What is Phenolphthalein

Phenolphthalein has the chemical formula C20H14O4 and its chemical name is 3,3-bis(4-hydroxyphenyl)-1(3h)-isobenzofuranon. Phenolphthalein CAS 77-09-8 is a white to slightly yellow crystalline powder, odorless and tasteless. The melting point is 260℃-263℃. It is soluble in ethanol and alkaline solutions, slightly soluble in ether, extremely slightly soluble in chloroform, and insoluble in water.

Chemical properties

Solubility: Insoluble in water, readily soluble in organic solvents such as ethanol and ether. In the experiment, ethanol solution is commonly used as the phenolphthalein indicator.

Acid-base color-changing property: This is the most core property of phenolphthalein, making it a classic acid-base indicator.

In acidic solutions (pH < 8.2) or neutral solutions, phenolphthalein exists in an internal ester structure, and the solution is colorless.

In weakly alkaline solutions (8.2 < pH < 10.0), the lactone structure will transform into a quinone structure, and the solution will turn pink.

In strongly alkaline solutions (pH > 10.0), the quinone structure will further decompose, the pink color gradually fades, and it returns to colorless.

Main uses

1. Phenolphthalein is a commonly used indicator in acid-base neutralization titration experiments, especially suitable for scenarios where a strong base titrates a strong acid or a weak acid. For instance, when titrating hydrochloric acid with sodium hydroxide solution, if the solution changes from colorless to light pink and does not fade within half a minute, the titration endpoint is reached, which can help accurately determine the moment when the reaction is completed.

2. In the pharmaceutical industry, it can be used as a raw material for medicine to treat habitual and stubborn constipation. However, due to the existence of side effects, its current use is restricted. Phenolphthalein can also be used in organic synthesis.

3. Phenolphthalein is mainly used in the synthesis of plastics, especially in the synthesis of polymers such as diazanaphthone and polyaryletherketone.

4. Phenolphthalein In some industrial production processes, phenolphthalein can be used to test the alkalinity of solutions. It can also be used as an intermediate in dye synthesis.

Safety and Precautions

Phenolphthalein itself has relatively low toxicity, but when used as an indicator, its ethanol solution should be kept away from skin contact (which may cause mild irritation).

When Phenolphthalein is used as a medicine, it is necessary to strictly follow the doctor’s advice and not use it for a long time or in excessive amounts.

Phenolphthalein should be stored in a sealed container away from light to prevent it from becoming ineffective due to oxidation or hydrolysis.

In conclusion, phenolphthalein, with its unique acid-base color-changing property, has become an important reagent in chemical experiments. Meanwhile, its application in the medical field has been constantly adjusted as research deepens, reflecting the dynamic development process of scientific cognition.