CAS Registry Number

The CAS Registry Number is a unique identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance listed in its database. The CAS Registry Number is a numerical identifier assigned to a unique chemical substance, which allows for easy and accurate identification of the substance by scientists, researchers, regulators, and industry. This identifier consists of up to 8 digits and is used globally as a standard identifier for chemical substances, allowing for quick and accurate communication between parties involved in scientific research, development, and regulation.


CAS Registry Number is a unique identifier assigned by the Chemical Abstracts Service (CAS), a division of the American Chemical Society. CAS Registry Numbers are widely used to identify and track specific chemical substances, especially in scientific and regulatory contexts. They provide a common, standardized way to identify and reference chemicals, helping to ensure that everyone is referring to the same substance. They are used in various scientific databases and in chemical labeling and packaging information, as well as in regulatory submissions and evaluations, and to track chemicals for environmental and health purposes. The use of CAS Registry Numbers ensures that chemical substances can be accurately and unambiguously identified and tracked, facilitating research, regulation, and communication about chemicals.


The CAS Registry Number (CAS RN) is a unique identifier assigned to chemical substances by the Chemical Abstracts Service (CAS). The number consists of three parts separated by hyphens, with the first part ranging from two to seven digits, the second part being two digits, and the third being a single digit serving as a check digit. The check digit is calculated based on a specific algorithm, which uses the last digit multiplied by 1, the preceding digit multiplied by 2, and so on, before taking the sum modulo 10.


The granularity of CAS Registry Numbers depends on the type of substance being assigned a number. For example:

  • Stereoisomers and racemic mixtures are assigned separate CAS Registry Numbers. For instance, L-epinephrine has the CAS Registry Number 51-43-4, D-epinephrine has 150-05-0, and racemic DL-epinephrine has 329-65-7.
  • Different phases of the same substance, such as liquid water and ice, do not receive different CAS Registry Numbers. In this case, both liquid water and ice have the CAS Registry Number 7732-18-5.
  • Different crystal structures of the same substance, however, are assigned different CAS Registry Numbers. For example, carbon in general has the CAS Registry Number 7440-44-0, graphite has 7782-42-5, and diamond has 7782-40-3.
  • Mixtures of known or unknown composition may also receive a CAS Registry Number, such as Leishman stain (12627-53-1) and mustard oil (8007-40-7).
  • Some chemical elements may be assigned different CAS Registry Numbers based on their oxidation state. For instance, the element chromium has the CAS Registry Number 7440-47-3, trivalent Cr(III) has 16065-83-1, and hexavalent Cr(VI) species have 18540-29-9.
  • In some cases, whole classes of molecules may receive a single CAS Registry Number. For example, the class of enzymes known as alcohol dehydrogenases has the CAS Registry Number 9031-72-5.

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