The impurities that matter most in dye intermediates are the ones that take part in your reaction: isomers, unreacted starting materials and side-reaction products. They change the shade or strength of the dye. Metals, insolubles and moisture matter next.
Inert impurities such as sodium chloride or sodium sulphate mostly dilute the product. They cost you money but rarely spoil a batch.
The main impurity groups
Most problems fall into one of these groups:
- Positional isomers, such as an ortho or meta amine in a para product, or a different naphthalene sulphonic acid in H-acid, which react and form off-shade dyes.
- Unreacted starting materials, such as residual nitro compound in an amine, which lower yield and can add colour.
- Over-reacted or side products, such as extra sulphonation or chlorination, which change solubility and shade.
- Iron, copper and other metals, often from reduction steps or equipment, which dull shades and can affect stability.
- Insoluble matter, which leaves specks in dyes and blocks filters and print screens.
- Excess moisture and hydrolysis products, which reduce activity and cause caking in storage.
Colour bodies
Some intermediates carry traces of tarry, coloured by-products from nitration or fusion steps. They may barely change the assay, but they can dull bright shades. A test of solution colour or absorbance on the intermediate can catch them before they reach your dye.
Impurities with regulatory weight
Some impurities matter for compliance, not just performance. Certain aromatic amines are restricted in textiles and leather because they can be released from azo dyes. If an intermediate carries one of these as an impurity, it can show up in your dye and then in your customer's fabric.
Our article on banned amines and how dyes are tested explains those limits. Heavy metal limits for finished colourants are covered in heavy metal limits for dyes and pigments.
How to control them
Name the impurities that matter in your specification, with a limit and a method for each. A general "purity 98% min" leaves the rest of the sample undefined.
Ask the supplier which route they use. The route predicts the likely impurities, and a change of route is the most common cause of a sudden new impurity. Your contract should require notice before any change of route or raw material source.
Keep a retained sample from each lot and compare HPLC profiles over time. A new peak is often the first sign that something has changed at the plant. Our guide on testing intermediate purity explains the methods.
Next step
We help you set impurity limits, check suppliers' routes during audits and have lots tested before dispatch. To discuss your intermediates, book a free sourcing review.