4-Hydroxybenzene Sulfonic Acid: An In-Depth Look at Properties, Structure, and Uses

What is 4-Hydroxybenzene Sulfonic Acid?

4-Hydroxybenzene Sulfonic Acid stands out among aromatic sulfonic acids, offering a unique blend of reactivity and stability. In my work with fine chemicals, I’ve seen this compound emerge as a crucial raw material, recognized for its dual functional groups: the hydroxyl group on the benzene ring ramps up solubility and reactivity, while the sulfonic acid group boosts stability, especially in aqueous and polar systems. Its formula, C6H6O4S, lays the groundwork for its chemistry, with a molecular weight of 174.18 g/mol.

Physical Properties and Appearance

Depending on purity and water content, 4-Hydroxybenzene Sulfonic Acid might appear as off-white to tan flakes, powder, crystalline solid, or even as colorless to slightly yellow pearls. Some labs prefer the liquid or solution forms for direct dosing or dilution, especially for large-scale operations. Its melting point usually settles around 110–120°C, which matters when choosing it for heat-sensitive syntheses. In my lab, weighing a dense pile of its pearls or crystals, I get a sense for its substantial density, typically close to 1.7 g/cm³—much heavier than many standard organics.

Chemical Structure and Molecular Characteristics

The structure shows a benzene ring substituted at the para position with both a hydroxyl (-OH) and a sulfonic acid (-SO3H) group. These groups are known to play well together in chemical reactions. The electron-withdrawing nature of the sulfonic acid group anchors the molecule, while the hydroxyl group remains reactive, letting the acid participate in coupling, condensation, and other position-based substitution. From a synthetic chemist’s viewpoint, this opens up plenty of reaction pathways—dye formation, resin production, or catalyst modification often rely on these reactive sites.

Specifications and HS Code

Commercial lots come with specifications for assay—often over 98% purity—so users know exactly what’s going into their processes. Impurities, moisture content, and sometimes pH or solubility data help customers size up the material’s suitability. For customs and trade, its HS Code typically falls under 2908.11, covering sulfonic acids and their salts. Accurate labeling streamlines cross-border shipment, prevents legal headaches, and, from my experience, cuts weeks off supply timelines.

Applications and Role as Raw Material

4-Hydroxybenzene Sulfonic Acid brings value to many sectors. The textile industry links it to certain azo dyes, where it feeds color-fastness and stability. Water-treatment chemists look for its raw material role in building blocks for ion-exchange resins. In polymer chemistry, it’s a common additive for conductivity, used in specialty plastics and polyaniline formation. In years past, I’ve mixed it into bath solutions while experimenting with electroplating baths for its effective pH control and surface activation. Fine chemical manufacturing counts on its intermediate properties, feeding the synthesis of complex organics, especially drugs and advanced materials.

Safety, Hazards, and Handling

Given its acidic nature, 4-Hydroxybenzene Sulfonic Acid demands respect in the lab or on the factory floor. Skin or eye contact can cause irritation, even burns; dust inhalation isn’t pleasant, and in my own handling, gloves, goggles, and good ventilation weren’t optional. Its powder and pearl forms spread easily—spilled crystals dissolve quickly in water, which means swift, careful cleanup to avoid spills spreading over benches. While it isn’t highly volatile or explosively reactive, I always checked for incompatibility with strong bases, oxidizers, and reducing agents. Safety Data Sheets bracket it as harmful, so waste gets collected by licensed handlers, never down the drain. Careful storage—sealed containers in cool, dry areas—prevents clumping and unwanted moisture absorption, keeping the crystals or powder free-flowing for the next use.

Environmental Considerations and Solutions

Waste disposal for sulfonic acids brings up plenty of environmental questions. Direct release to waterways isn’t an option. Most labs and factories neutralize waste streams, filtering out solids before chemical treatment removes the acidic load. In my experience, collaboration with hazardous waste companies gives the best results, keeping groundwater and ecosystems clean. As regulations tighten, companies look for greener chemistries—sometimes swapping out harsh sulfonic precursors for milder alternatives or investing in process upgrades that minimize waste at the source.

Summary Table: Key Facts of 4-Hydroxybenzene Sulfonic Acid