Expiration dates are an important part of chemical labeling.
However, many laboratory chemicals and reagents are actually manufactured by a company other than the one that puts the label on the bottle. Final packaging from bulk or a larger stock can occur months, or even years, after chemicals are manufactured. This extra step in handling presents the risk of contamination of the end product, essentially resulting in arbitrary expiration dates. These dates may prove to be more useful than the actual expiration date indicated on the original container label. According to the American Chemical Society, the principal criteria for assigning time spans to chemicals are the conditions in storage, the rates at which the compounds are oxidized by oxygen, the rates at which the compounds react with moisture and, in some cases, the ways in which they may polymerize.