H-acid is a naphthalene intermediate used mainly as a coupling component in azo dyes. Its chemical name is 1-amino-8-naphthol-3,6-disulphonic acid, and it is normally sold as the monosodium salt, a grey to off-white powder.
Its largest use is in reactive dyes for cotton, above all reactive blacks and navies. It is also used in acid dyes for wool, nylon and leather, and in some direct dyes.
Why dye makers use it
H-acid has an amino group and a hydroxyl group on the same naphthalene ring. That lets it couple with a diazo compound twice: once under acid conditions next to the amino group, and once under alkaline conditions next to the hydroxyl group.
Coupling twice gives a large molecule that absorbs light across a wide range, which is why so many deep blacks and blues are built on H-acid. The two sulphonic acid groups make the dye soluble in water.
A well-known example is Reactive Black 5. It is made by coupling two molecules of diazotised vinyl sulphone para base ester onto one molecule of H-acid. Our article on vinyl sulphone explains the other half of that dye.
How it is made
H-acid is made from naphthalene in several steps: sulphonation, nitration, reduction, isolation and an alkali fusion step. The route produces large volumes of acidic, coloured effluent and inorganic salts.
This is why H-acid supply is sensitive to environmental enforcement. When a plant's effluent treatment or consent is in question, output can stop with little notice. See how environmental rules affect intermediate supply in Gujarat.
What to check when you buy it
H-acid quality varies between plants because the process has many steps where by-products form. These are the points to pin down before you order:
- The basis of the assay, usually stated as purity by nitrite titration on the monosodium salt, and whether it is on an as-is or dry basis.
- The HPLC purity and the limits on named impurities, such as other naphthalene sulphonic acid isomers.
- Moisture, insoluble matter and inorganic salt content, which dilute the active product.
- Iron and other metals, which can dull the final shade.
- Physical form, particle size and packing, which affect handling and caking in storage.
Our guide on how to write a specification for H-acid turns these points into a document you can send to suppliers.
Next step
H-acid is a commodity name, but the product behind it is not uniform. We find and audit manufacturers, compare samples against your specification and have lots tested before loading. To start, book a free sourcing review.