Nitrosylsulfuric Acid
A diazotization reagent supplied as a solution in sulfuric acid, for weakly basic aromatic amines that cannot be diazotized in aqueous mineral acid.
What is Nitrosylsulfuric Acid?
Nitrosylsulfuric Acid is a nitrosyl group bound to a hydrogen sulfate group. It is not traded as the pure solid but as a pale yellow to greenish solution in concentrated sulfuric acid, most commonly at 40% strength.
It exists to solve one specific problem. Ordinary anilines diazotize happily with sodium nitrite in dilute aqueous acid, but the deactivated amines that give the best pigments — nitroanilines, chloronitroanilines and their relatives — are barely basic enough to protonate and barely soluble in that medium, so the reaction stalls. Nitrosylsulfuric acid supplies the same nitrosating chemistry in a strongly acidic, essentially non-aqueous system in which those amines dissolve and react to a clean end point.
At Chelora Dyes, NSA is supplied to a controlled assay in acid-resistant packaging, with trace-heated bulk delivery available for plants that dose it continuously.
- Appearance
- Pale yellow liquid
- Odour
- Sharp, acidic
- Reactivity
- Violent with water
- Function
- Diazotization
- Grade
- 40% in sulfuric acid
A nitrosyl ester of sulfuric acid
- Molecular formulaNOHSO4
- Molecular weight127.07 g/mol
- IUPAC nameNitrosylsulfuric acid
- Functional groupsNitrosyl, sulfate ester
- CAS number7782-78-7
Technical product information
Standard specification for Chelora Dyes' 40% Nitrosylsulfuric Acid solution. Alternative strengths are available on request.
| Chemical name | Nitrosylsulfuric Acid |
|---|---|
| Synonyms | Nitrosyl sulfuric acid, Nitrosylsulphuric acid, NSA |
| CAS number | 7782-78-7 |
| Molecular formula | NOHSO4 |
| Molecular weight | 127.07 g/mol |
| Supplied as | Solution in concentrated sulfuric acid |
| Assay (NSA content) | 40.0% ± 1.0% |
| Appearance | Pale yellow to greenish clear liquid |
| Density | Approx. 1.83 g/cm³ at 20°C |
| Crystallisation behaviour | Crystallises on cooling; heated storage required (see COA for grade figure) |
| Free sulfuric acid | Balance of solution |
| Iron (as Fe) | ≤ 50 ppm |
| Water reactivity | Hydrolyses violently; evolves nitrogen oxides and heat |
| Transport classification | UN 2308, nitrosylsulfuric acid, liquid (verify against current documentation) |
| Packaging | Lined and glass-lined drums, heated IBCs, road tankers |
| Shelf life | 6 months in sealed heated storage |
Applications of Nitrosylsulfuric Acid
Diazotization of weak amines
The standard reagent where nitroanilines and other deactivated amines must be diazotized.
Azo pigment manufacture
Used across orange and red pigment production where the diazo component is deactivated.
Disperse and azoic dyes
Applied in disperse dye and azoic systems built on poorly basic aromatic amines.
Optical brightener chemistry
Supports diazotization steps in brightener and stilbene derivative manufacture.
Fine chemical synthesis
Used as a nitrosating reagent in pharmaceutical and agrochemical intermediate routes.
Pilot and development use
Available in drum quantities for process development and scale-up trials.
Why choose Chelora Dyes
Controlled assay
NSA content held to a tight window so dosing and end point stay predictable.
Full documentation
COA, SDS, and transport classification supplied with every shipment.
Heated logistics
Trace-heated drums, IBCs and tankers to keep the solution above crystallisation.
Technical support
Dosing, storage, and compliance guidance from our chemistry team.
Storage and guidelines
- Store in sealed, acid-resistant containers under trace heating above the crystallisation point.
- Keep the system closed and absolutely dry — water ingress is the principal hazard.
- Segregate from water, alkalis, organics, reducing agents and combustible materials.
- Use glass-lined, PTFE or other acid-resistant materials of construction; avoid bare metals.
- Vent and scrub storage and transfer systems for nitrogen oxides.
- Bund the storage area and have acid spill neutralisation materials on hand.
Handling precautions
Nitrosylsulfuric Acid causes severe skin burns and serious eye damage on contact, and it reacts violently with water, liberating heat and toxic nitrogen oxides.
Never add water to the acid, and never allow washings, condensate or damp equipment into a vessel containing it.
Handle only in closed, dry systems by trained operators. Use full acid-resistant personal protective equipment including face shield, apron and gauntlets, with local exhaust ventilation and gas detection where nitrogen oxides may accumulate.
Ensure emergency showers and eyewash are immediately available at the point of use, and refer to the current Safety Data Sheet before any handling operation.
Frequently asked questions
What is Nitrosylsulfuric Acid used for?
It is the diazotization reagent used for aromatic amines that are too weakly basic to be diazotized in ordinary aqueous mineral acid. Nitroanilines, chloronitroanilines and similar deactivated amines are handled this way, which makes it a standard reagent in azo pigment and disperse dye manufacture.
Why not just use sodium nitrite in aqueous acid?
For ordinary anilines that route works well. Weakly basic amines, however, are barely protonated and poorly soluble in dilute aqueous acid, so the reaction is slow and incomplete. Nitrosylsulfuric acid delivers the same nitrosating species in a strongly acidic non-aqueous medium in which those amines dissolve and react cleanly.
In what form is it supplied?
As a solution in sulfuric acid, most commonly at 40% strength. The pure compound is a solid and is not traded as such, so all specifications and dosing are quoted on the solution.
Why does it need heated storage?
The solution crystallises as it cools, and crystallised material is difficult to redissolve and to dose accurately. Storage and transfer lines are normally trace heated and held above the crystallisation point of the grade supplied. Confirm the figure for your grade on the Certificate of Analysis.
What happens if water gets in?
It hydrolyses, releasing nitrogen oxides and generating a great deal of heat. Water ingress is the principal process hazard with this material: keep the system closed and dry, and never add water to the acid.
What packaging is available?
Lined and glass-lined drums, heated IBCs, and road tankers for bulk supply. Trace-heated delivery can be arranged for customers without on-site heated storage.
How should it be stored, and for how long?
In sealed, heated, acid-resistant containers in a dry, well-ventilated store away from water, alkalis, organics and metals. Shelf life is 6 months from manufacture when heated storage is maintained, and assay should be checked before use on older stock.
Is it hazardous to handle?
Yes, severely. It causes severe skin burns and eye damage, reacts violently with water, and liberates toxic nitrogen oxides. It must be handled in a closed, dry system by trained operators with full acid personal protective equipment, and the current Safety Data Sheet must be reviewed before any operation.
