Finding the Right Chemical by CAS Number: Common Mistakes

The same chemical can have dozens of names, but only one CAS number. Quoting the CAS number in your order greatly reduces the risk of receiving the wrong product. Read incorrectly, however, a CAS number can also lead to the wrong product. This guide summarises how to use CAS numbers correctly.

What is a CAS number?

A CAS number (CAS Registry Number) is a unique identifier assigned to each chemical substance by Chemical Abstracts Service, a division of the American Chemical Society. Product names vary by language, manufacturer and habit; the CAS number does not. For example, "sodium hydroxide", "caustic soda" and "NaOH" are the same substance: 1310-73-2.

Format and check digit

A CAS number has three parts separated by hyphens: 2–7 digits, 2 digits and 1 check digit (for example 7732-18-5, water). The last digit is calculated from the others, so typing errors are easy to catch.

How to check it: excluding the check digit, multiply the digits from right to left by 1, 2, 3… and add them up; the remainder after dividing by 10 must equal the last digit. For water: 8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 = 105 → remainder 5 ✓

Common mistakes

• Hydrate and anhydrous forms are different substances. Copper(II) sulfate anhydrous: 7758-98-7; pentahydrate: 7758-99-8. Zinc sulfate anhydrous: 7733-02-0; heptahydrate: 7446-20-0. The formula, molecular weight and amount to use change too.
• An acid and its salt are different. EDTA (free acid): 60-00-4; EDTA disodium salt dihydrate: 6381-92-6.
• Isomers are different. L-alanine: 56-41-7; D-alanine: 338-69-2; DL-alanine: 302-72-7.
• Solutions and mixtures. Solutions are usually referred to by the CAS number of the solute; concentration and solvent must be stated separately. Mixtures may have several CAS numbers.
• A CAS number does not indicate grade. AR, LR and HPLC grades share the same CAS number; state the grade and pack size separately (see the grades guide).
• Typing errors. A single digit can point to another substance. For example, the CAS number of ammonium dihydrogen phosphate is 7722-76-1; 7727-76-1 is invalid and fails the check-digit test.

When ordering

• Give the CAS number together with the product name.
• State clearly whether you need the hydrate or anhydrous form, the salt and the isomer.
• Add the grade (AR, LR, HPLC…), pack size and quantity.
• At delivery, check that the CAS number on the SDS and CoA matches your order.