Phenol red is used as a pH indicator. A solution of phenol red exhibits a gradual transition from yellow to red over the pH range 6.8 to 8.2. Above pH 8.2, phenol red turns a bright pink (fuchsia) color. The sodium salt of phenol red is used widely in culture media to identify changes from neutral to acidic pH values. It is typically used in cell culture media at 11mg/L. Acid end products from carbohydrate fermentation lower the pH causing the pH indicator phenol red to turn from red (alkaline) to yellow (acid). Phenol red in tissue culture media can act as a weak estrogen, especially with human breast cancer cells. Lipophilic impurities, not the phenol red dye itself, account for the estrogenic activity. 95-99% of these impurities can be removed from the sodium salt of phenol red with a reduction in estrogen-like activity. P4040 appropriate for use in cell culture applications. Phenol red added as a component to tissue culture media can be autoclaved.
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