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2-Aminonaphthalene-1-sulphonic acid is an aminonaphthalene sulfonic acid best known by the established industrial name Tobias acid.
The molecule combines a primary aromatic amino group and a sulfonic acid group on adjacent positions of a naphthalene framework, giving it the dual reactivity and ionic character required in important azo dye and pigment chemistry.
2-Aminonaphthalene-1-sulphonic acid is principally used as a diazo intermediate for colorants, particularly Tobias-acid-derived azo pigments and dyes including the Pigment Red 49 family and Acid Yellow 19.
CHEMICAL IDENTITY AND COMMON NAMES
2-Aminonaphthalene-1-sulphonic acid contains an amino group at the 2-position and a sulfonic acid group at the adjacent 1-position of the naphthalene ring system.
The positional relationship of these groups is important because other aminonaphthalenesulfonic acid isomers possess separate CAS numbers, different traditional names, and different dye-intermediate applications.
The established common name Tobias acid is widely used in dyestuff and pigment manufacturing.
2-Naphthylamine-1-sulfonic acid is another recognized structural name for the same chemical identity.
2-Aminonaphthalene-1-sulphonic acid should not be confused with 1-aminonaphthalene-2-sulfonic acid, naphthionic acid, Laurent acid, Dahl acid, Bronner acid, Cleve acids, or other positional isomers within the aminonaphthalenesulfonic acid family.
The exact substitution pattern is essential when selecting a material for diazotization and subsequent azo coupling.
2-Aminonaphthalene-1-sulphonic acid should also be distinguished from 2-aminonaphthalene itself.
The sulfonic acid group changes water and alkali behaviour, molecular weight, processability, environmental mobility, and the chemistry of downstream colorant manufacture.
Synonyms and Common Names: Tobias acid, Tobias' acid, 2-Amino-1-naphthalenesulfonic acid, 2-Amino-1-naphthalenesulphonic acid, 2-Naphthylamine-1-sulfonic acid, 2-Naphthylamine-1-sulphonic acid, 1-Naphthalenesulfonic acid, 2-amino-, 1-Naphthalenesulphonic acid, 2-amino-, 1-Sulfo-2-naphthylamine, 1-Sulpho-2-naphthylamine, 2-Aminonaphthalene-1-sulfonic acid, 2-Aminonaphthalene-1-sulphonic acid, 2-Aminonaphthalenesulfonic acid, 2-Naphthylaminesulfonic acid, NSC 5523, Tobiassäure
TECHNICAL IDENTIFICATION
CAS Number: 81-16-3
EC / EINECS Number: 201-331-5
Molecular Formula: C10H9NO3S
Molar Mass: 223.25 g/mol
Chemical Class: Aminonaphthalenesulfonic acid
Functional Groups: Primary aromatic amine, sulfonic acid
Common Industrial Name: Tobias acid
PubChem CID: 6670
InChIKey: GWIAAIUASRVOIA-UHFFFAOYSA-N
PHYSICAL AND CHEMICAL PROPERTIES
Appearance: White to light-coloured crystalline solid or powder
Physical State: Solid
Molecular Formula: C10H9NO3S
Molar Mass: 223.25 g/mol
Melting Behaviour: High-melting solid; commercial references commonly report values around 180 °C or higher depending on product form and test conditions
Water Solubility: Limited in the free-acid form; solubility increases in hot water and after conversion to suitable salts
Alkali Solubility: Readily converted into more water-compatible sulfonate salts under alkaline conditions
Log Pow: Approximately -1.16
Volatility: Very low under normal ambient conditions
Acid Functionality: Sulfonic acid
Amine Functionality: Primary aromatic amine
Chemical Character: Amphoteric aromatic sulfonic acid
Diazotization Reactivity: Readily converted at the amino group to the corresponding diazonium intermediate under controlled acidic conditions
Combustion Behaviour: Thermal decomposition or combustion can generate irritating nitrogen- and sulfur-containing decomposition products
The sulfonic acid group gives 2-Aminonaphthalene-1-sulphonic acid strong acidic character and makes salt formation an important part of its process chemistry.
Neutralization with suitable alkali produces corresponding sulfonate forms with improved compatibility in aqueous reaction systems.
The amino group provides the principal reaction site for diazotization.
Conversion to the diazonium form allows subsequent coupling with aromatic coupling components to produce strongly conjugated azo colorants.
The fused naphthalene ring contributes aromatic conjugation and provides a larger chromophoric framework than simple aniline sulfonic acid intermediates.
This structural characteristic is particularly important in the development of red and related azo colorants.
FUNCTIONAL CHARACTERISTICS
2-Aminonaphthalene-1-sulphonic acid functions primarily as a diazo component in azo colorant manufacture.
The primary aromatic amino group can be transformed into a diazonium group using controlled nitrosation under acidic conditions.
The resulting diazonium intermediate readily undergoes azo coupling with electron-rich aromatic compounds.
2-Naphthol is one of the most important coupling components associated with Tobias-acid pigment chemistry.
The sulfonic acid group remains incorporated into many downstream azo structures.
It contributes ionic character and enables preparation of sodium, calcium, barium, strontium, and other metal-salt forms of selected azo colorants.
This metal-salt-forming behaviour is especially important for lake pigment manufacture.
Conversion of a soluble or partially soluble azo sulfonate into an appropriate metal salt can change pigment insolubility, shade, crystal structure, dispersibility, and application behaviour.
The 1-sulfonic acid group also influences the regioelectronic behaviour of the naphthalene ring.
Its position relative to the amino group is therefore an essential part of the characteristic colour chemistry of 2-Aminonaphthalene-1-sulphonic acid.
PRODUCTION AND COMMERCIAL FORM
A well-established production route to 2-Aminonaphthalene-1-sulphonic acid begins with 2-naphthol.
Controlled sulfonation produces 2-hydroxynaphthalene-1-sulfonic acid, historically known as oxy-Tobias acid or Stebbins acid.
The hydroxy intermediate is converted to the amino compound through Bucherer-type amination.
Ammonia together with a sulfur-dioxide-derived reagent enables replacement of the hydroxy group by an amino group while preserving the sulfonic acid substitution pattern.
Industrial processes can perform this conversion in aqueous ammoniacal media under elevated temperature and pressure.
Reaction conditions are selected to obtain useful conversion while limiting desulfonation, residual 2-naphthol, unwanted positional isomers, and coloured by-products.
The resulting 2-Aminonaphthalene-1-sulphonic acid can be isolated by controlled acidification of its soluble salt.
Precipitation, filtration, washing, and drying then provide the free-acid product.
Industrial material may be supplied as dry powder, crystalline solid, wet cake, or another process-compatible form.
The moisture basis and active-content basis are therefore important when comparing commercial grades.
Purity is particularly important for colorant manufacture because residual naphthol, oxy-Tobias acid, positional isomers, inorganic salts, and coloured impurities can influence diazotization, coupling, filtration, shade, and pigment performance.
APPLICATIONS AND INDUSTRIES
AZO DYE INTERMEDIATE
2-Aminonaphthalene-1-sulphonic acid is an established intermediate for azo dye manufacture.
Its amino group provides the diazotizable functionality required to create an azo bond with a suitable coupling component.
Diazotization converts 2-Aminonaphthalene-1-sulphonic acid into a highly reactive diazonium intermediate.
Controlled addition to an activated aromatic coupling component then produces an extended conjugated azo structure.
The sulfonic acid group frequently remains part of the final dye molecule.
This can increase ionic character and support processing in aqueous dye-manufacturing systems.
The final colour depends on the coupling component, additional substituents, salt form, pH, conjugation pattern, and molecular environment around the azo group.
2-Aminonaphthalene-1-sulphonic acid therefore provides a defined diazo component from which several technically distinct colorants can be produced.
PIGMENT RED 49 AND LITHOL RED CHEMISTRY
One of the most important established uses of 2-Aminonaphthalene-1-sulphonic acid is the manufacture of the Pigment Red 49 family, historically known as Lithol Red pigments.
These pigments are formed through diazotization of 2-Aminonaphthalene-1-sulphonic acid followed by coupling with 2-naphthol.
The initial azo structure contains the sulfonate functionality originating from 2-Aminonaphthalene-1-sulphonic acid.
Conversion into selected metal salts produces the characteristic lake pigments used within the Pigment Red 49 family.
Sodium, barium, calcium, and strontium forms are historically important members of this chemistry.
Changing the counter-ion affects crystal form, insolubility, colour characteristics, dispersibility, and performance in pigment applications.
Pigment Red 49 materials have been associated with printing inks, paints, coatings, artists' colours, and other pigmentation systems.
2-Aminonaphthalene-1-sulphonic acid serves as the upstream chemical intermediate rather than being incorporated directly into these finished products as an unreacted additive.
LAKE PIGMENT MANUFACTURE
2-Aminonaphthalene-1-sulphonic acid is particularly important in classical azo lake pigment chemistry.
After diazotization and coupling, the sulfonated azo dye can be converted into a sparingly soluble metal salt to obtain a pigment.
Calcium, barium, strontium, manganese, and other suitable cations have been used in lake-pigment chemistry depending on the specific colorant.
Metal-ion selection can influence particle formation, hue, transparency, crystal modification, heat response, and resistance properties.
Lake formation also permits a water-processable azo intermediate to be converted into a pigment suitable for dispersion in non-aqueous binders.
This transformation historically helped extend Tobias-acid chemistry into printing inks, coatings, paints, and related pigment applications.
PRINTING INK PIGMENTS
Pigments derived from 2-Aminonaphthalene-1-sulphonic acid have long been associated with printing-ink colouration.
Lithol Red pigment salts provide red shades that can be dispersed into suitable ink vehicles.
Performance in ink depends on pigment particle size, crystal form, counter-ion, surface treatment, dispersion, binder compatibility, and printing process.
The quality of 2-Aminonaphthalene-1-sulphonic acid can influence the purity and consistency of the azo intermediate from which the pigment is produced.
Impurities that interfere with diazotization or coupling can affect pigment shade and strength.
Reliable assay and control of related aromatic compounds are therefore useful for consistent ink-pigment manufacture.
PAINT AND COATING PIGMENTS
Tobias-acid-derived azo pigments have also been used in paint and coating systems.
The lake-pigment form provides the low aqueous solubility required for dispersion as a particulate colorant.
Pigment performance is determined by more than colour alone.
Particle morphology, counter-ion, crystal structure, binder interaction, light exposure, chemical environment, and processing conditions influence practical coating performance.
2-Aminonaphthalene-1-sulphonic acid is consequently selected as a synthesis intermediate according to the pigment chemistry required downstream.
High-quality intermediate control supports more reproducible pigment precipitation and final colour.
ACID YELLOW 19
2-Aminonaphthalene-1-sulphonic acid is an established intermediate associated with the manufacture of C.I. Acid Yellow 19.
Its diazotizable aromatic amine functionality provides one of the building blocks needed for the azo chromophore.
Acid-dye production uses aqueous reaction chemistry in which pH, diazotization temperature, nitrite addition, coupling conditions, and salt concentration require careful control.
The sulfonic acid group supports the ionic character needed for processing and use of appropriate downstream dye structures.
TEXTILE COLORANT MANUFACTURE
2-Aminonaphthalene-1-sulphonic acid contributes indirectly to textile coloration through dyes manufactured from its diazonium chemistry.
The final dye structure determines fibre affinity, application pH, exhaustion, shade, wet fastness, and other textile properties.
Its use is therefore concentrated at the intermediate-manufacturing stage.
Textile formulators generally work with the completed dye rather than with unreacted 2-Aminonaphthalene-1-sulphonic acid.
ORGANIC PIGMENT INDUSTRY
2-Aminonaphthalene-1-sulphonic acid is useful to organic-pigment producers that manufacture sulfonated monoazo lake pigments.
The combination of a diazotizable naphthylamine group and a metal-binding sulfonate functionality makes the molecule particularly suitable for this class of pigment chemistry.
Process control at the intermediate stage influences the subsequent diazo solution.
Unreacted starting material, particulate impurities, excess mineral acid, inorganic salts, and side products can affect coupling efficiency and pigment precipitation.
SPECIALTY AZO SYNTHESIS
Beyond its major historical colorants, 2-Aminonaphthalene-1-sulphonic acid can be used as a defined aromatic diazo building block in specialty azo synthesis.
Selection of different coupling partners permits preparation of compounds with modified absorption, ionic behaviour, and functional properties.
These uses are most relevant where the specific 2-amino-1-sulfonated naphthalene framework is required.
Closely related aminonaphthalenesulfonic acid isomers cannot be substituted automatically because positional isomerism changes coupling chemistry and downstream molecular structure.
CHEMICAL RESEARCH
2-Aminonaphthalene-1-sulphonic acid is used in research involving aromatic diazotization, azo coupling, naphthalene sulfonic acid chemistry, and structure-colour relationships.
Its established substitution pattern makes it a useful representative of industrial aminonaphthalenesulfonic acids.
Research applications can examine diazonium stability, coupling kinetics, pH effects, chromophore formation, metal-salt precipitation, and spectroscopic behaviour.
High-purity material is useful where unidentified isomers or process impurities would interfere with interpretation.
ANALYTICAL REFERENCE AND PROCESS CONTROL
2-Aminonaphthalene-1-sulphonic acid can serve as an analytical reference in colorant manufacturing and environmental testing.
Chromatographic techniques can distinguish it from related naphthalene sulfonic acids, residual starting materials, and downstream reaction products.
Process laboratories can monitor residual 2-Aminonaphthalene-1-sulphonic acid during diazotization or coupling to assess reaction completion.
This information supports optimization of raw-material ratios and reduction of unreacted intermediate in final products.
Analytical measurement can also support wastewater management at dye and pigment facilities.
Tracking the parent intermediate helps evaluate material loss, recovery opportunities, and treatment performance.
DYE-MANUFACTURING WASTEWATER MONITORING
Sulfonated aromatic intermediates can enter process wastewater through filtration, washing, transfer, and incomplete reaction.
2-Aminonaphthalene-1-sulphonic acid can therefore be an appropriate target analyte when assessing wastewater from facilities using Tobias-acid chemistry.
Its sulfonic acid functionality strongly influences environmental behaviour.
Ionized forms can display different mobility and sorption behaviour from nonsulfonated aromatic amines.
Monitoring can help distinguish parent intermediate from finished dyes, pigments, coupling components, and transformation products.
This is useful when optimizing treatment systems or evaluating recovery from concentrated process liquors.
RECOVERY FROM PROCESS STREAMS
Industrial processes can recover 2-Aminonaphthalene-1-sulphonic acid or its salts from suitable aqueous streams.
Acid-base control, precipitation, extraction of organic impurities, adsorption, membrane separation, or related operations may be incorporated depending on stream composition.
Recovery can improve raw-material efficiency while reducing organic load in wastewater.
The economic value depends on concentration, purity, salt content, contaminant profile, and whether the recovered material can be returned to the intended process.
GRADE SELECTION AND PRODUCT SUITABILITY
Assay is a principal purchasing parameter for 2-Aminonaphthalene-1-sulphonic acid used in dye or pigment synthesis.
Accurate active content supports controlled diazotization stoichiometry and reproducible coupling.
Moisture is particularly important because industrial material can be supplied as dry powder or wet cake.
A nominal mass of wet material contains less active 2-Aminonaphthalene-1-sulphonic acid than the same mass of dry product.
Specifications should therefore identify whether assay is stated on an as-is, dry, or active basis.
This distinction is necessary for accurate raw-material charging and commercial comparison.
Residual 2-naphthol can be important because it originates from upstream production and can interfere with downstream azo chemistry.
Uncontrolled residual coupling component may affect colour development or introduce additional organic impurities.
Oxy-Tobias acid is another relevant process-related impurity.
Its hydroxyl group does not undergo diazotization in the same manner as the required amino functionality, so excessive residual precursor reduces effective reactive content.
Positional isomers are also important.
Aminonaphthalenesulfonic acid isomers possess similar elemental compositions but generate different diazonium and azo structures.
Colour and appearance provide practical supplementary quality information.
Unexpectedly dark material can indicate elevated coloured by-products, oxidation products, or incomplete purification.
Inorganic salts may remain from neutralization, reduction, acidification, or washing operations.
Excess salts can affect active-content calculations, dissolution, diazotization medium, filtration, and wastewater loading.
FORMULATION AND PROCESS CONSIDERATIONS
Diazotization of 2-Aminonaphthalene-1-sulphonic acid requires controlled acidic conditions.
The amino group is converted to the corresponding diazonium species using nitrous acid generated from an appropriate nitrite source.
Temperature control is important because aromatic diazonium compounds can decompose when exposed to unsuitable conditions.
Industrial processes commonly keep diazotization systems cool and monitor nitrite consumption and reaction completion.
pH determines both amine protonation and the behaviour of the sulfonate group.
Acid concentration must therefore support effective diazotization without unnecessarily promoting side reactions or difficult downstream neutralization.
Coupling with 2-naphthol or another coupling component generally requires a pH range appropriate to activation of the coupling partner.
The diazo solution and coupling solution are therefore frequently prepared separately before controlled combination.
Addition rate can influence local concentration and pigment nucleation.
Uniform mixing helps prevent zones of excess diazonium compound or coupling component that could produce variable shade or particle structure.
Lake-pigment manufacture introduces a further precipitation stage.
Counter-ion concentration, temperature, pH, salt content, agitation, aging, and filtration can all influence formation of the final pigment.
Free-acid 2-Aminonaphthalene-1-sulphonic acid has limited cold-water solubility.
Conversion to an appropriate salt or controlled alkaline dissolution can improve aqueous processability before reaction.
Excessively alkaline or severe thermal conditions should be avoided when they can promote unwanted changes in the aromatic intermediate.
Process design should maintain the substitution pattern required for downstream colour chemistry.
QUALITY, SPECIFICATIONS AND DOCUMENTATION
Relevant quality parameters for 2-Aminonaphthalene-1-sulphonic acid include assay, appearance, moisture, residual 2-naphthol, residual hydroxy precursor, positional isomers, insoluble matter, and inorganic salt content.
The exact specification should reflect the intended dye or pigment synthesis.
Assay provides the principal measure of available Tobias-acid content.
Accurate assay is especially important because diazotization consumes the amino group on a molar basis.
Moisture analysis supports correct conversion between gross product weight and active chemical content.
This is particularly important for wet-cake grades.
Chromatographic methods are useful for distinguishing the parent compound from related aromatic impurities.
HPLC can provide valuable information about residual precursors and positional isomers that cannot be assessed reliably from total nitrogen or simple titration alone.
Diazotization-value or equivalent functional analysis can also provide useful process information.
Such testing measures the reactive aromatic amine content relevant to downstream azo synthesis.
Appearance and colour can complement quantitative chemical tests.
A clean white to light-coloured product generally supports colour-sensitive downstream processing more effectively than material containing substantial dark organic impurities.
A Certificate of Analysis provides batch-specific values for the parameters included in the agreed product specification.
A Technical Data Sheet provides relevant product characteristics, while the Safety Data Sheet provides workplace handling, exposure-control, storage, transport, and emergency information.
SAFETY AND REGULATORY CONSIDERATIONS
2-Aminonaphthalene-1-sulphonic acid is classified in current supplier safety documentation as causing serious eye irritation.
Industrial handling should therefore prevent dust or particulate material from contacting the eyes.
Dust generation should be minimized during charging, weighing, sampling, transfer, and packaging.
Effective local exhaust ventilation is useful where dry powder can become airborne.
Direct skin contact should also be minimized through appropriate gloves, protective clothing, and workplace hygiene.
Contaminated clothing should be removed and cleaned before reuse.
2-Aminonaphthalene-1-sulphonic acid is a separate chemical identity from 2-aminonaphthalene.
The presence of a sulfonic acid group changes the physicochemical and toxicological profile and the two materials should not be treated as interchangeable substances.
The material is not generally classified as a dangerous good for routine transport in current supplier documentation.
Normal chemical packaging should nevertheless protect the product from moisture, contamination, physical damage, and uncontrolled release.
Thermal decomposition can produce irritating gases and vapours.
Fire response should therefore use appropriate respiratory protection and extinguishing media suitable for the surrounding fire.
German water-hazard classification assigns 2-Aminonaphthalene-1-sulphonic acid to WGK 1.
Process discharge should still be controlled, particularly in dye and pigment operations where wastewater can contain multiple aromatic intermediates and colorants.
FIRST AID
Inhalation: Move the exposed person to fresh air and keep at rest.
Obtain medical attention if respiratory discomfort or other symptoms occur.
Skin Contact: Remove contaminated clothing and wash affected skin thoroughly with plenty of water and soap.
Obtain medical attention if irritation develops or persists.
Eye Contact: Immediately rinse cautiously with plenty of clean water for at least several minutes, including beneath the eyelids.
Remove contact lenses when easy to do, continue rinsing, and obtain medical attention if irritation persists.
Ingestion: Rinse the mouth thoroughly with water.
Obtain medical advice if a significant quantity has been swallowed or symptoms develop.
Note to Physicians: Treat according to the route of exposure and observed clinical condition.
Eye irritation is the principal classified acute hazard in current supplier safety information.
HANDLING AND STORAGE
Handling: Avoid generating 2-Aminonaphthalene-1-sulphonic acid dust and prevent direct eye and unnecessary skin contact.
Use controlled charging, sampling, and transfer procedures.
Ventilation: Provide effective general ventilation and local exhaust ventilation where powder handling can generate airborne particulate matter.
Storage: Store 2-Aminonaphthalene-1-sulphonic acid in tightly closed containers in a cool, dry, well-ventilated location.
Protect the material from moisture, contamination, excessive heat, and incompatible reactive substances.
Incompatibilities: Keep separated from strong oxidizing agents and other materials capable of undesirable reaction with aromatic amines or sulfonic acids.
Packaging: Use clean, dry, chemically compatible packaging that prevents contamination, moisture uptake, and dust release during transport and storage.
PACKAGING AND PROCUREMENT CONSIDERATIONS
2-Aminonaphthalene-1-sulphonic acid is primarily purchased as an industrial dye and pigment intermediate.
The most important commercial parameters normally relate to reactive content, impurity profile, moisture, physical form, and consistency between batches.
CAS Number 81-16-3 should be specified clearly in procurement documentation.
This avoids confusion with other aminonaphthalenesulfonic acid positional isomers that may share similar common naming patterns.
Tobias acid is an established and useful commercial name, but registry identity remains important when purchasing internationally.
The location of the amino and sulfonic acid groups determines downstream colour chemistry.
Dry-powder grades provide high active-content density and easier active-material calculations.
Wet-cake grades can reduce drying requirements at the production site and may offer practical advantages where the receiving process already uses aqueous or alkaline dissolution.
Moisture must be considered when comparing these forms commercially.
A wet-cake specification should state the active-content basis so that buyers can calculate actual chemical consumption accurately.
Pigment Red 49 manufacturers can place particular emphasis on assay, diazotizable amine content, residual 2-naphthol, positional isomers, and coloured impurities.
These parameters influence coupling yield, lake formation, pigment shade, and batch reproducibility.
Acid-dye manufacturers can additionally emphasize water-compatible processing, inorganic salt content, solution clarity after neutralization, and control of impurities that interfere with diazotization.
Particle size can affect charging, dissolution rate, dust generation, and filtration.
A consistent physical form supports more predictable plant handling even when particle size is not a primary chemical specification.
Packaging size should correspond to production scale and batch consumption.
Appropriate liners and moisture-resistant outer packaging help maintain product quality during warehousing and transportation.
Procurement documentation can include assay, test basis, moisture, appearance, relevant organic impurity limits, inorganic salts, insoluble matter, physical form, packaging, and required quality documents.
The specification should reflect the downstream colorant rather than imposing unrelated analytical limits.
Ataman Kimya can support enquiries for 2-Aminonaphthalene-1-sulphonic acid concerning grade selection, assay, moisture basis, impurity requirements, dye and pigment applications, documentation, packaging, and supply requirements.
For product and procurement information, contact Ataman Kimya at +90 216 577 10 10 or [info@atamankimya.com](mailto:info@atamankimya.com).
