When One Ingredient Shows Two Different CAS Numbers

You pull the SDS from your citric acid supplier and it lists CAS 77-92-9. Then you check an old certificate of analysis from a different lot and it says 5949-29-1. Same jar, same INCI name, two numbers staring back at you. This trips up a lot of makers who assume a CAS number is as fixed as a barcode. It mostly is, but not always, and the exceptions follow a pattern once you see it.

Why one substance can have more than one CAS

The Chemical Abstracts Service assigns a registry number to a specific chemical entity, and "specific" is doing a lot of work in that sentence. A few situations legitimately produce more than one valid number for what a formulator would call the same ingredient:

• Hydrate versus anhydrous forms. Citric acid anhydrous is 77-92-9. Citric acid monohydrate, which has a water molecule built into its crystal structure, is 5949-29-1. Chemically these are different registered substances even though both function as citric acid in your formula.
• Isomers. Some ingredients exist as distinct spatial arrangements of the same atoms, and each isomer can get its own number, even when the INCI name doesn't distinguish them.
• Salt forms of the same acid or base. Sodium, potassium, and calcium salts of an acid are separate substances with separate numbers, even though a supplier data sheet might describe them all under a similar trade name.
• Free acid versus its common salt. Similar to the point above, but worth calling out separately because it's the single most common source of confusion in preservative and pH-adjuster ingredients.

None of this means the CAS system is broken. It means CAS numbers track molecular identity, not marketing names, and your raw material can genuinely change identity between hydrate and anhydrous states while your label copy stays the same.

Why this matters for your filing

In a Canadian Cosmetic Notification Form, ingredients are declared by INCI name with a concentration or range, and the CAS number rides along as an identifier. Health Canada's Hotlist restrictions are written against specific substances, and in some cases a hydrate and its anhydrous counterpart are treated the same way for safety purposes, but you should never assume that without checking. The safer habit:

1. Confirm which physical form you actually purchased, not which form you assume is "standard."
2. Pull the CAS number straight off the current SDS section 3 (composition), not from an old spec sheet or a supplier's marketing PDF.
3. If your supplier's documentation is ambiguous or conflicting, ask them directly which registered substance is in the drum, and get it in writing.

A quick example

INCI name	Form	CAS number
Citric Acid	Anhydrous	77-92-9
Citric Acid	Monohydrate	5949-29-1

Both rows say "Citric Acid" on a label. Both function identically in a toner or a bath bomb. But if you're building a compliance file and your paperwork only ever shows one of these numbers, that's a sign someone copied a template rather than checking the actual lot.

What to do when you genuinely can't tell

Some botanical extracts don't have a CAS number at all, since they're complex mixtures rather than single defined substances, and that's normal, not an error. The problem case is different: an ingredient that clearly should have one number, but your paperwork shows something inconsistent from batch to batch. When that happens:

• Don't guess and don't average the two numbers together, that's not how registry identity works.
• Ask the supplier for a current, lot-specific SDS rather than a generic one that may be years old.
• If two suppliers give you different CAS numbers for what they both call the same INCI name, treat that as a real discrepancy worth resolving before you file, not a rounding error.

This is exactly the kind of detail that's easy to skim past when you're assembling a formula sheet under deadline, and it's also exactly the kind of detail a reviewer catches. Cosmetic Comply's matching step maps each ingredient to its INCI name and CAS number from your actual supplier documentation, flags mismatches like hydrate versus anhydrous before they reach a filing, and has a compliance reviewer confirm the result. It's a small thing until it's the reason your notification gets bounced back.

Keep a simple internal note next to each raw material record: which form you buy, which CAS number matches that form, and the date you last confirmed it against a current SDS. It takes five minutes per ingredient and saves you from re-checking your entire formula sheet the next time a supplier swaps their manufacturing site without telling you.