Phenolphthalein is an acid–base indicator. In solutions of pH 8.5, it is deep red-purple. Account for the change in color.

Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy

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Bruice 8th Edition

Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy

Problem 70

Chapter 14, Problem 70

Phenolphthalein is an acid–base indicator. In solutions of pH < 8.5, it is colorless; in solutions of pH > 8.5, it is deep red-purple. Account for the change in color.

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Phenolphthalein is a pH-sensitive compound that exists in different structural forms depending on the pH of the solution. These structural forms have different colors due to their distinct electronic configurations and absorption of light.

At a pH below 8.5, phenolphthalein exists predominantly in its protonated (acidic) form. This form does not absorb visible light in a way that produces color, so the solution appears colorless.

As the pH increases above 8.5, the solution becomes more basic, and phenolphthalein undergoes deprotonation. This deprotonation leads to the formation of a conjugated structure, which absorbs light in the visible spectrum, resulting in a red-purple color.

The transition from colorless to red-purple occurs because the deprotonated form of phenolphthalein has an extended conjugated π-electron system. This system lowers the energy gap between the ground and excited states, allowing the molecule to absorb light in the visible range.

The observed color change is a result of the equilibrium shift between the protonated (colorless) and deprotonated (red-purple) forms of phenolphthalein, which is influenced by the pH of the solution.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Base Indicators

Acid-base indicators are substances that change color in response to changes in pH. They are typically weak acids or bases that exhibit different colors in their protonated and deprotonated forms. Phenolphthalein, for example, is colorless in acidic solutions and turns deep red-purple in basic solutions, making it useful for determining the acidity or basicity of a solution.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 to 14. A pH of 7 is considered neutral, values below 7 indicate acidity, and values above 7 indicate basicity. The pH level affects the ionization of indicators like phenolphthalein, which is why its color changes at specific pH thresholds.

Ionization of Phenolphthalein

Phenolphthalein undergoes ionization depending on the pH of the solution. In acidic conditions (pH < 7), it remains in its protonated form, which is colorless. As the pH increases and the solution becomes basic (pH > 8.5), phenolphthalein loses protons and transitions to its deprotonated form, resulting in a deep red-purple color. This transition is what allows it to serve as an effective pH indicator.

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