Oleum 23%
Sulphuric acid has as much water as it can hold. Oleum has less — it carries free sulphur trioxide dissolved in the acid, which is why its strength is quoted above 100%. At 23% free SO3, equivalent to about 105.2% H2SO4, it does two jobs ordinary acid cannot: it sulphonates, and it takes water out of a reaction as fast as the reaction makes it.
Chelora Chemicals supplies Oleum 23% to dye, detergent, pharmaceutical and nitroaromatic producers worldwide, in road tankers, ISO tanks and drums, with a batch COA and GHS safety data sheet on every consignment.
Product identity
- CAS Number
- 8014-95-7
- Common name
- Fuming sulphuric acid
- Composition
- H2SO4 + 23% free SO3
- Total acidity
- ≈ 105.2% as H2SO4
- EC / EINECS
- Listed as sulphuric acid, fuming*
- UN Number
- 1831 · Class 8 (6.1) · PG I
- Appearance
- Oily fuming liquid
- HS Code
- 2807 00*
* Oleum is an inventory-listed substance under the fuming sulphuric acid entry rather than a single defined compound, and its identifiers vary between regional inventories. Confirm the exact listing and HS heading for your destination with your customs broker — we will supply the identity data you need for the filing.
What Oleum 23% is used for
Oleum is a colourless to brownish, oily, fuming liquid — a solution of sulphur trioxide in concentrated sulphuric acid. The fuming is literal: exposed to humid air, the free SO3 grabs atmospheric moisture and throws off a dense white mist of sulphuric acid. That behaviour is both the reason it is useful and the reason it is handled the way it is.
Two applications account for most of the volume. In sulphonation, the free SO3 attaches a sulphonic acid group to an aromatic ring — the step that turns para nitrotoluene into PNTOSA, alkylbenzene into detergent acid, and a long list of dye intermediates into their soluble forms. In mixed-acid nitration, oleum is not there to nitrate at all; it is there to absorb the water the nitration produces, keeping the acid strong enough for the reaction to continue. Beyond those, it is used to boost the strength of weak or spent sulphuric acid and as a dehydrating agent in fine chemical synthesis.
How the grades differ
- Oleum 20–25% — the common commercial band, including this 23% grade. Liquid at ordinary ambient temperatures, which is what makes it practical to move and store.
- Oleum 65% — much higher SO3 content for users who want to ship SO3 rather than acid weight; needs heated handling and dedicated equipment.
- Sulphuric acid 98% — no free SO3 at all. It will not sulphonate efficiently and it cannot scavenge water, which is precisely why nitration plants buy oleum.
- Liquid SO3 — the pure reagent, with a far sharper hazard and handling profile than any oleum grade.
Specify the strength on the purchase order, not just "oleum" — see the full product range.
Appearance and handling character
Oleum is heavier than water, violently reactive with it, corrosive to tissue on contact and fatal if the mist is inhaled. It is not flammable, but it attacks many metals once diluted and can liberate hydrogen in the process. There is no version of this product that is suitable for an improvised installation — the safety section below should be read in full before any handling is planned.
| Parameter | Typical value |
|---|---|
| Free SO3 | 23.0% ± 0.5 |
| Total acidity (as H2SO4) | ≈ 105.2% |
| Iron (Fe) | Reported per batch |
| Sulphur dioxide (SO2) | Reported per batch |
| Non-volatile residue | Reported per batch |
| Chloride | Reported per batch |
| Nitrogen oxides (as N2O3) | Reported per batch |
| Appearance | Clear, free of suspended matter |
Important: the figures above are indicative of commercial grade material and are offered for planning only. The batch certificate of analysis is the governing document. On this product the strength figure decides your charge calculation and the iron and nitrogen oxide figures decide your product colour. Tell us which matter most in your process and we will quote against them.
| Property | Value |
|---|---|
| Physical state | Oily fuming liquid |
| Relative density | ≈ 1.9 |
| Freezing point | Near ambient-low; strength-dependent |
| Reaction with water | Violent, strongly exothermic |
| Behaviour in humid air | Fumes — dense H2SO4 mist |
| Flammability | Not combustible; oxidising towards organics |
| Action on metals | Corrosive when diluted; may liberate hydrogen |
| Materials of construction | Dedicated carbon steel, dry service |
Oleum freezing points do not fall on a straight line with strength — the commercial grades sit in the low troughs of that curve, which is why 20–25% is the band that moves by tanker. Confirm the handling temperature window for your exact strength with us before you design the installation.
Where our customers put this acid to work
Two jobs dominate: putting a sulphonic acid group onto a ring, and taking water out of a reaction that is busy making it.
Aromatic sulphonation
The free SO3 attaches a sulphonic acid group directly to the ring. This is the step that produces PNTOSA from para nitrotoluene and a long list of soluble dye intermediates from their parent aromatics.
- Strength consistency drives charge accuracy
- See PNTOSA
- Low iron grades for colour-critical work
Mixed-acid nitration
Nitration makes water, and water kills the reaction. Oleum in the mixed acid absorbs it as it forms, holding the acid strength where the nitration needs it — the role it plays behind nitrobenzene, the nitrotoluenes and the nitrochlorobenzenes.
- Water scavenger, not the nitrating agent
- See nitrobenzene
- Consistent free SO3 matters more than purity
Detergent & surfactant acid
Sulphonation of alkylbenzene and related substrates produces the sulphonic acids that become the active matter in household and industrial detergents.
- Colour and iron specifications supported
- Bulk contracted supply available
- Scheduled tanker call-off
Acid strength boosting
Adding oleum to weak or partially spent sulphuric acid restores its strength without adding volume — a standard move in plants that recycle their own acid rather than discarding it.
- Reduces spent acid disposal load
- Strength calculated from the COA figure
- Technical desk will check your balance
Fine chemical & pharma synthesis
Where a route calls for a strongly dehydrating acidic medium — cyclisations, condensations, certain rearrangements — oleum provides it with a free SO3 content you can calculate against.
- Tight impurity specifications available
- Consistent lot-to-lot strength
- Full analytical documentation available
Contract & scheduled supply
Dedicated oleum tankers are specialised equipment and there are not many of them. Buyers who contract a volume and a schedule get the equipment; buyers who call at short notice compete for it.
- Annual contracts with scheduled call-off
- Equipment reserved against the programme
- Second source held in reserve
How we supply Oleum 23%
Chelora Chemicals is a supplier and sourcing partner, not a manufacturer. On a bulk acid, the value we add is not chemistry — it is compliant equipment and reliable scheduling. Dedicated oleum tankers and ISO tanks are scarce, Packing Group I paperwork is unforgiving, and the difference between a smooth delivery and a refused one is usually administrative.
We place your order with whichever qualified producer best matches your strength tolerance, your impurity limits and your delivery window.
We work to keep a second qualified source on this line. When a plant takes its contact unit down for maintenance, your sulphonation line switches source rather than stopping.
- Strength confirmed before we quote. 20%, 23%, 25% or 65% are four different products with four different handling requirements — we settle which you mean in the first reply.
- Impurities reported individually. Iron, SO2, chloride, nitrogen oxides and residue each get their own line on the COA, because each one shows up differently in your process.
- Equipment arranged, not assumed. Dedicated tankers and ISO tanks are booked with the order, with the previous-cargo position checked rather than taken on trust.
- Packing Group I handled properly. The highest danger category leaves no room for an incomplete declaration, and we file before the booking rather than after.
- Receiving facilities discussed first. If your site is not set up for an oleum delivery, we would rather find out at enquiry stage than at your gate.
- Export machinery handled in-house. UN 1831 classification, approved tanks, placarding, freight booking and customs paperwork are ours to manage.
Moved in dedicated equipment
This is a bulk acid, not a drummed speciality. What you can receive at your end usually decides the format — so tell us what your unloading point looks like before we quote.
Dedicated road tankers
The standard domestic format. Carbon steel tankers in dedicated oleum service, placarded for UN 1831, delivered direct into your storage tank on a scheduled slot.
Export tank containers
Approved dedicated tank containers for sea movement, with the equipment position often the longest lead item in the plan — which is why we book it with the order.
MS drums
For smaller users and batch plants, where local dangerous-goods rules permit drummed oleum. Sealed, placarded and palletised, for closed transfer only.
Rail tank wagons
Where your site has a rail connection and volume justifies it, rail movement in dedicated acid wagons is the lowest-cost and lowest-exposure option for large programmes.
Call-off programmes
Contracted tonnage released against your production calendar, with tanker slots reserved in advance instead of competed for at short notice.
Incoterms to suit you
Quoted on the Incoterm your purchasing team works to. Note that oleum is forbidden on passenger aircraft, so air freight is not an available route on this product.
Water is the hazard people underestimate
Oleum causes severe skin burns and eye damage on contact, and its mist can be fatal to inhale. But the incidents that escalate usually start with water — a wet hose, a damp vessel, rain on a spill — because oleum reacts with it violently and exothermically. The summary below is orientation only — the safety data sheet supplied with your consignment is the authoritative document.
Classified hazards
H314Causes severe skin burns and eye damage.H330Fatal if inhaled.H335May cause respiratory irritation.H290May be corrosive to metals.EUH014Reacts violently with water.
Hazard statements reflect classifications commonly published for fuming sulphuric acid. Strong inorganic acid mists containing sulphuric acid are additionally the subject of occupational carcinogenicity classifications in several jurisdictions. Always read the SDS issued with your specific consignment.
Spill & emergency orientation
Plan your response before the tanker arrives. Sections 4, 5 and 6 of the SDS are the authoritative instructions and should be posted at the unloading point.
- Never apply water to the spill. It reacts violently and spatters acid. Contain with dry sand, soda ash or lime and neutralise cautiously, from the edge inward.
- Treat the mist as the main casualty risk. Fuming in humid air produces a dense acid aerosol that can be fatal to inhale — set a downwind exclusion zone, not just a bund.
- Watch for hydrogen. Oleum diluted by a wet spill attacks steel and can liberate hydrogen, turning a corrosion event into a flammability event. Eliminate ignition sources.
- Acid to water, never water to acid. The rule exists because the reverse boils the water and throws acid out of the vessel.
- Emergency showers and eye wash within seconds of the unloading point, tested and unobstructed. With H314, response time is the control.
Storage
- Dedicated carbon steel in dry service. Oleum is normally stored in mild steel, but only while dry — moisture ingress changes both the corrosion picture and the hazard picture.
- Blanket and dry the vents. Dry air or nitrogen blanketing with suitable vent driers stops the tank breathing in humid air and fuming continuously.
- Bund to full capacity with an acid-resistant, sealed bund. Keep water services, drains and rainwater out of the bunded area.
- Segregate absolutely from water, organics, combustibles, chlorates, permanganates, carbides, metal powders and bases. This is a segregation list to design around, not to post on a wall.
- Restrict access. Oleum unloading should be a permit operation performed by trained people with the right PPE staged at the point of use.
Handling & PPE
- Closed transfer only. Fixed lines, dedicated couplings and a checked unloading procedure. There is no acceptable way to decant oleum by hand.
- Full acid-resistant suit, face shield over goggles, gauntlets and boots for unloading and sampling — the splash is not survivable through ordinary overalls.
- Respiratory protection appropriate to an acid mist, available and fit-tested, for unloading, sampling and any breaking of containment.
- Verify the line is dry before connecting. The commonest escalation in acid handling is water left in a hose.
- Never discharge to drain. Neutralise and dispose of as regulated chemical waste, and check your discharge consent before the first delivery.
Transport
Oleum travels as UN 1831, Class 8 corrosive with a Class 6.1 subsidiary risk, Packing Group I — the highest danger category. We classify each consignment, arrange approved dedicated equipment, apply correct placarding and file the dangerous-goods declaration before the booking is made. It is forbidden on passenger aircraft — ask us for the current DG paperwork when you plan the shipment.
What travels with every consignment
A chemical shipment is only as good as the file that travels with it. On a Packing Group I acid, the file is also what keeps the consignment moving.
Certificate of Analysis
Batch-specific free SO3, total acidity, iron, SO2, chloride, nitrogen oxides and residue — checked against your purchase specification before release.
GHS Safety Data Sheet
Full 16-section SDS in GHS format covering hazards, exposure controls, handling, storage, spill response, reactivity and disposal — issued in the language your destination requires.
Dangerous Goods Declaration
UN 1831, Class 8 with 6.1 subsidiary risk and Packing Group I, declared and filed before the movement is booked, with correct placarding and marking on the equipment.
Tank & equipment approvals
Tank certification, test dates and last-cargo records supplied with the movement, so your receiving team can verify the equipment rather than assume it.
Certificate of Origin
Chamber-attested where required, plus preferential-origin forms where a trade agreement gives your import duty relief at the destination.
Third-party inspection
SGS, Intertek or Bureau Veritas sampling and inspection arranged on request before loading, giving you independent verification of strength and impurities.
More SO3 than water to hold it
Sulphuric acid is SO3 and water in balance. Push more SO3 in than the water can hydrate and the excess stays dissolved as free SO3 — that is oleum, and that free SO3 is what does the work. The outline below is the general industrial pattern, not a process disclosure.
Burn sulphur to SO2
Elemental sulphur is burned in dry air to sulphur dioxide, or SO2 is recovered from metallurgical off-gas at a smelter.
Convert to SO3
Catalytic oxidation over vanadium pentoxide in a multi-pass converter takes the dioxide to sulphur trioxide at high conversion.
Absorb into strong acid
The SO3 is absorbed into circulating concentrated sulphuric acid. Absorbing more than the acid can hydrate is what produces oleum rather than 98% acid.
Adjust to grade
The circuit is blended and trimmed to the sold strength — 20%, 23%, 25% or 65% free SO3 — and the figure on your COA is the one your charge is calculated from.
Why the small numbers on the COA matter
On most products the assay is the headline. On oleum the strength is a composition and the impurities are where the surprises live:
- Free SO3 is the number your stoichiometry uses. A half-point drift is a half-point error in every charge until someone notices.
- Iron carries straight into sulphonated products and shows up as colour. On dye and detergent chemistry it is often the deciding specification.
- Nitrogen oxides left from a sulphur-burning circuit can nitrosate sensitive substrates and discolour the product — worth a limit if your chemistry is delicate.
- Chloride attacks stainless steel in downstream equipment even where the oleum itself is handled in mild steel.
- SO2 and residue indicate how well the absorption circuit is running and how clean the source plant's acid loop is.
That is why we quote against your written limits rather than a generic grade, and why the batch COA is shared for approval before dispatch rather than sent with the invoice.
| Grade | Character & use |
|---|---|
| Oleum 23% this product · ≈105.2% acidity | Liquid at ambient Sulphonation & mixed acid |
| Oleum 20 – 25% the commercial band | Tanker-friendly General industrial use |
| Oleum 65% high SO3 | Heated handling SO3-intensive duty |
| Sulphuric acid 98% no free SO3 | Will not scavenge water Different product entirely |
We supply the acids and the aromatics they react with — see the full product range. Tell us the strength and the duty and we will confirm the correct grade before quoting.
What actually moves the price of this acid
Knowing what drives the quote helps you time a purchase and read whether an offer is realistic.
Sulphur and smelter acid
The raw material is sulphur or smelter off-gas. When metal production slows, by-product acid supply tightens regionally — and oleum follows the sulphuric acid balance it is made from.
- Track the regional acid balance
- Smelter outages move the market quickly
- We flag feedstock-driven moves in the quote
Freight is a big share
A dense, low-value, highly regulated liquid is dominated by logistics cost. Distance from the plant matters more here than on almost any product we supply.
- Compare on delivered cost, never ex-works
- Nearest qualified source usually wins
- We quote FOB, CIF, CFR or DDP as you prefer
Equipment availability
Dedicated oleum tankers and ISO tanks are a limited pool. On export lanes the tank, not the acid, is usually what sets your lead time and a chunk of your cost.
- Book equipment with the order, not after
- Contracted programmes reserve capacity
- Thin lanes need longer notice
Packing Group I compliance
The highest danger category brings approvals, placarding, routing restrictions and carrier acceptance conditions. Each of those is a cost and a potential delay.
- Forbidden on passenger aircraft
- Carrier acceptance varies by lane
- We check destination rules before quoting
Energy and conversion cost
Contact plants are energy integrated and usually export steam. Energy pricing therefore cuts both ways, and the net effect on oleum is not always the one the headline suggests.
- Steam export credits offset conversion cost
- Regional power pricing feeds through
- Ask us what is moving in your region
Source availability
A contact plant turnaround or a converter catalyst change takes capacity offline for weeks at a time. Single-sourced buyers on a bulk acid feel that immediately.
- We work to hold a second qualified source
- Qualify the alternate before you need it
- Ask us about dual-source contracts
| If your end use is… | Specify |
|---|---|
| Sulphonation of nitroaromatics | Tight free SO3; low iron and nitrogen oxides |
| Detergent sulphonation | Low iron and colour; consistent strength |
| Mixed-acid nitration | Consistent free SO3; low nitrogen oxides |
| Acid strength boosting | Commercial grade; accurate strength certificate |
| Pharma / fine chemical | Low iron, chloride and residue; full impurity profile |
Not sure which column you are in? Describe the duty and our technical desk will recommend the grade — including telling you when 98% sulphuric acid would do the job and save you the handling burden entirely.
Support that goes beyond dropping the tanker
What our technical desk will do
- Confirm the strength before anything else. "Oleum" on a purchase order is not a specification — 20, 23, 25 and 65% are four different handling propositions.
- Check your receiving arrangements. Unloading point, blanket gas, vent driers, bunding and emergency facilities — we would rather discuss them at enquiry stage than at your gate.
- Be straight about the hazard profile. Severe burns, a mist that can be fatal to inhale, violent reaction with water — stated up front so your EHS team can plan properly.
- Tell you when you do not need oleum. If 98% acid will do the duty, we will say so. It is a far easier material to live with.
Regulatory & compliance notes
- Major-hazard thresholds. Bulk oleum storage can bring a site within process-safety or major-accident regimes in several jurisdictions. Check your inventory against the thresholds before you size the tank.
- Reporting and notification duties apply to sulphuric acid and oleum storage in many regions, including emergency planning obligations with local authorities.
- Workplace exposure limits for sulphuric acid mist are set in most jurisdictions, and strong inorganic acid mists carry occupational carcinogenicity classifications that trigger additional duties.
- Effluent rules apply to neutralised washings and bund water. Check your discharge consent before the first delivery.
None of the above is legal advice — your own regulatory affairs team owns the final compliance position for your import and your workplace. What we commit to is accurate product identity data, an honest statement of the hazard classification and complete documentation to work from.
From enquiry to delivered tanker
Send the specification
Strength required, impurity limits, quantity, delivery format, your unloading arrangements and the destination site or port.
Receive a firm quote
Pricing, lead time, equipment and Incoterm confirmed in writing within 24 hours — our sourcing desk is staffed around the clock.
Approve and schedule
Batch COA shared for your approval, equipment booked and the delivery slot agreed against your production calendar.
Ship and track
UN 1831 declaration filed, approved tank placarded and dispatched, and the movement tracked through transit and clearance to your site.
What buyers ask us about Oleum 23%
What does 23% oleum actually mean?
Why 23% rather than 20%, 25% or 65%?
What specification do you normally supply?
What happens if water gets in?
What is the minimum order quantity?
How long does delivery take?
How should we store it?
Is this a dangerous good? What paperwork comes with it?
- Batch certificate of analysis
- GHS-format safety data sheet
- Dangerous-goods declaration with UN number, class, subsidiary risk and packing group
- Tank or packaging approval documentation
- Commercial invoice, packing list and bill of lading
- Certificate of origin and any destination-specific forms
What should the emergency plan cover?
Do you manufacture this yourself?
Request pricing on Oleum 23%
Send your required strength, impurity limits, quantity, delivery format, unloading arrangements and destination. Our sourcing desk is available 24 hours a day, 7 days a week, and comes back with pricing, lead time and equipment options — whether that is a single tanker or a contracted programme for the year.