Phenol Red

4,4′-(3H-2,1-Benzoxathiol-3-ylidene)diphenol S,S-dioxide; phenolsulfonphthalein; C₁₉H₁₄O₅S = 354.4 (143-74-8)

General reagent grade of commerce.

Produces a yellow colour in neutral and very faintly acidic solutions and a red colour in weakly alkaline solutions.