Why is Phenolphthalein used?

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Why is Phenolphthalein used?

Phenolphthalein is a chemical that is used as an indicator substance for acids and bases. It is written as ph ph” in shorthand notation. Phenolphthalein is used in titrations mostly to indicate the pH of the solution. If acidic solutions are added with this chemical they become colorless. If the acidic solutions are already colorless, they remain colorless after its addition too. Strong acidic solutions appear in orange color with phenolphthalein. The basic solutions appear pink in color. If the concentration of phenolphthalein in solution is more, then it can give purple color to the basic solutions. The pink color of this indicator in basic solutions gradually fades away and the solution becomes colorless after sometime.

Phenolphthalein exists in 4 structural forms each of them having its own properties. The four forms are represented in ionic form as ln+, H2ln, ln2-, ln (OH) 3- . The pH of each of the phenolphthalein forms are <0, 0 to 8.2, 8.2 to 12, and > 12.0 respectively. The state of the solution in each of the forms is known as strongly acidic, acidic or neutral, basic and strongly basic respectively. The color of each of the forms is orange, colorless, pink, and colorless respectively. The color of the pink solution slowly fades into colorless having the ion ln (OH) 3- and this ion is used to study the reaction kinetics of the chemical reaction.

Phenolphthalein is not soluble in water and hence the solution of it is generally prepared using alcohol. Phenolphthalein solution is itself a weak acid and loses hydrogen ion in the solution. Its molecular form is colorless and its ionic form is pink colored. It was observed that when a base is added to its solution, the equilibrium between its molecules and ions will be shifted to its right. It means that when a base is added to phenolphthalein solution, more hydrogen ions will be released which was predicted by Le Chatelier’s principle. Apart from being used as acid-base indicator, phenolphthalein is used as laxative, in the Kastle-Meyer test, in the toys, and in developing magical graphical patterns.

Author: Hari M

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