A CAS number is a unique numeric identifier given to a chemical substance by Chemical Abstracts Service, a division of the American Chemical Society. Each number points to one substance, however many names that substance has.
You need it because chemical names are unreliable. One substance can have a systematic name, a common name and several trade names. A CAS number lets you, your supplier, your regulator and your lab talk about exactly the same thing.
How the number is built
A CAS number has up to ten digits in three parts, separated by hyphens. The last digit is a check digit. Water is 7732-18-5 and sodium hydroxide is 1310-73-2.
You can check the last digit yourself. Take the digits before it, multiply the rightmost by 1, the next by 2, and so on, then add them up. The last digit of the total should equal the check digit. For water, the sum is 105, so the check digit is 5. A number that fails this test has been mistyped.
Where you will use it
- REACH and UK REACH searches, where you look up registrations by CAS or EC number, as explained in what EU REACH requires.
- The US TSCA Inventory, where listing is checked by CAS number, as described in checking a substance under TSCA.
- Section 3 of the safety data sheet, which lists the CAS numbers of hazardous components.
- Customs classification, where the CAS number helps a broker find the right HS code.
- Export control screening, where controlled chemicals are often listed by CAS number.
- Your purchase specification and contract, so the supplier cannot substitute a related substance.
Pitfalls to watch
Related forms have different numbers. A free acid and its sodium salt, an anhydrous form and a hydrate, or two isomers usually have separate CAS numbers. If the supplier's number does not match the form you specified, ask why before you order.
Mixtures do not have a single CAS number. A formulated product is identified by the numbers of its components. A supplier that quotes one number for a blend is either wrong or describing only its main component.
A CAS number identifies a substance, not a grade. Two products with the same number can differ widely in purity and impurities. Purity belongs in the specification and the certificate of analysis.
EU documents also use the EC number, a separate seven-digit identifier for substances on the EU market. Ask for both if you sell into Europe.
Put the CAS number in your purchase order and contract as well as the chemical name. If the goods that arrive test as a different substance, the contract gives you a clear basis for a claim.
Next step
We confirm CAS and EC numbers with the manufacturer and check them against the SDS and COA before you commit. To start, book a free sourcing review.