CAS Registry Numbers Explained: How to Find the Right One
CAS registry numbers are unique identifiers for chemical substances, essential for accurate sourcing and regulatory compliance. Each number is assigned by the Chemical Abstracts Service and consists of up to 10 digits separated by hyphens. Use official databases like CAS SciFinder or Reaxys to verify and locate the correct number for a given compound.
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Type I water
CAS 7732-18-5 · H2O -
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2,4-Dinitrophenol (25%-30% wetted)
CAS 51-28-5 · C6H3(NO2)2OH -
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1,4-Dinitrobenzene
CAS 100-25-4 -
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L-Ascorbic acid (Vitamin C)
CAS 50-81-7 · C6H8O6
CAS registry numbers are unique, non-sequential identifiers assigned by the Chemical Abstracts Service (CAS) to chemical substances. Each number is a distinct, unambiguous reference for a specific chemical entity, including its structure, isotopic composition, and molecular formula. This system is critical in scientific communication, regulatory documentation, and supply chain traceability across industries such as pharmaceuticals, biotechnology, and fine chemicals.
How are CAS numbers structured?
A CAS number consists of up to 10 digits, divided into three parts by hyphens: the first part has 2–7 digits, the second has 2 digits, and the third is a single check digit. For example, the CAS number for water is 7732-18-5. The check digit is calculated using a specific algorithm to ensure data integrity. The number is not predictive of chemical structure or properties, but it is globally recognised as a definitive identifier.
Where can I find a CAS number for a compound?
The most reliable sources for CAS numbers are official databases maintained by CAS, such as SciFinder and Reaxys. These platforms provide access to the CAS Registry, which contains over 200 million unique chemical substances as of 2023. For public access, the CAS website offers a free search tool at, which allows users to search by name, molecular formula, or structure. However, full access to detailed data, including purity specifications and regulatory status, requires a subscription.
What should I check when verifying a CAS number?
When sourcing chemicals, always verify the CAS number against authoritative databases. A mismatch can lead to incorrect material identification, regulatory non-compliance, or experimental failure. For example, two compounds with similar names—such as 2,4-dinitrophenol (CAS 51-28-5) and 2,6-dinitrophenol (CAS 100-25-4)—have different toxicological profiles and regulatory classifications. Using the wrong CAS number may result in safety or compliance issues under REACH, TSCA, or GHS frameworks.
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Additionally, confirm that the CAS number corresponds to the correct isomer, salt form, or hydrate. For instance, the CAS number for L-ascorbic acid is 50-81-7, while D-ascorbic acid is 134-03-2. Even small structural differences can significantly alter biological activity and regulatory status.
How do CAS numbers support regulatory compliance?
Regulatory frameworks such as REACH (EU), TSCA (USA), and GHS (global) rely on CAS numbers for substance identification. For example, under REACH, substances must be registered with their CAS number to be legally manufactured or imported into the European Economic Area. Similarly, US EPA uses CAS numbers in the Toxic Substances Control Act (TSCA) Inventory to track chemical substances.
In pharmaceutical development, CAS numbers are used in regulatory submissions to the FDA and EMA. The USP (United States Pharmacopeia), BP (British Pharmacopoeia), and EP (European Pharmacopoeia) also reference CAS numbers in monographs to ensure consistency in quality control. For instance, the USP monograph for acetylsalicylic acid (aspirin) cites CAS 50-78-2.
What is the purpose of a CAS number?
A CAS number uniquely identifies a chemical substance, ensuring unambiguous communication across scientific, regulatory, and industrial contexts.
Can two different chemicals have the same CAS number?
No. Each CAS number is assigned to one specific chemical entity, including its isotopic composition and structure. If two substances are identical in all respects, they share the same number.
How do I find the CAS number for a compound not in my supplier’s catalogue?
Use the CAS Common Chemistry database or search SciFinder/Reaxys with the compound’s name, molecular formula, or structure.
Are CAS numbers required on safety data sheets (SDS)?
Yes. According to GHS and ISO 11014, the CAS number must be included on an SDS to ensure accurate hazard communication and regulatory compliance.
Keywords
CAS registry number, chemical identification, REACH, TSCA, GHS, USP, BP, EP, SciFinder, Reaxys, chemical safety, regulatory compliance, molecular formula, isomer, hydrate, salt form
Frequently asked
What is the purpose of a CAS number?
A CAS number uniquely identifies a chemical substance, ensuring unambiguous communication across scientific, regulatory, and industrial contexts.
Can two different chemicals have the same CAS number?
No. Each CAS number is assigned to one specific chemical entity, including its isotopic composition and structure. If two substances are identical in all respects, they share the same number.
How do I find the CAS number for a compound not in my supplier’s catalogue?
Use the CAS Common Chemistry database or search SciFinder/Reaxys with the compound’s name, molecular formula, or structure.
Are CAS numbers required on safety data sheets (SDS)?
Yes. According to GHS and ISO 11014, the CAS number must be included on an SDS to ensure accurate hazard communication and regulatory compliance.
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