Phenol red is an acid-base indicator. It is made by condensing two moles of phenol with one mole of o-sulfobenzoic acid anhydride.
Phenol Red is used as a pH indicator in cell culture applications. A solution of phenol red will have a yellow color at a pH of 6.4 or below and a red color at a pH of 8.2 and above. Phenol red in tissue culture media can act as a weak estrogen, especially with human breast cancer cells. Phenol red can be used to measure hydrogen peroxide in cultured macrophages in multiwell plates.