There is no single purity that suits every dye intermediate. The right figure depends on the product, its route and what your own reaction can tolerate. What matters most is that the purity is stated with its test method and basis, and backed by limits on the impurities that hurt you.
Two suppliers can both quote "98%" and ship very different material. One may mean HPLC area percentage, the other a titration on an as-is basis with high moisture.
How purity is expressed
Ask every supplier to state purity in the same way. These are the points that make figures comparable:
- The test method, such as nitrite titration, acid-base titration, HPLC or GC.
- The basis, either as-is or on a dry basis, with moisture stated separately.
- The form, for example free acid or sodium salt, since the molecular weight differs.
- For HPLC, whether the figure is area percentage or an assay against a reference standard.
Our article on testing intermediate purity by HPLC and titration explains why these methods give different numbers for the same sample.
Setting the right level
Liquid intermediates made by distillation, such as many amines and nitro compounds, are often sold at high purity measured by GC. Solid sulphonic acids such as H-acid are often sold at lower assay because the product carries water and inorganic salt from isolation.
Lower assay is not always a problem. Salt and water add weight but may not harm your reaction. What harms you is an active impurity: an isomer, an unreacted starting material or a by-product that couples or reacts like the main product.
Start from your own process. Look at the purity of the material you already use successfully and at your past batch records. Set the minimum assay a little below what good batches showed, and set impurity limits from what caused problems before.
If you are switching suppliers, test the new material side by side with your current one in a small production run. Two COAs can match while the impurity profiles differ.
What else belongs in the specification
Purity alone is not enough. A useful specification also covers moisture, insoluble matter, ash or inorganic salt, iron and named impurities. For some uses, colour or the appearance of a solution is also worth fixing.
Our guide to the impurities that matter most in intermediates goes through the usual problem groups.
Next step
We help you set a specification you can test against, then have supplier samples and production lots checked against it. To discuss your intermediates, book a free sourcing review.