Phenol red is a water-soluble dye used as a pH indicator, changing from yellow to red over pH 6.6 to 8.0, and then turning a bright pink color above pH 8.1. As such, phenol red can be used as a pH indicator dye in various medical and cell biology tests.
After incubation the liquid in the tube turns yellow (indicated by the change in the color of the phenol red indicator). It indicates that there is drop in the pH because of the production of the acid by the fermentation of the carbohydrate (sugar) present in the media.
Phenol Red Indicator Solution: Dissolve 0.1 g of phenol red in 2.82 ml of 0.1 M sodium hydroxide and 20 ml of ethanol (95 percent). After the solution is effected, add sufficient water to produce 100 ml.
Its solubility is 0.77 grams per liter (g/l) in water and 2.9 g/l in ethanol. It is a weak acid with pKa = 8.00 at 20 °C (68 °F). A solution of phenol red is used as a pH indicator, often in cell culture.
