When you order a chemical, the name alone is often ambiguous. Acetic acid, ethanoic acid and glacial acetic acid are the same substance. Sodium sulfate anhydrous and sodium sulfate decahydrate are not. The CAS Registry Number cuts through this ambiguity. This article explains what CAS numbers are, how they are structured, how to check one for typing errors, and where they fall short.
What a CAS number is
A CAS Registry Number is a unique identifier assigned by the Chemical Abstracts Service, a division of the American Chemical Society, to a specific chemical substance recorded in the CAS Registry. The Registry covers well over two hundred million organic and inorganic substances, as well as biological sequences, and it continues to grow daily. A CAS number itself carries no chemical meaning: it is a sequential identifier, not a code derived from structure. Its value is that it is unique and widely used across regulations, catalogues and safety documents.
Structure of a CAS number
A CAS number has up to ten digits divided into three parts by hyphens:
- The first part has two to seven digits.
- The second part has exactly two digits.
- The final part is a single check digit.
Water is 7732-18-5, ethanol is 64-17-5 and sodium chloride is 7647-14-5. Leading zeros are not written.
How the check digit works
The check digit lets you catch most typing errors. To calculate it:
- Take all the digits except the check digit.
- Starting from the rightmost of these digits, multiply it by 1, the next digit to the left by 2, the next by 3, and so on.
- Add the products together.
- The check digit is the remainder when that sum is divided by 10.
Worked example: water, 7732-18-5
The digits before the check digit are 7, 7, 3, 2, 1, 8. Working from the right:
| Digit | Position weight | Product |
|---|---|---|
| 8 | 1 | 8 |
| 1 | 2 | 2 |
| 2 | 3 | 6 |
| 3 | 4 | 12 |
| 7 | 5 | 35 |
| 7 | 6 | 42 |
The sum is 105. The remainder after dividing by 10 is 5, which matches the check digit.
Worked example: ethanol, 64-17-5
Digits from the right: 7 × 1 = 7, 1 × 2 = 2, 4 × 3 = 12, 6 × 4 = 24. The sum is 45, the remainder is 5, and the check digit is 5.
A valid check digit does not prove that a CAS number refers to the substance you intend, only that it is well formed. Transposing two digits usually changes the check result, which is why the scheme catches common typos, but a completely wrong number with a valid check digit will pass.
What a CAS number does and does not identify
A CAS number identifies a substance at a defined level of detail, which is both its strength and its trap. Related forms have different numbers:
- Salts and free bases: an API and its hydrochloride salt have different CAS numbers.
- Hydrates: copper(II) sulfate anhydrous and copper(II) sulfate pentahydrate have different numbers, and different molecular weights, 159.61 and 249.69 g/mol respectively.
- Stereoisomers: a racemate and each single enantiomer usually have distinct numbers.
- Isotopically labelled compounds: deuterated analogues typically have their own numbers, though not always.
Conversely, some substances of variable composition, such as mixtures, polymers and natural extracts, have CAS numbers that cover a range of materials. And some registered numbers have been superseded or merged, so older documents may cite a deleted number.
Why CAS numbers matter when ordering
For a supplier, a CAS number removes most ambiguity from an enquiry. It distinguishes anhydrous from hydrated forms, salts from free bases, and similar-sounding names. It also drives regulatory checks: inventory status under TSCA or REACH, controlled substance lists, dangerous goods classification and customs classification often start from the CAS number. When you send a request for quotation, include the CAS number wherever possible, along with the grade and quantity.
Other identifiers worth knowing
| Identifier | What it is | Best for |
|---|---|---|
| IUPAC name | Systematic name derived from structure | Unambiguous description in documents |
| SMILES | Line notation of structure | Sending structures by email or into software |
| InChI and InChIKey | Open, structure-derived identifiers | Database matching and web searching |
| EC number | European Community inventory number | EU regulatory context |
| HS or HSN code | Customs tariff classification | Import, export and tax |
Because the InChIKey is computed from structure, two people drawing the same molecule get the same key, which makes it a useful cross-check when a CAS number is missing or disputed.
Practical tips
- Validate CAS numbers in spreadsheets with a check-digit formula before sending bulk enquiries.
- Always check whether the number refers to the salt, hydrate or stereoisomer you need.
- Cross-check the molecular formula and weight on the COA against the CAS number.
- For novel compounds without a CAS number, send a structure as SMILES or a drawing.
- When a document cites a CAS number you cannot find, check whether it has been deleted or replaced. Database records usually point to the current number.
- Keep the CAS number in your purchasing and inventory records so that reorders and regulatory checks refer to exactly the same substance.
FAQs
Can two different chemicals have the same CAS number?
No. Each CAS number identifies one substance record. However, related forms such as salts, hydrates and isomers each have their own numbers.
Does a correct check digit mean the CAS number is right?
It only shows the number is well formed. A wrong but valid-looking number will still pass, so cross-check the name and formula.
Do all chemicals have a CAS number?
Most commercial substances do, but novel research compounds may not. For those, supply a structure as SMILES or a drawing.