Kalmar “litmus” suna ne da ke nufin rini mai narkewa da ruwa da aka samu daga wasu lichens, musamman Roccella tinctoria. Ana amfani dashi azaman alamar pH, yana juya ja a ƙarƙashin yanayin acidic da shuɗi a ƙarƙashin asali ko yanayin alkaline. Hakanan ana iya amfani da kalmar "litmus" don komawa ga gwaji mai mahimmanci ko mai nuna yanayi ko halin wani abu.
