A blue coloring matter obtained from Roccella tinctoria and other species of lichens, the principal component of which is azolitmin; used as an indicator (reddened by acids and turned blue again by alkalies). [a corruption of lacmus, fr. Dutch lakmoes]
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lit·mus 'lit-məs n a coloring matter from lichens that turns red in acid solutions and blue in alkaline solutions and is used as an acid-base indicator
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lit·mus (litґməs) a pigment prepared from Roccella tinctoria and other lichens, used as a test for acidity and alkalinity. It has a pH range of 4.5 to 8.3. Crude fractions are azolitmin, erythrolitmin, and erythrolein.Medical dictionary. 2011.