Phenolsulfonphthalein

Phenol red

Identifiers
CAS Number	143-74-8 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:31991 Y
ChemSpider	4602 Y
ECHA InfoCard	100.005.100
PubChem CID	4766
InChI InChI=1S/C19H14O5S/c20-15-9-5-13(6-10-15)19(14-7-11-16(21)12-8-14)17-3-1-2-4-18(17)25(22,23)24-19/h1-12,20-21H Y Key: BELBBZDIHDAJOR-UHFFFAOYSA-N Y
SMILES O=S2(=O)OC(c1ccccc12)(c3ccc(O)cc3)c4ccc(O)cc4
Properties
Chemical formula	C19H14O5S
Molar mass	354.38 g·mol−1
Pharmacology
ATC code	V04CH03 (WHO)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Phenol red (also known as phenolsulfonphthalein or PSP) is a pH indicator frequently used in cell biology laboratories.

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 pK_a = 8.00 at 20 °C (68 °F).

A solution of phenol red is used as a pH indicator, often in cell culture. Its color exhibits a gradual transition from yellow (λ_max = 443 nm) to red (λ_max = 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 H+
2PS−
and is orange-red. If the pH is increased (pK_a = 1.2), the proton from the ketone group is lost, resulting in the yellow, negatively charged ion denoted as HPS⁻. At still higher pH (pK_a = 7.7), the phenol's hydroxide 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.