CAS registry number
The CAS registry number of any chemical element, compound, polymer, alloy, or other chemical substance is its unique, nine-digit identifier assigned by the Chemical Abstract Service of the American Chemical Society.
The CAS Registry
The CAS Registry is a database of every chemical substance described in the chemical literature. This registry came about because different scientists often describe the same chemical by different names. Having a registry of all chemicals makes searching for papers about those chemicals much easier.
As of October 12, 2007 at 13:51:40 EDT (17:51:40 UTC), the CAS Registry had 32,773,931 entries for organic and inorganic substances and 59,372,514 entries for sequences. The Registry adds approximately 4,000 new entries every day. (See CAS Registry Number and Substance Counts for up-to-date counts at any time.)
The CAS Registry has different numbers not only for different elements but also for different isotopes of the same element, and also for different isomers (including stereoisomers) of the same compound.
A CAS registry number has as many as nine digits, divided into three parts, separated by hyphens. The first part has up to six digits, the second exactly two, and the third part, one. That one digit is a check digit to verify that the number is valid. The checksum is a modulo 10 number. To compute it, multiply each digit to the left of the check digit by its number of places to the left of the check digit. Then add these products together, divide by ten, and take the remainder (which actually is the last digit of the sum).
For example, the CAS registry number for water is 7732-18-5. The checksum is:
(8*1 + 1*2 + 2*3 + 3*4 + 7*5 + 7*6) mod 10 = 105 mod 10 = 5.
The following free resources are available for searching for CAS registry numbers:
- R&D Chemicals
- NIH ChemIDplus
- NIST Chemistry WebBook
- NCI Database Browser
- European chemical Substances Information System (ESIS) - useful for finding EC numbers.