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 color under normal conditions. Typically, 15 mg/L are used for cell culture. 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. In addition, the waste products produced by the mammalian cells themselves will slowly decrease the pH, gradually turning the solution orange and then yellow. This color change is an indication that even in the absence of contamination, the medium needs to be replaced (generally, this should be done before the medium has turned completely orange). Since the color of phenol red can interfere with some spectrophotometric and fluorescent assays, many types of tissue culture media are also available without phenol red. Estrogen mimic Phenol red is a weak estrogen mimic, and in cell cultures can enhance the growth of cells that express the estrogen receptor. It has been used to induce ovarian epithelial cells from post-menopausal women to differentiate into cells with properties of oocytes (eggs), with potential implications for both fertility treatment and stem cell research. Use in swimming pool test kits Phenol red, sometimes labelled with a different name, such as "Guardex Solution #2", is used as a pH indicator in home swimming pool test kits. Chlorine can result in the bleaching of the dye in the absence of thiosulfate to inhibit the oxidizing chlorine. High levels of bromine can convert phenol red to bromophenol red (dibromophenolsulfonephthalein, whose lowered pKa results in an indicator with a range shifted in the acidic direction – water at pH 6.8 will appear to test at 7.5). Even higher levels of bromine (>20 ppm) can result in the secondary conversion of bromophenol red to bromophenol blue with an even lower pKa, erroneously giving the impression that the water has an extremely high pH despite being dangerously low. References External links Video of phenol red activity demonstration and medium preparation PH indicators Triarylmethane dyes Phenols Benzenesulfonates