The active ingredient of Litmus is called Erythrolitmin, and commercial litmus dye is chemically p-Dimethylaminoazobenzenesulfonic acid, sodium salt. There is no chemical equation as such for litmus other than the structure for Dimethylaminoazobenzenesulfonic acid. This editor is limited in it's functionality or I would provide the structure for Dimethylaminoazobenzenesulfonic acid.