Dye used to determine acidity (6)

I believe the answer is:

litmus

(Other definitions for litmus that I've seen before include "A dye used as a rough indicator of acid" , "It turns red with acids and blue with alkalies" , "Colouring material used as a rough acid-base indicator" , "Material from lichens used to test for acidity" , "Acid-alkali testing substance" .)

I've seen this clue in The Guardian.

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