4-Nitro Aniline-2 Sulfonic Acid belongs to the aromatic amine sulfonic acids family. Its bright yellow to brown shade stands out in both powdered and crystalline forms. Over years of handling, I have found that the powder structure often clumps, which responds sharply once handling technique slips—use gloves, mind lab coats, and keep moisture out of reach. The smell tends toward faint must at first, but after being left open, it thickens into a sharper odor. Synthesis uses aniline and sulfuric acid, leading to a compound that carries both nitro and sulfonic acid groups attached to the benzene ring. Out in the field, you notice granules collect static; store it away from electronics and at controlled humidity.
Its molecular formula is C6H6N2O5S, and sometimes, chemists render it as C6H5N2O5S when accounting for residual moisture. The structure features a benzene core with nitro (-NO₂) at the 4-position and sulfonic acid (-SO₃H) at the 2-position. This arrangement makes it both electron-deficient and good at attracting polar solvents. During lab work, the nitro group draws attention, both because it flags hazard and because its strong electron-withdrawing nature raises questions about stability.
4-Nitro Aniline-2 Sulfonic Acid appears in forms including crystalline flakes, granules, or fine powder. Density ranges from roughly 1.7 to 1.9 g/cm³ depending on water content. No matter the form, direct skin contact brings irritation, so always double glove. Its melting point can sit above 260℃, but decomposition kicks in before true melting sets in—expect a colored, acrid smoke as the first warning sign. It dissolves in water to produce a bright yellow solution, though solubility remains modest compared to straight sulfonic acids. As a solid, it resists many nonpolar solvents, which helps during cleanup; a little ethanol sometimes removes any stubborn residue from benchtop surfaces. The HS Code for global trade sits under 292142 as part of aromatic amines and their derivatives.
Manufacturers supply it mainly as dense flakes or free-flowing powder. Flakes break up easily under gentle pressure. In powder form, static charges make it stick to glass and gloves—an experience shared by any chemist pouring it from a brown glass bottle. Pearls sometimes indicate a more purified product, but impurities can scatter throughout the batch. Crystals form after a slow, controlled cooling in saturated solutions, and these catch the light in a pale yellow tint. In solution, it displays a translucent golden color; concentrations matter for handling, so label everything, as accidental mixing poses risks. Liquid product remains rare, mostly used in larger, continuous processing plants that demand steady feedstocks.
The chemical puts both environmental and health risks front and center. Dust in the air causes throat and eye discomfort, and ingestion results in nausea, so never lower safety glasses for even a quick look. Prolonged skin exposure can lead to rashes or deeper tissue irritation. Chronic inhalation—something you only realize after years—affects upper respiratory tracts. Handling always calls for N95 masks or better, lab aprons, and a well-ventilated fume hood. Waste follows protocols for both nitro and sulfonic organic compounds, so secure containers and double-bagging before disposal. Fire risk seems low, though strong oxidizers can trigger wild reactions that you never want to see outside a blast shield.
Used mainly as an intermediate in dye manufacturing, this acid features in the synthesis of azo dyes and complex pigments for textiles and inks. It enters further steps—in many cases diazotization—before attachment to aromatic rings to form various colorants. Experience shows that purity creates clarity in final hues; traces of by-products result in muddy tones after even careful application. Detergent labs sometimes test it for specialized surfactants, where its sulfonic side brings water solubility while the nitro group changes molecular interaction rates. In agricultural labs, it rarely appears, but specialty pesticide tests use it as a model for building blocks.
Regulatory agencies, including REACH and OSHA, mark the acid as hazardous. Shipping requires all proper documentation and UN status for hazardous organics. Labels must state potential harm, from inhalation to aquatic toxicity, with real enforcement—customs officials detain shipments for unclear or missing labels. Lab training programs hammer home exposure routes and spill responses: dry sand for spills, sodium thiosulfate for neutralization if needed, and isolation from acids and active metals. Experience repeatedly underlines: each handover, inventory check, and disposal turns into a checkpoint of responsibility.
