Oleum 23%
Fuming sulfuric acid supplied as Oleum 23% (23% free SO3) for use as a sulfonation reagent across dye, pigment and intermediate manufacture.
What is Oleum 23%?
Oleum 23%, also called fuming sulfuric acid, is a solution of sulfur trioxide (SO3) dissolved in concentrated sulfuric acid, held at a nominal 23% free SO3 content.
The dissolved SO3 is a powerful electrophile, making oleum a far more aggressive sulfonating agent than concentrated sulfuric acid alone. It is used to introduce sulfonic acid (–SO3H) groups onto aromatic rings, a key step in building water-solubilising groups into dyes and dye intermediates.
At Chelora Dyes, Oleum 23% is manufactured and analysed to a controlled free-SO3 specification so sulfonation reactions downstream give a consistent, reproducible degree of substitution batch to batch.
- Appearance
- Colourless to dark, fuming oily liquid
- Odour
- Pungent, acrid
- Solubility
- Miscible with sulfuric acid; reacts violently with water
- Function
- Sulfonation reagent
- Grade
- Technical / process chemical
Sulfur trioxide dissolved in sulfuric acid
- CompositionH2SO4 with dissolved SO3
- Free SO3 content22–24% by weight
- Common nameFuming sulfuric acid, Oleum 23%
- Reactive speciesSO3 (electrophilic sulfonating agent)
- CAS number8014-95-7
Technical product information
Standard specification for Chelora Dyes' Oleum 23% (fuming sulfuric acid). Custom free-SO3 strengths and bulk specifications are available on request.
| Chemical name | Oleum 23% (Fuming sulfuric acid) |
|---|---|
| Synonyms | OL23, Sulfuric acid fuming 23% free SO3 |
| CAS number | 8014-95-7 |
| Composition | H2SO4 / SO3 mixture |
| Free SO3 content | 22–24% by weight |
| Appearance | Colourless to dark, fuming oily liquid |
| Density (20°C) | Approx. 1.87–1.89 g/cm³ |
| Iron content | Held to low ppm limits for colour-sensitive applications |
| Reactivity | Reacts violently and exothermically with water |
| Packaging | Acid-resistant drums and IBC; bulk tanker on request |
| Shelf life | 12 months in sealed original packaging |
Applications of Oleum 23%
Aromatic sulfonation
Introduces sulfonic acid groups onto aniline, naphthalene and other aromatic rings for dye intermediate synthesis.
Water-solubilising groups
Builds –SO3H functionality into dyes and pigment dispersants to control water solubility.
Sulfonic acid dye intermediates
Used in the manufacture of sulfonated azo and reactive dye intermediates.
Detergent & surfactant intermediates
Applied in the broader sulfonation chemistry used for anionic surfactant manufacture.
Nitration & mixed-acid processes
Used in combination with nitric acid in mixed-acid nitration systems.
General acid-process reagent
Applied across various acid-catalysed condensation and dehydration reactions.
Why choose Chelora Dyes
Consistent strength
Free SO3 content controlled and verified before dispatch.
Full documentation
COA, SDS, and regulatory data supplied with every shipment.
Flexible volumes
From acid-resistant drums to bulk IBC and tanker quantities.
Technical support
Process, sulfonation, and compliance guidance from our chemistry team.
Storage and guidelines
- Store in tightly sealed, acid-resistant original containers.
- Keep in a cool, dry, well-ventilated area, away from direct sunlight.
- Store away from water and moisture at all times.
- Segregate from organic materials, bases, and reducing agents.
- Use compatible containers: steel, glass-lined or PTFE-lined vessels.
- Re-seal containers promptly after use to prevent moisture ingress and fuming loss.
Handling precautions
Oleum 23% is strongly corrosive to skin, eyes and the respiratory tract, and reacts violently and exothermically with water, releasing dense fumes.
Use acid-resistant gloves, full face and eye protection, and a chemical apron during transfer and charging operations.
Always add oleum to the reaction medium slowly with cooling and agitation; never add water to the acid.
Refer to the full Safety Data Sheet for exposure limits, spill containment, disposal, and first-aid measures before use.
Frequently asked questions
What is Oleum 23% used for?
Oleum 23% is used as a sulfonation reagent to introduce sulfonic acid groups onto aromatic rings during dye, pigment and surfactant intermediate manufacture.
Why is oleum a stronger sulfonating agent than sulfuric acid?
The free SO3 dissolved in oleum is a far more reactive electrophile than sulfuric acid alone, allowing faster and more complete sulfonation of aromatic substrates.
Is Oleum 23% soluble in water?
It reacts violently and exothermically with water rather than simply dissolving, so it must always be added slowly to water or the reaction medium, never the reverse.
What free SO3 strength do you supply?
Standard grade is supplied at 22–24% free SO3 by weight; other oleum strengths, including 65%, are also available.
What packaging is available?
Acid-resistant drums and IBC totes as standard, with bulk tanker supply available on request.
How should it be stored, and for how long?
In sealed, acid-resistant original containers in a cool, dry, ventilated store, strictly isolated from water and moisture. Shelf life is 12 months from manufacture when storage conditions are maintained.
Is it hazardous to handle?
Yes, it is strongly corrosive and reacts violently with water, so it must be handled with full acid-resistant PPE and adequate ventilation. Review the Safety Data Sheet before any handling operation.
Do you provide a Certificate of Analysis?
Yes. Each batch ships with a Certificate of Analysis and Safety Data Sheet covering free SO3 content, density, and appearance.