Problem 25 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

Phenolphthalein is used as an indicator in acid-base titrations. In solutions at high \(\mathrm{pH}\), it is a bright magenta colour with a peak at \(553 \mathrm{nm}\) in the absorption spectrum; at low pH phenolphthalein is colourless. (Section 10.6 ) (a) In terms of absorption and transmission of light, explain the colour of phenolphthalein at high pH. (b) At high pH, phenolphthalein is ionized; it is unionized at low pH. Suggest a reason for the colour change. (c) Does the absorption move to higher energy or lower energy at low pH?

Step-by-Step Solution

Answer

Phenolphthalein is magenta at high pH because it absorbs green light. The color change is due to ionization affecting light absorption. Absorption moves to different energies, less in the visible spectrum, at low pH.

Step 1: Understanding Absorption and Transmission

At high pH, phenolphthalein appears magenta because it absorbs light in the opposite part of the spectrum, specifically at 553 nm, which corresponds to the green region. This absorption allows the complementary magenta color to be observed due to the transmission of other wavelengths.

Step 2: Effect of Ionization

At high pH, phenolphthalein is ionized, which alters its electronic structure. This allows it to interact with light, producing intense color through absorption. Conversely, the unionized form at low pH does not absorb visible light significantly, making the solution appear colorless.

Step 3: Analyzing Energy and Absorption

At low pH, phenolphthalein does not absorb visible light significantly, suggesting that any absorption occurs outside the visible region, likely at lower energy (infrared) or higher energy (ultraviolet). Compared to the visible absorption at high pH, there is a shift to either lower energy or higher energy due to absence of visible absorption.

Key Concepts

Phenolphthalein

Phenolphthalein is a well-known acid-base indicator commonly used in chemistry. It is typically utilized in titration experiments to visually signify the completion of the chemical reaction through a color change. In its ionized form, which occurs at high pH levels, phenolphthalein displays a striking magenta color. However, when the pH is low, phenolphthalein remains in its unionized form, rendering it colorless. This visible change happens in a narrow pH range, roughly between 8.2 to 10, where it shifts from colorless to pink as the pH increases. This unique property makes it highly valuable in detecting the endpoint of reactions where a shift from acidic to basic conditions is indicated by the appearance of a color.

pH Levels in Chemistry

The pH scale is a measure of the acidity or basicity of a solution. It ranges from 0 to 14, with 7 being neutral. Values below 7 indicate acidic conditions, while values above 7 are basic (alkaline).
Phenolphthalein is particularly responsive to changes in pH, which is why it is employed in reactions that involve shifts between acidic and basic states. In acidic solutions, phenolphthalein's molecular structure does not interact with visible light, leading to a colorless appearance. Conversely, when the pH climbs to basic levels, ionization occurs, altering the electron configuration to allow absorption of certain light wavelengths, manifesting as a magenta color.

Light Absorption and Transmission

When phenolphthalein appears magenta at high pH, it's due to specific interactions with light. The process involves absorption of light, particularly around 553 nm, which corresponds to the green region of the spectrum. This means that phenolphthalein absorbs the green light and does not transmit it.

• As green light is absorbed, its complementary color, magenta, is transmitted, which is what our eyes perceive.
• The range of light absorbed indicates the alterations in the electronic structure of phenolphthalein based on its ionization state.

By this means, color indicators like phenolphthalein serve as practical tools in laboratory settings to visually follow chemical reactions and transformations.

Ionization in Chemistry

Ionization refers to the process in which an atom or molecule gains or loses electrons, acquiring a net charge. For phenolphthalein, this occurs when the pH is high.
The ionized form of phenolphthalein has a different electron distribution compared to its non-ionized form at low pH. This difference in electronic states affects how the molecule absorbs light, leading to the visible color shift.

• In its ionized, magenta form, phenolphthalein's altered electron structure enables the absorption of specific wavelengths.
• Conversely, the unionized form, which appears colorless, does not absorb these visible wavelengths effectively.

This distinct behavior of phenolphthalein in response to pH changes illustrates the importance of ionization in altering the properties of molecules and their interactions with light.