Crossword Genius

Dye indicating acidity (6)

Ross

I believe the answer is:

litmus

(Other definitions for litmus that I've seen before include "Substance used to distinguish between acid and alkali" , "It turns red in acid and blue in alkali" , "paper for chemistry test" , "Chemical tester" , "Substance used in test for acidity" .)

I've seen this clue in The Guardian.
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