CAS Registry Number

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The CAS Registry Number, also referred to as CASRN or simply CAS Number, is a unique numerical identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance described in the open scientific literature (currently including all substances described from 1957 through the present).

As of 2018, there are more than 141 million unique organic and inorganic substances and 67 million protein and DNA sequences with CAS number, plus additional information about each substance. It is updated with around 15,000 additional new substances daily.[1]


A CAS Registry Number has no inherent meaning, but is assigned in sequential, increasing order when the substance is identified by CAS scientists for inclusion in the CAS REGISTRY database. A CASRN is separated by hyphens into three parts, the first consisting from two up to seven digits, the second consisting of two digits, and the third consisting of a single digit serving as a check digit. This current format gives CAS a maximum capacity of 1,000,000,000 unique identifiers.[2]

Different phases do not receive different CASRNs (liquid water and ice both have 7732-18-5), but different crystal structures do (carbon in general is 7440-44-0, graphite is 7782-42-5 and diamond is 7782-40-3). Commonly encountered mixtures of known or unknown composition may receive a CASRN (ex: mustard oil). Some metals with distinct oxidation states, like chromium are assigned different CASRN: elemental Cr has 7440-47-3, Cr(III) has 16065-83-1 and the hexavalent Cr(VI) ion has 18540-29-9. Individual stereoisomers and racemic mixtures are assigned discrete CAS Registry Numbers.



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