical goggles, and appropriate respiratory protection are required for any transfer operation.
In some jurisdictions oleum is subject to site security, threshold reporting, or chemical-of-interest requirements. Confirm your national obligations before ordering.
Refer to the full Safety Data Sheet for exposure limits, spill neutralisation, fire response, and first-aid measures before use.
Frequently asked questions
What is oleum?
Sulfuric acid holding dissolved sulfur trioxide. It is used where sulfonation or dehydration needs more strength than 98% sulfuric acid can provide, because it contains no free water and can supply SO3 directly.
Why do Oleum 23 and Oleum 65 share the same CAS number?
Because they are the same substance at different compositions. CAS 8014-95-7 covers fuming sulfuric acid generally; the grade number tells you the free SO3 content, which is what actually differs. Always order by grade as well as CAS.
What does the grade number mean?
It is the percentage of free SO3 by weight. Grade 23 contains 23% free SO3 and Grade 65 contains 65%. The rest is sulfuric acid.
How do I convert free SO3 to sulfuric acid equivalent?
Multiply the free SO3 percentage by 0.225 and add 100. Grade 23 gives about 105.2% and Grade 65 about 114.6%. The figure exceeds 100% because the dissolved SO3 would form additional sulfuric acid if water were added.
Which grade should I use?
It depends on the duty. Lower grades are common where moderate additional strength is needed and easier handling matters. Higher grades deliver much more SO3 per tonne and are chosen for demanding sulfonation, but bring harsher fuming and tighter temperature requirements.
Why does it fume?
Free SO3 escapes from the surface and reacts immediately with moisture in the air to form a dense sulfuric acid mist. The fume is visible, corrosive, and dangerous to breathe, which is why closed handling and scrubbing are standard.
Can water be used on a spill?
No. Water drives a violent reaction that generates heat and a large acid mist cloud. Spill response uses dry inert absorbent or dry soda ash, and fire response uses dry chemical or carbon dioxide.
Why does oleum need heated storage?
Several oleum compositions solidify at or near ambient temperature, and freezing followed by expansion can damage lines and vessels. Heated, insulated storage keeps the product liquid and the installation intact.
How does oleum differ from 98% sulfuric acid?
98% acid contains a small amount of water and cannot sulfonate to completion without producing more. Oleum contains no free water and supplies SO3 directly, which is why it is used where the reaction must be driven fully.
What materials of construction are suitable?
Carbon steel is used for many oleum services, but the correct choice depends on grade, temperature, and velocity. Confirm materials against your specific grade and duty before commissioning any transfer system.
Is oleum regulated?
In many jurisdictions it carries strict transport classification and may fall under site security, threshold reporting, or chemical-of-interest requirements. Confirm what applies where you operate before ordering.
Do you provide a Certificate of Analysis?
Yes. Every consignment ships with a Certificate of Analysis and Safety Data Sheet stating the grade, free SO3 content, total acidity, iron, and appearance, together with the required transport documentation.