Most dye intermediates are tested with both titration and HPLC, because the two answer different questions. Titration tells you how much reactive material is present. HPLC tells you what else is in the sample and how much of each.
Neither is better in every case. The method matters because the same batch can give different figures by each one.
Titration
Titration measures a chemical property of the sample by reacting it with a solution of known strength. For aromatic amines and amino-naphthalene sulphonic acids, the usual test is nitrite titration, which measures the amount of diazotisable amine.
Titration is cheap, fast and close to how the product behaves in your reaction. Its weakness is that it cannot tell the main product from impurities that react the same way. An amine isomer, for example, also consumes nitrite and is counted as product.
Ask how the lab detects the end point, by eye or with an electrode. Potentiometric end points are usually more repeatable, which helps when your lab and the supplier's lab compare results.
Acid-base titration is used for free acids and bases, and Karl Fischer titration measures water.
HPLC and GC
High-performance liquid chromatography separates the components of a sample and measures each one. It shows the main peak and the impurity peaks, so you can see isomers and by-products that titration misses.
HPLC purity is often reported as area percentage. That is the main peak's share of all peaks detected. It ignores water, salt and anything the detector does not see, so area percentage is usually higher than the true content.
A true HPLC assay compares the sample with a reference standard of known purity. It is more work but gives a real content figure. Volatile liquids such as many amines and nitro compounds are tested by gas chromatography instead.
What to ask for on the COA
Agree the methods before the first order and keep them fixed. These are the points to cover:
- The assay method and basis, such as nitrite titration on an as-is basis.
- HPLC or GC purity, with the main named impurities listed separately.
- Moisture by Karl Fischer or loss on drying.
- Insoluble matter, ash and iron where they matter to your process.
Ask for the chromatogram with the COA for the first few lots. It lets your lab compare peaks, not just one number. Our guide on reading a lab certificate covers the wider checks, and what purity intermediates should have covers the limits.
Next step
We check COAs against your agreed methods and have lots tested independently before they ship. To set this up for your intermediates, book a free sourcing review.