Nitrosyl Sulfuric Acid
A diazotising agent supplied as a solution in sulfuric acid, used where weakly basic aromatic amines cannot be diazotised in aqueous acid.
What is Nitrosyl Sulfuric Acid?
Nitrosyl Sulfuric Acid — NSA — is supplied as a colourless to yellow solution of the nitrosyl compound in sulfuric acid. It is not handled as a solid in normal industrial practice.
It exists to solve one specific problem. Ordinary diazotisation uses sodium nitrite in dilute aqueous acid, which works well for anilines but fails for weakly basic amines that will not dissolve or protonate properly — the nitroanilines, dichloroanilines, and heavily substituted colour bases that dominate modern pigment chemistry. NSA carries the nitrosyl group in a strongly acidic, water-poor medium where those amines can be diazotised cleanly.
At Chelora Chemicals, NSA is supplied to a controlled assay with free acid and stability specified. It is a highly corrosive material that decomposes on warming and must be kept cold and used promptly.
- Appearance
- Colourless to yellow solution in sulfuric acid
- Odour
- Pungent, acidic
- Solubility
- Reacts with water; miscible with sulfuric acid
- Function
- Diazotising agent
- Grade
- Solution in sulfuric acid
Formula diagram
A nitrosyl group carried on a sulfuric acid backbone
- Molecular formulaNOHSO4
- Molecular weight127.08 g/mol
- IUPAC nameNitrosylsulfuric acid
- Functional groupsNitrosyl, sulfate ester acid
- CAS number7782-78-7
Technical product information
Standard specification for Chelora Chemicals' solution grade Nitrosyl Sulfuric Acid. Custom-grade and bulk specifications are available on request.
| Chemical name | Nitrosyl Sulfuric Acid |
|---|---|
| Synonyms | NSA, Nitrosylsulphuric acid, Nitrosyl hydrogen sulfate, Chamber crystals |
| CAS number | 7782-78-7 |
| Molecular formula | NOHSO4 |
| Molecular weight | 127.08 g/mol |
| Form supplied | Solution in sulfuric acid — concentration to order |
| Assay (NOHSO4) | To agreed specification, typically 40% class |
| Appearance | Colourless to yellow clear solution |
| Free sulfuric acid | Balance of solution |
| Storage temperature | Cool, controlled — see handling notes |
| Packaging | 250 kg drums, IBC, dedicated tanker |
| Shelf life | Limited — use promptly, re-assay before extended storage |
Applications of Nitrosyl Sulfuric Acid
Diazotisation of weak amines
Diazotises nitroanilines, dichloroanilines, and other weakly basic colour bases that fail in aqueous nitrite systems.
Azo pigment manufacture
The standard diazotising route for heavily substituted pigment intermediates.
Azo dye manufacture
Used across dye synthesis where the amine will not diazotise conventionally.
Heterocyclic amine diazotisation
Applied to aminothiazoles and related weakly basic heterocycles.
Specialty synthesis
Used in Sandmeyer and related transformations requiring a non-aqueous diazonium route.
Fine chemical manufacture
Supplied where controlled, strongly acidic diazotisation is required.
Why choose Chelora Chemicals
Every batch of Nitrosyl Sulfuric Acid 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
Assay and free acid controlled to the agreed solution specification and verified before dispatch.
Full documentation
COA, SDS, and regulatory data supplied with every shipment.
Flexible volumes
From 250 kg drums to IBC and dedicated tanker supply.
Technical support
Process, handling, and compliance guidance from our chemistry team.
Storage and guidelines
- 01Store cold and in dedicated, clearly labelled, bunded containers — the product decomposes on warming.
- 02Exclude moisture; contact with water hydrolyses the product and releases nitrogen oxides.
- 03Vent storage safely: decomposition generates gas, and sealed containers can pressurise.
- 04Segregate rigorously from water, alkalis, organic materials, reducing agents, and combustibles.
- 05Use appropriate materials: glass, glass-lined steel, or PTFE. Avoid ordinary carbon steel in wet service.
- 06Order to consumption and use promptly; this is not a product to hold in long-term inventory.
Handling precautions
Nitrosyl sulfuric acid causes severe skin burns and eye damage and is a strong oxidiser. It is for industrial use by trained personnel only.
It decomposes on warming and on contact with water, releasing nitrogen oxides. NOx exposure is dangerous and its effects can be delayed by several hours, so anyone with a suspected inhalation exposure needs medical observation even if they feel well.
Because decomposition generates gas, containers and reactors must be adequately vented. A sealed, warming drum is a pressure hazard.
Contact with organic material, reducing agents, and combustibles can be vigorous. Diazotisation reactions using NSA are exothermic and must be run with controlled addition, effective cooling, and temperature monitoring.
Diazonium compounds formed in these reactions are themselves potentially unstable and should never be isolated dry or allowed to warm.
Full acid suit, face shield, chemical goggles, and appropriate respiratory protection are required. Emergency showers and eyewash must be immediately accessible.
Refer to the full Safety Data Sheet for exposure limits, decomposition behaviour, spill response, and first-aid measures before use.
Frequently asked questions
What is nitrosyl sulfuric acid used for?
Diazotisation of weakly basic aromatic amines. Where sodium nitrite in dilute aqueous acid fails, NSA supplies the nitrosyl group in a strongly acidic, water-poor medium that gets the reaction to work.
Why can't sodium nitrite be used for these amines?
Conventional diazotisation needs the amine dissolved and protonated in dilute aqueous acid. Nitroanilines, dichloroanilines, and similar bases are too weakly basic and too insoluble for that, so the reaction stalls or gives poor conversion.
Which amines typically need NSA?
Nitroanilines, chloro- and dichloro-nitroanilines, heavily substituted colour bases, and weakly basic heterocyclic amines such as aminothiazoles. Many of the pigment bases in this catalogue fall into that group.
Is it supplied as a solid?
No. Although the pure compound is crystalline, it is supplied and used industrially as a solution in sulfuric acid, which is safer to handle and easier to meter into a reaction.
Why must it be kept cold?
It decomposes on warming, losing assay and releasing nitrogen oxides. Cold, vented, dedicated storage and prompt use are the practical controls, and long inventory holds are not advisable.
What happens on contact with water?
It hydrolyses, generating nitrogen oxides and heat. That is why moisture exclusion matters both for product quality and for safety, and why water is not used in spill response.
Why is NOx exposure treated so seriously?
Nitrogen oxides damage the deep lung, and the effects can be delayed by hours after an exposure that seemed minor at the time. Anyone with a suspected exposure needs medical observation regardless of how they feel initially.
What are the process safety concerns in use?
The diazotisation itself is exothermic and needs controlled addition and effective cooling. The diazonium products formed are potentially unstable, must be kept cold and in solution, and should never be isolated dry.
What materials of construction are suitable?
Glass, glass-lined steel, and PTFE. Avoid ordinary carbon steel where moisture may be present, and confirm gasket and seal compatibility before commissioning any transfer line.
What concentration do you supply?
Solution concentration is set to order against your process requirement. Specify the assay and free acid you need on enquiry, and the Certificate of Analysis will state both.
What packaging is available?
250 kg drums, IBCs, and dedicated tankers. Supply requires confirmation of the receiving site's cold storage, venting, and emergency arrangements.
Do you provide a Certificate of Analysis?
Yes. Every consignment ships with a Certificate of Analysis and Safety Data Sheet stating assay, free sulfuric acid, appearance, and the transport documentation required.