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| Identifiers | |
|---|---|
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3D model (JSmol) | |
| ChEBI | |
| ChEMBL | |
| ChemSpider |
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| ECHA InfoCard | 100.005.100 |
| UNII | |
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| Properties | |
| C19H14O5S | |
| Molar mass | 354.38 g·mol−1 |
| Pharmacology | |
| V04CH03 (WHO) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Phenol red (also known asphenolsulfonphthalein orPSP) is apH indicator frequently used incell 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 inethanol.[1] It is aweak acid withpKa = 8.00 at 20 °C (68 °F).
A solution of phenol red is used as apH indicator, often in cell culture. Its color exhibits a gradual transition from yellow (λmax = 443 nm[2]) to red (λmax = 570 nm[3]) over the pH range 6.8 to 8.2. Above pH 8.2, phenol red turns a bright pink (fuchsia) color.[4][5]
| Phenol red(pH indicator) | ||
| below pH 6.8 | above pH 8.2 | |
| 6.8 | ⇌ | 8.2 |
Incrystalline form, and in solution under very acidic conditions (low pH), the compound exists as azwitterion as in the structure shown above, with thesulfate group negatively charged, and theketone group carrying an additional proton. This form is sometimes symbolically written asH+
2PS−
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), thephenol'shydroxy group loses its proton, resulting in the red ion denoted as PS2−.[6]
In several sources, the structure of phenol red is shown with thesulfur atom being part of a cyclic group, similar to the structure ofphenolphthalein.[1][7] However, this cyclic structure could not be confirmed byX-ray crystallography.[8]
Several indicators share a similar structure to phenol red, includingbromothymol blue,thymol blue,bromocresol purple,thymolphthalein, and phenolphthalein. (A table of other common chemical indicators is available in the article onpH indicators.)
Phenol red was used byLeonard Rowntree and John Geraghty in thephenolsulfonphthalein test to estimate the overall blood flow through thekidney in 1911.[9] 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 administeredintravenously; theurine produced is collected. By measuring the amount of phenol red excretedcolorimetrically, kidney function can be determined.[10]
Mostliving tissues prosper at a near-neutral pH—that is, a pH close to 7. The pH ofblood ranges from 7.35 to 7.45, for instance. Whencells are grown intissue 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 dividingmammalian cells can be quickly overgrown bybacterial contamination. This usually results in anacidification of the medium, turning it yellow. Manybiologists 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 somespectrophotometric andfluorescent assays, many types of tissue culture media are also available without phenol red.
Phenol red is a weakestrogen mimic, and in cell cultures can enhance the growth of cells that express the estrogen receptor.[11] It has been used to induceovarianepithelial cells from post-menopausal women todifferentiate into cells with properties ofoocytes (eggs), with potential implications for bothfertility treatment and stem cell research.[12]
Phenol red, sometimes labelled with a different name, such as "Guardex Solution #2", is used as a pH indicator in homeswimming pool test kits.[13]
Chlorine can result in the bleaching of the dye in the absence ofthiosulfate to inhibit the oxidizing chlorine. High levels ofbromine can convert phenol red tobromophenol red (dibromophenolsulfonephthalein, whose loweredpKa 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 tobromophenol blue with an even lower pKa, erroneously giving the impression that the water has an extremely high pH despite being dangerously low.[14]
