Oleum 23%
Fuming sulfuric acid carrying 23% free sulfur trioxide, supplied as the general purpose sulfonation reagent for dye and pigment intermediate manufacture.
What is Oleum 23%?
Oleum is sulfuric acid with sulfur trioxide dissolved in it. The percentage quoted is the free SO3 content, not a dilution, so 23% oleum is stronger than 100% sulfuric acid — roughly 105.2% total acidity expressed as H2SO4.
That free SO3 is the working part of the reagent. It attacks an aromatic ring directly and installs a sulfonic acid group, which is how the water-soluble backbone of the dye industry gets built: naphthalene sulfonic acids, sulfonated anilines and toluenes, and the sulfonated intermediates that carry through into acid, direct and reactive dyes. At 23% strength the reagent is powerful enough for the great majority of routine sulfonations while remaining the more manageable of the two grades.
At Chelora Dyes, Oleum 23% is supplied to a controlled free SO3 window with iron held low, in drums or by dedicated tanker for plants with the receiving facilities to take it in bulk.
- Appearance
- Oily fuming liquid
- Odour
- Sharp, choking
- Reactivity
- Violent with water
- Function
- Sulfonation
- Grade
- 23% free SO3
Sulfuric acid carrying dissolved sulfur trioxide
- CompositionH2SO4 with free SO3
- Free SO323.0% ± 0.5%
- Total acidityApprox. 105.2% as H2SO4
- Active speciesSulfur trioxide
- CAS number8014-95-7
Technical product information
Standard specification for Chelora Dyes' Oleum 23%. Alternative free SO3 strengths, including 65%, are available on request.
| Chemical name | Sulfuric acid, fuming (Oleum) |
|---|---|
| Synonyms | Oleum 23%, Fuming sulfuric acid, Disulfuric acid solution |
| CAS number | 8014-95-7 |
| Composition | H2SO4 with dissolved free SO3 |
| Free SO3 | 23.0% ± 0.5% |
| Total acidity (as H2SO4) | Approx. 105.2% |
| Appearance | Colourless to pale brown oily fuming liquid |
| Density | Approx. 1.91 g/cm³ at 20°C |
| Crystallisation behaviour | Solidifies at low ambient temperatures; lagged and trace-heated storage recommended |
| Iron (as Fe) | ≤ 100 ppm |
| Residue on ignition | ≤ 0.05% |
| Water reactivity | Reacts violently and exothermically |
| Transport classification | UN 1831, sulfuric acid, fuming (verify against current documentation) |
| Packaging | Lined drums; road tankers and ISO tanks for bulk supply |
| Shelf life | 12 months in sealed, dry, vented storage |
Applications of Oleum 23%
Sulfonation of intermediates
The general purpose reagent for introducing sulfonic acid groups onto aromatic dye intermediates.
Naphthalene sulfonic acids
Used across the naphthalene sulfonation chain that underpins acid and direct dye chemistry.
Sulfonated anilines
Applied where amines and toluenes must be sulfonated to give water-soluble components.
Optical brightener chain
Supports the sulfonation steps feeding stilbene brightener manufacture.
Drying and dehydration
Used as a strong dehydrating medium in process duty where water must be removed.
Acid strength adjustment
Charged to raise the strength of spent or dilute sulfuric acid systems back to specification.
Why choose Chelora Dyes
Controlled strength
Free SO3 held to a tight window so sulfonation charges stay predictable.
Full documentation
COA, SDS, and transport classification supplied with every shipment.
Compliant logistics
Dedicated tankers and dangerous goods paperwork handled end to end.
Technical support
Charging, materials of construction, and compliance guidance from our team.
Storage and guidelines
- Store in closed, dry carbon steel or lined vessels, vented to a scrubber.
- Lag and trace heat tanks and lines where ambient temperatures approach solidification.
- Keep the entire system absolutely dry — never allow water, washings or condensate in.
- Segregate from water, alkalis, organics, oxidisable materials and metals.
- Bund the storage area to full capacity and keep neutralising media on hand.
- Use closed transfer only; never open-pour or sample without a dedicated closed system.
Handling precautions
Oleum causes rapid, deep chemical burns to skin and eyes and destroys tissue on contact. It chars organic material, including clothing and skin, almost immediately.
It reacts violently and exothermically with water; the reaction can boil and spatter acid. Acid is always added to water in a controlled dilution, never the reverse, and dilution of oleum should only be done under a written, engineered procedure.
The free SO3 fume is corrosive and severely damaging to the respiratory tract. Systems must be closed, vented and scrubbed, with gas detection where fume may accumulate.
Sulfuric acid and oleum are subject to regulatory reporting and control in several jurisdictions. Confirm your obligations, ensure only trained operators with full acid personal protective equipment handle the material, and refer to the current Safety Data Sheet before any operation.
Frequently asked questions
What is Oleum 23% used for?
It is the standard sulfonation reagent for dye and pigment intermediates. The free sulfur trioxide dissolved in the acid introduces sulfonic acid groups onto aromatic rings, which is how water-soluble dye intermediates such as naphthalene sulfonic acids and sulfonated anilines are made.
What does 23% actually refer to?
It is the free sulfur trioxide content dissolved in the sulfuric acid, not a dilution. Oleum is therefore stronger than 100% sulfuric acid, and 23% oleum corresponds to a total acidity of roughly 105.2% expressed as H2SO4.
How does it differ from 65% oleum?
Both are fuming sulfuric acid; the difference is how much free sulfur trioxide is dissolved. The 23% grade is the general purpose sulfonation strength used for most routine work, while 65% is reserved for harder sulfonations that will not proceed at the lower strength or that need less water in the system.
Why does it fume?
The dissolved sulfur trioxide has a real vapour pressure and escapes above the liquid, where it combines instantly with atmospheric moisture to form a dense mist of sulfuric acid. That visible fume is corrosive and severely irritant, which is why oleum systems must be closed and vented to a scrubber.
Can it solidify in storage?
Yes. Oleum solidifies at temperatures that are reachable in an unheated tank in cold weather, and the exact point depends on the SO3 content. Tanks and lines are normally lagged and trace heated, and the crystallisation figure for your grade is given on the Certificate of Analysis.
What packaging and bulk options are available?
Lined drums for smaller users, and dedicated road tankers and ISO tanks for bulk supply. Bulk delivery requires suitable receiving facilities, and we will review your offloading arrangements before first shipment.
How should it be stored?
In closed, dry, vented carbon steel or lined vessels, lagged and trace heated where ambient temperatures are low, with the vent routed to a scrubber. Keep the system absolutely free of water and segregated from organics, alkalis and oxidisable materials.
Is it hazardous to handle?
Yes, severely. It causes rapid and deep chemical burns, reacts violently and exothermically with water, chars organic material on contact and evolves corrosive sulfur trioxide fume. It is also subject to regulatory controls in several jurisdictions. Only trained operators with full acid protection and closed transfer systems should handle it, and the current Safety Data Sheet must be reviewed first.
