Practice question

Phenol red chemically phenolsulfonphthalein, sulfonephthalein class dye, serves as visual pH indicator deliberately incorporated at 5 to 15 mg per L in many mammalian culture media including Dulbecco Modified Eagle Medium, RPMI 1640 and Minimum Essential Medium to allow non-invasive monitoring of acid-base status. Color transition results from protonation equilibrium with pKa approximately 7.9 at 37 Celsius and ionic strength of media: acidic protonated form absorbs at 430 nanometer appearing yellow below pH 6.8, partially deprotonated neutral form appears orange-red around pH 7.4 physiological condition, fully deprotonated di-anion absorbs at 560 nanometer appearing fuchsia-purple above pH 8.2 alkaline indicating CO2 deficiency or ammonia accumulation. Rapid yellowing suggests lactate build-up from glycolysis or bacterial contamination producing acids. Although generally considered non-toxic at standard concentrations, studies by Berthois et al. 1986 revealed weak estrogenic activity via binding to estrogen receptor, stimulating proliferation of MCF-7 breast cancer cells, prompting development of phenol red-free formulations for endocrine research. No nutritional, antibiotic or growth factor role exists; sole function remains optical pH sentinel preventing hidden acidosis harming viability.