Question

Phenolphthalein indicator is used in the pH range

• A. $3.2-4.4$
• B. $4.8-6.0$
• C. $6.8-8.4$
• D. $8.2-10.0$

Answer 1

The correct option is D

$8.2-10.0$

Explanation of the correct option:

Option (D): $8.2-10.0$

• Phenolphthalein is an organic compound with a chemical formula ${\mathrm{C}}_{20}{\mathrm{H}}_{14}{\mathrm{O}}_4$
• It is used as an indicator to distinguish between acidic and basic mediums.
• It works only in a basic medium because in the acidic medium it remains colorless.
• It Changes its color to pink in a basic solution of pH greater than $8.5$
• As the pH increases more than $9$ the color of the solution changes from pink to deep red.
• At room temperature $(25°\mathrm{C})$ the range of pH for phenolphthalein is $8-10$.
• As temperature increases the pH range of phenolphthalein decreases because pK_in decreases with an increase in temperature.
• In an acidic solution, it remains colorless so we cannot use it in the acid test.
• From the given option only option D has the pH range of the basic medium.

The explanation for the incorrect options:

Option(A): $3.2-4.4$

The given pH range is of the acidic medium in which phenolphthalein remains colorless

Option (B): $4.8-6.0$

The given pH range is of the acidic medium in which phenolphthalein remains colorless

Option(C): $6.8-8.4$

• The given pH range is not entirely basic as the pH from 6.8 to 7 will be acidic so phenolphthalein will remain colorless in this range.
• And the change of color in the basic medium can only be observed at a pH greater than $8.5$

So, Option (D) is correct.