Phenol red is 3H-2,1-Benzoxathiole 1,1-dioxide in which both of the hydrogens at position 3 have been substituted by 4-hydroxyphenyl groups. A pH indicator changing colour from yellow below pH 6.8 to bright pink above pH 8.2, it is commonly used as an indicator in cell cultures and in home swimming pool test kits. It is also used in the (now infrequently performed) phenolsulfonphthalein (PSP) test for estimation of overall blood flow through the kidney. It has a role as a two-colour indicator, an acid-base indicator and a diagnostic agent. It is a 2,1-benzoxathiole, a member of phenols, a sultone and an arenesulfonate ester.