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 20 °C.
A solution of phenol red is used as a pH indicator: its color exhibits a gradual transition from yellow to red over the pH range 6.8 to 8.2. Above pH 8.2, phenol red turns a bright pink (fuchsia) color.
| Phenol red (pH indicator) | ||
| below pH 6.8 | above pH 8.2 | |
| 6.8 | ↔ | 8.2 |
This solution contains a pH indicator which helps in monitoring of the pH changes in the cell culture. It will change from red to yellow color when the pH value decreases.
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 H2+PS− 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 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.
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.)
Read more about this topic: Phenol Red
Famous quotes containing the words chemical, structure and/or properties:
“If Thought is capable of being classed with Electricity, or Will with chemical affinity, as a mode of motion, it seems necessary to fall at once under the second law of thermodynamics as one of the energies which most easily degrades itself, and, if not carefully guarded, returns bodily to the cheaper form called Heat. Of all possible theories, this is likely to prove the most fatal to Professors of History.”
—Henry Brooks Adams (1838–1918)
“... the structure of our public morality crashed to earth. Above its grave a tombstone read, “Be tolerant—even of evil.” Logically the next step would be to say to our commonwealth’s criminals, “I disagree that it’s all right to rob and murder, but naturally I respect your opinion.” Tolerance is only complacence when it makes no distinction between right and wrong.”
—Sarah Patton Boyle, U.S. civil rights activist and author. The Desegregated Heart, part 2, ch. 2 (1962)
“A drop of water has the properties of the sea, but cannot exhibit a storm. There is beauty of a concert, as well as of a flute; strength of a host, as well as of a hero.”
—Ralph Waldo Emerson (1803–1882)