Phenol red (also known as phenolsulfonphthalein or PSP) is a pH indicator frequently used in cell biology laboratories. Chemical structure and properties Phenol red exists as a red crystal that is stable in air. 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 . A solution of phenol red is used as a pH indicator, often in cell culture. Its color exhibits a gradual transition from yellow (λmax = 443 nm) to red (λmax = 570 nm) over the pH range 6.8 to 8.2. Above pH 8.2, phenol red turns a bright pink (fuchsia) color. In crystalline form, and in solution under very acidic conditions (low pH), the compound exists as a zwitterion as in the structure shown above, with the sulfate group negatively charged, and the ketone group carrying an additional proton. This form is sometimes symbolically written as and is orange-red. If the pH is increased (pKa = 1.2), the proton from the ketone group is lost, resulting in the yellow, negatively charged ion denoted as HPS−. At still higher pH (pKa = 7.7), the phenol's hydroxy group loses its proton, resulting in the red ion denoted as PS2−. In several sources, the structure of phenol red is shown with the sulfur atom being part of a cyclic group, similar to the structure of phenolphthalein. However, this cyclic structure could not be confirmed by X-ray crystallography. 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.) Uses Phenolsulfonphthalein test Phenol red was used by Leonard Rowntree and John Geraghty in the phenolsulfonphthalein test to estimate the overall blood flow through the kidney in 1911. It was the first test of kidney function and was used for almost a century but is now obsolete. The test is based on the fact that phenol red is excreted almost entirely in the urine. Phenol red solution is administered intravenously; the urine produced is collected. By measuring the amount of phenol red excreted colorimetrically, kidney function can be determined. Indicator for cell cultures Most living tissues prosper at a near-neutral pH—that is, a pH close to 7. The pH of blood ranges from 7.35 to 7.45, for instance. When cells are grown