Chapter U6.123, Problem 1E

Interpretation Introduction

Interpretation:

The work of an acid-base indicator to show the pH of a solution needs to be explained.

Concept Introduction :

The substances which respond to an alteration in the hydrogen ion concentration of a solution are known as acid-base indicators. Through a color change, one can recognize the change in pH of a solution.

Answer to Problem 1E

Phenolphthalein is a colorless, weak acid which dissociates in water forming pink anions and its pH range is 8.3 - 10.0.

Other indicators are methyl orange, it turns yellowish orange, and its pH range is 3.1 - 4.4.

Explanation of Solution

Whether a solution is acidic or basic, one can identify by a numeric scale known as pH. A pH value smaller than 7 is acidic, whereas pH values higher than 7 is basic. A pH equal to 7 is neutral.

When an acid-base indicator mixes with an unknown solution, a change in equilibrium to the weak acid and its conjugate base takes place. This change results in the change in pH. Due to change in pH, when change in the hydrogen ion concentration takes place, a shift in equilibrium is observed.

For example:

Phenolphthalein is a colorless, weak acid which dissociates in water and formation of pink anions takes place and its pH range is 8.3 - 10.0.

Other indicators are methyl orange, it turns yellowish orange, and its pH range is 3.1 - 4.4.