C.I. Pigment Red 49 (10-13-5) Physical and Chemical Properties
C.I. Pigment Red 49
Monosodium sulfonated azo pigment supplied as a red powder for industrial colorant formulations, suitable for ink, coating and plastics applications.
| CAS Number | 10-13-5 |
| Family | Sulfonated azo dye / pigment (sodium salt) |
| Typical Form | Powder or crystalline solid |
| Common Grades | EP |
C.I. Pigment Red 49 is an organic azo sulfonate pigment in the class of naphthol-based azo dyes; structurally it is the monosodium salt of a 1-naphthalenesulfonic acid bearing a 2-(2-hydroxy-1-naphthyl)azo substituent. The molecule contains two fused aromatic (naphthyl) systems linked by an azo (-N=N-) chromophore, a phenolic OH group adjacent to the azo linkage and a sulfonate substituent that is present as the sodium salt. These features produce a conjugated π-system responsible for strong visible absorption and the characteristic red hue, while the sulfonate introduces a permanently ionizable functionality that increases aqueous dispersibility in the sodium salt form.
Electronically, the azo linkage and adjacent hydroxy group provide resonance stabilization of the dye’s chromophore and permit intramolecular charge delocalization; the sulfonate is a strong electron-withdrawing, highly polar substituent. Acid–base behavior is dominated by the sulfonic acid (as anion in the sodium salt) and the phenolic OH (which may deprotonate at sufficiently high pH), so the material exists predominantly as a water-dissociated anion paired with Na+ under neutral conditions. The molecule is polar overall (topological polar surface area and multiple hydrogen-bond acceptors), with limited conformational flexibility (rotatable bond count = 3), characteristics that favor colloidal dispersion rather than partitioning into nonpolar media.
Commercially and industrially, Pigment Red 49 has been used as a colorant in coatings, inks and earlier cosmetic/drug color additive applications; the free acid and metal-complex “lake” forms are used where insoluble pigment particles are required. As a representative azo-sulfonate dye, it is chemically more resistant to simple hydrolysis than aliphatic sulfonates but is susceptible to reductive cleavage of the azo bond and to photodegradation under strong UV exposure. Common commercial grades reported for this substance include: EP.
Basic Physical Properties
Solubility and Hydration
C.I. Pigment Red 49 is the monosodium sulfonate salt of an azo-naphthol; as such the sodium form is significantly more dispersible in water than the corresponding free acid. The sulfonate group confers ionic character and promotes aqueous solubility or colloidal dispersion at typical working concentrations used in dyeing and ink formulations. The free acid (parent) is substantially less soluble in water and is typically isolated or used in lake/insoluble pigment forms for applications requiring particulate color.
No quantitative solubility or hydration data are available in the current data context.
Thermal Stability and Decomposition
Azo pigments of this structural class are generally thermally stable under moderate processing temperatures encountered in printing and coating curing; however, they can undergo decomposition at elevated temperatures where cleavage of the azo linkage or oxidation of the aromatic system becomes thermodynamically accessible. For use as pigments in polymers or coatings, thermal stability is often evaluated on a formulation-by-formulation basis.
No experimentally established decomposition temperature or detailed thermal data are available in the current data context.
Chemical Properties
Complex Formation and Coordination
The molecule contains a phenolic OH ortho to an azo group and a sulfonate functionality; such structural motifs permit formation of metal-complex “lake” pigments when treated with polyvalent metal salts (e.g., Al3+, Ca2+, or other mordants) to produce insoluble, pigmentary complexes. The sulfonate provides strong ionic anchoring for counterions, while the azo–hydroxy system can participate in coordination or hydrogen-bonding interactions that influence hue and fastness properties of manufactured lakes.
Reactivity and Stability
Reactivity is dominated by the azo linkage (reduction-prone) and the aromatic hydroxyl (electrophilic substitution and conjugate acid–base behavior). In reducing environments (chemical reductants, certain anaerobic biological systems), azo cleavage can yield aromatic amines; in strongly oxidative or photolytic conditions, the chromophore can be degraded, producing color loss. The sulfonate functionality confers resistance to simple hydrolysis but increases susceptibility to ion-exchange and salt formation. Under normal handling and ambient storage, the sodium salt is chemically stable; compatibility testing is recommended when formulating with strong reducing agents, strong oxidizers, or highly alkaline systems.
Molecular Parameters
Molecular Weight and Composition
- Molecular formula: C20H13N2NaO4S (as supplied)
- Molecular weight: 400.4 \(\mathrm{g}\,\mathrm{mol}^{-1}\) (reported)
- Exact mass: 400.04937236 Da (reported)
- Monoisotopic mass: 400.04937236 Da (reported)
- Heavy atom count: 28
- Covalently-bonded unit count: 2 (sodium counterion plus organic anion)
Additional computed descriptors: - Topological polar surface area (TPSA): 111 \(\text{Å}^2\) (reported) - Complexity: 653 - Formal charge: 0 - Hydrogen-bond donor count: 1 - Hydrogen-bond acceptor count: 6 - Rotatable bond count: 3
These parameters are consistent with a relatively large, planar, conjugated organic anion bearing high polarity from the sulfonate and multiple heteroatoms, promoting aqueous dispersion and limiting passive diffusion into nonpolar matrices.
LogP and Ionization State
No experimentally established octanol–water partition coefficient (log P) is available in the current data context. Qualitatively, the sodium sulfonate form is expected to have low affinity for nonpolar solvents (negative partitioning into octanol relative to water) because of the anionic sulfonate group and high TPSA; the free acid would be relatively more lipophilic.
Ionization state: the commercial form is the monosodium salt; in aqueous solution at neutral pH the compound exists predominately as the naphthalenesulfonate anion paired with Na+. The phenolic OH remains partly or fully protonated under neutral to acidic conditions and may deprotonate at elevated pH, affecting color and solubility.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS number: 10-13-5
- InChIKey: VVNRQZDDMYBBJY-UHFFFAOYSA-M
- InChI: InChI=1S/C20H14N2O4S.Na/c23-18-12-10-13-5-1-3-7-15(13)19(18)22-21-17-11-9-14-6-2-4-8-16(14)20(17)27(24,25)26;/h1-12,23H,(H,24,25,26);/q;+1/p-1
- SMILES: C1=CC=C2C(=C1)C=CC(=C2N=NC3=C(C4=CC=CC=C4C=C3)S(=O)(=O)[O-])O.[Na+]
- UNII: IY4YV0PVDA
- EC number: 214-998-2 (reported)
- DSSTox substance ID: DTXSID2021230
Synonyms and Structural Names
Reported synonyms and trade-style names (selected from recorded identifiers) include: - sodium 2-[(2-hydroxynaphthalen-1-yl)diazenyl]naphthalene-1-sulfonate - Sodium 2-((2-hydroxynaphthyl)azo)naphthalenesulphonate - Sodium lithol - Lithol Red R; Lithol Red; Lithol Red sodium salt; Lithol Toner - Resamine Red RC; Resamine Red RB - D&C Red No. 10; D and C Red No. 10 (historical/delistings noted) - CI 15630; C.I. Pigment Red 49 - Pigment red 49 (c.i. 15630) - Sodium Lithol Red - 1-Naphthalenesulfonic acid, 2-((2-hydroxy-1-naphthalenyl)azo)-, monosodium salt
(These synonyms are representative and include common names, trade names and Color Index identifiers used in industrial contexts.)
Industrial and Commercial Applications
Use as Salt Form or Excipient
As the monosodium sulfonate salt, Pigment Red 49 is used where anionic water-dispersible red colorants are required. The sodium salt form is chosen when aqueous solubility or colloidal dispersion is desired (e.g., inks, some textile applications), whereas the free acid or metal-complex lake forms are used to produce insoluble pigment particles suitable for plastics, coatings and paper systems.
Representative Use Cases
- Cosmetic and personal-care formulation history: historically listed as a cosmetic colorant and as a D&C (drug and cosmetic) red number in earlier regulatory classifications; many jurisdictions now have restricted or delisted food and some cosmetic uses.
- Pigment manufacture: conversion to lake pigments (metal complexation) to obtain insoluble pigments for inks, plastisols, and coatings.
- Coloration of specialty paper, inks and nonfood consumer products where a stable, bright red is required.
No concise application summary beyond these class-level uses is available in the current data context; in practice selection is driven by desired solubility (salt vs. lake), substrate compatibility and regulatory acceptance for the intended end use.
Safety and Handling Overview
Toxicological Considerations
Reported hazard classifications for this substance indicate "Not Classified" under standard GHS hazard categories in available classifications. A human-health tier 1 assessment has been reported for the chemical identity. As with many azo dyes, potential concerns under certain conditions include the formation of aromatic amine cleavage products under reductive degradation; therefore, formulation and end-use assessments should consider the potential for reductive breakdown and the toxicity profiles of any resulting fragments. For occupational exposure, dust control and prevention of inhalation, dermal contact minimization and avoidance of ingestion are standard precautions.
Storage and Handling Guidelines
General industrial precautions appropriate for solid azo sulfonate salts apply: store in a cool, dry, well-ventilated area away from strong oxidizers and strong reducing agents; keep container tightly closed to avoid moisture uptake and dust generation. Use appropriate personal protective equipment (gloves, eye protection, protective clothing) and engineering controls (local exhaust ventilation) to limit inhalation and dermal exposure. Avoid uncontrolled release to waterways. For detailed hazard, transport and regulatory information, users should refer to the product-specific Safety Data Sheet (SDS) and applicable local legislation.
