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Main Uses Of Phenolphthalein

Oct 20, 2023

1. Used as an acid-base indicator and an indicator for determining the acid value of oil

Phenolphthalein has a very interesting color-changing reaction. Under acid-near-neutral conditions (pH0-8.2), phenolphthalein molecules do not dissociate and are colorless. In the alkaline environment (pH8.2-12), a phenol hydroxyl group is dissociated into a quinone structure, and the lactone bond is broken, the carbon atom in the molecular center changes from sp3 hybridization to sp2 hybridization, all carbon atoms are in the same plane, the entire molecular conjugated declaration forms a 19-center 19-electron large π bond, and the free electrons in the system can absorb specific wavelength electromagnetic waves. It gives off a pink-burgundy. In the strong acid environment, the phenolphthalein molecule is extremely protonated, and the lactone bond breaks, forming a large π bond with 19 centers and 18 electrons, which is orange red. This property of phenolphthalein is often used to detect acid and base.

Because phenolphthalein has a certain fat solubility, it is especially suitable as an indicator for non-aqueous titration, such as an indicator for the determination of acid value in fats.


2. Used as a pharmaceutical laxative

Phenolphthalein forms soluble sodium in the alkaline environment of the intestine, stimulates the nerve plexus in the intestinal wall, directly acts on the intestinal smooth muscle, and increases intestinal peristalsis. At the same time, phenolphthalein also inhibits the absorption of water in the intestine, so that water and electrolyte accumulate in the colon, resulting in a laxative effect.

The laxative effect of phenolphthalein depends on the alkalinity of intestinal fluid. Under normal circumstances, its effect is mild and rarely causes intestinal spasm. Therefore, it is suitable for habitual stubborn constipation, or for intestinal cleaning during colon and rectal endoscopy and X-ray examination. There are tablets, suppositories and other dosage forms.

Taking phenolphthalein diarrhea defecation, reduce the absorption of food, so there is a certain weight loss effect, but if taken for a long time, it will lead to fluid and electrolyte disorders due to excessive diarrhea, cause human nutrition loss, damage the intestinal nervous system.

On October 27, 2017, the World Health Organization’s International Agency for Research on Cancer included phenolphthalein in the list of Group 2B carcinogens.