A good H-acid specification names the exact product and form, then sets limits for assay, purity, impurities, moisture, insolubles and metals, each with a test method. It also covers packing and documents.
Base your limits on what works in your own dye process. If you have good past lots, use their results as the starting point. Our article on what H-acid is used for gives the background.
Identity and form
Start by fixing what you are buying. State the chemical name, 1-amino-8-naphthol-3,6-disulphonic acid, and the form, usually the monosodium salt. Add the CAS number for that form, so there is no confusion with the free acid.
Describe the appearance you expect, such as a grey to off-white free-flowing powder. Say whether you accept a wet cake or need dried material.
If you use the H-acid for a particular dye, say so. It helps the supplier understand which impurities matter most to you and which tests to watch.
The test parameters
These are the parameters most H-acid buyers set:
- Assay, with the method stated, usually nitrite titration, and the basis, as-is or dry, and the molecular weight used for the calculation.
- HPLC purity, stated as area percentage or as an assay against a reference standard.
- Named impurity limits, such as other amino-naphthol sulphonic acid isomers and unreacted precursors, with an HPLC method.
- Moisture, by Karl Fischer or loss on drying.
- Insoluble matter, as a percentage after dissolving in a stated alkaline solution.
- Iron content, in parts per million.
- Inorganic salt or ash, if it matters to your process.
- A performance test, such as dyeing a standard dye made from the lot against a reference, if your lab can run one.
Write the method for each parameter, or reference a standard method. A limit without a method cannot be enforced. See how intermediate purity is tested for why the method changes the result.
Packing, labelling and documents
Specify the pack, such as 25 kg HDPE bags with a sealed inner liner, and the palletising. Ask for the batch number, manufacture date and net weight on every pack. State the allowed variation in net weight per pack and per shipment.
Require a COA for each batch, showing results for every parameter in your specification, and a current SDS. State the minimum remaining shelf life at dispatch.
Change control and sampling
Add a clause that the supplier must tell you in writing before changing route, raw material source or site. Agree how samples are drawn for testing and how long the supplier keeps retained samples. Attach the specification to your purchase contract.
Next step
We help you turn your process needs into a specification, then test supplier samples and production lots against it. To start, book a free sourcing review.