A purplish red coloring matter obtained from natural litmus or synthesized by oxidizing orcinol in the presence of ammonia, lime, and potash; used as a broad indicator of pH (red at 4.5, blue at 8.3).
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azo·lit·min .az-ō-'lit-mən, .āz- n a dark red nitrogenous coloring matter obtained from litmus and used as an acid-base indicator
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azo·lit·min (az″o-litґmin) a coloring principle, from litmus; it is used as a pH indicator, being red at a pH of 4.5 and blue at 8.3.Medical dictionary. 2011.