Phenol red is a pH indicator that is frequently used in cell biology laboratories. A synonym for phenol red is phenolsulfonephthalein.

Chemical structure and properties

Phenol red exhibits a gradual transition from yellow to red over the pH range 6.6 to 8.0. Its pK_a is 8.00 at 20°C. Its chemical formula is C₁₉H₁₄O₅S.

Several indicators share a similar structure to phenol red, including bromothymol blue, thymol blue , bromocresol purple , thymolphthalein, and phenolphthalein. (A table of other common chemical indicators is available in the article on pH indicators.)

Biological applications

Most living tissues prosper at a near-neutral pH; that is, a pH close to 7. Blood's pH ranges from 7.35 to 7.45, for instance. When cells are grown in tissue culture, the medium in which they grow is held close to this physiological pH. A small amount of phenol red added to this growth medium will have a pink-red colour under normal conditions.

In the event of problems, waste products produced by dying cells or overgrowth of contaminants will cause a change in pH, leading to a change in indicator color. For example, a culture of relatively slowly-dividing mammalian cells can be quickly overgrown by bacterial contamination. This usually results in an acidification of the medium, turning it yellow. Many biologists find this a convenient way to rapidly check on the health of tissue cultures.

It should be noted that phenol red is a weak estrogen mimic, and can enhance the growth in culture of cells that express the estrogen receptor.

External links

• Berthois Y. et al. (1986) Phenol red in tissue culture media is a weak estrogen: implications concerning the study of estrogen-responsive cells in culture. Proc. Nat. Acad. Sci. 83(8):2496-500.