Phenol red (phenolsulfonphthalein) is used as a pH indicator. A solution of phenol red will have a yellow color at a pH of 6.4 or below, and a pH of 8.2 and above. Phenolsulfonphthalein is used widely in culture media, to identify changes from neutral to acidic pH values. It is typically used in a cell culture of 11mg/L.