© Springer Science+Business Media, LLC, part of Springer Nature 2019. Working with redox compounds needs to take into account the oxidation and reduction state of the compound under study. This redox state can be influenced by the media in which the compound is found, but will also be influenced by local environments. For example, this may be dictated perhaps by the locality of amino acids in the three dimensional structure of a protein. Therefore, historically, equations have been developed to enable either the redox poise of the environment to be determined, or the redox state of the compound of interest. If a compound is found in the wrong redox state—perhaps inactive—in a cell this has significant ramifications for its role, for example in cell signaling. Here, the use of such equations is discussed, with examples of the relevance to modern redox biology.