{"id":4025,"date":"2026-10-13T07:00:00","date_gmt":"2026-10-13T05:00:00","guid":{"rendered":"https:\/\/modafinil.pl\/?p=4025"},"modified":"2026-10-08T14:12:06","modified_gmt":"2026-10-08T12:12:06","slug":"cas-number-what-it-identifies-and-what-it-does-not-guarantee","status":"publish","type":"post","link":"https:\/\/modafinil.pl\/en\/cas-number-what-it-identifies-and-what-it-does-not-guarantee\/","title":{"rendered":"CAS Number: What It Identifies and What It Doesn’t Guarantee"},"content":{"rendered":"

The CAS Registry Number is the most commonly used identifier of a chemical substance and, at the same time, the most often over-interpreted one.<\/strong> It is sometimes read as a guarantee: if the number matches, the material is the right one. In reality, the number answers one very narrow question \u2014 which substance is meant<\/em> \u2014 and makes no statement on any other subject.<\/p>\n

This text shows what the number settles unambiguously, what it does not tell you at all, and how to check in a dozen or so seconds whether a recorded number contains a typo. The last of these is a surprisingly practical skill, because an incorrect number in a document looks exactly the same as a correct one.<\/p>\n

What a CAS Registry Number is, and what it is not<\/h2>\n

A CAS Registry Number is an entry in a registry \u2014 a sequential identifier assigned to a substance at the moment it enters the database. It does not encode the structure of the molecule, does not follow from its formula, and cannot be “derived” from the structure. Substances of similar structure have completely dissimilar numbers, and numerically adjacent numbers have nothing to do with each other.<\/p>\n

This distinguishes it from structural identifiers such as InChIKey or SMILES, which are<\/em> derived from structure and can be generated from the structure alone. A CAS Registry Number is a label assigned from outside, so its only function is to point: “this is the registry entry in question”.<\/p>\n

This arbitrariness yields an advantage and a drawback at the same time. The advantage is unambiguity: one number is one registry entry, regardless of how many names the substance goes by. The drawback \u2014 the absence of any semantic redundancy: a miscopied number still looks like a number, it just points to something else or to nothing.<\/p>\n

The checksum, or how to catch a typo<\/h2>\n

Fortunately, the CAS Registry Number has a built-in safeguard that few people know about: the last digit is a check digit<\/strong>, calculated from the others. The rule is simple enough to apply on paper.<\/p>\n

The number consists of three groups separated by hyphens. We take all the digits from the first two groups, read them from right to left<\/em>, multiply them successively by 1, 2, 3, 4 and so on, and add them up \u2014 and the remainder of dividing this sum by 10 must equal the last digit.<\/p>\n

Take modafinil, number 68693-11-8<\/a>. The digits from the first two groups are 6869311. Read from the right: 1, 1, 3, 9, 6, 8, 6.<\/p>\n