563. The Chemical Abstract Service (CAS) maintains a computerized filing system that contains two main index files. The chemical abstract file provides bibliographic information referencing chemicals appearing in over 9,000 journals, papers, and symposiums from 1967 to the present. The chemical abstract file is an important tool for people interested in learning about the research, patents, and uses for specific chemicals. The chemical registry number file assigns CAS registry numbers to unique chemicals for purposes of identification. Assigning a CAS number to a particular chemical facilitates managing and regulating that chemical by universally identifying it with a specific number. Only one CAS number is assigned to each chemical and under EPCRA section 313, only one CAS number is listed per toxic chemical. If chemicals are to be assigned only one CAS number, why are some chemicals listed with multiple Chemical Abstract Service (CAS) numbers in 40 CFR Table 302.4 and the Title III List of Lists (EPA 550-B-98-017)?
There are two possible reasons for a chemical to have multiple numbers. The CAS numbers could refer to different forms of a chemical where each is considered unique for its particular properties and characteristics. The CAS registry number file includes the registry number, synonyms, chemical structure, and molecular formula for each chemical recorded in the file. If specific research has been done on a particular form of a chemical, a separate CAS number may be assigned to that particular form to facilitate the search process in the CAS file. For example, sodium hypochlorite is listed with two CAS numbers, 7681-52-9 and 10022-70-5. The former refers to the sodium salt form of hypochlorous acid, sodium hypochlorite, while the latter refers to the pentahydrate form of sodium hypochlorite. Both forms could be called sodium hypochlorite, thus sodium hypochlorite has, in effect, two CAS numbers.
A chemical may also be listed with multiple CAS numbers when multiple numbers have been inadvertently assigned to the same chemical. This multiple assignment can occur when forms of a chemical are originally believed to be unique, but after further review by chemists, are identified as the same chemical. In this case, all the CAS numbers are cross-referenced, allowing the chemical to be located with any assigned number. The misassigned numbers are deleted as registry numbers, but remain on file for referencing purposes. The CAS number first assigned is the more accurate number to use when denoting the chemical. Although all of the CAS numbers will find the chemical, only the more accurate number will prompt the CAS registry file system to display the name, synonyms, and characteristics associated with the chemical. Chromic acid, listed with CAS numbers 1115-74-5 and 7738-94-5, illustrates this situation. After further review by chemists, CAS number 1115-74-5 was deleted as a registry number, but remains on file for future reference. CAS number 7738-94-5 is the more accurate number to identify chromic acid because it was the first registry number assigned.