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Chapter 9: Problem 10

A quantitative analysis for aniline \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}, K_{\mathrm{b}}=3.94 \times 10^{-10}\right)\) is carried out by an acid-base titration using glacial acetic acid as the solvent and \(\mathrm{HClO}_{4}\) as the titrant. A known volume of sample that contains \(3-4 \mathrm{mmol}\) of aniline is transferred to a \(250-\mathrm{mL}\) Erlenmeyer flask and diluted to approximately \(75 \mathrm{~mL}\) with glacial acetic acid. Two drops of a methyl violet indicator are added, and the solution is titrated with previously standardized \(0.1000 \mathrm{M} \mathrm{HClO}_{4}\) (prepared in glacial acetic acid using anhydrous \(\mathrm{HClO}_{4}\) ) until the end point is reached. Results are reported as parts per million aniline. (a) Explain why this titration is conducted using glacial acetic acid as the solvent instead of using water. (b) One problem with using glacial acetic acid as solvent is its relatively high coefficient of thermal expansion of \(0.11 \% /{ }^{\circ} \mathrm{C}\). For example, \(100.00 \mathrm{~mL}\) of glacial acetic acid at \(25^{\circ} \mathrm{C}\) occupies \(100.22 \mathrm{~mL}\) at \(27^{\circ} \mathrm{C}\). What is the effect on the reported concentration of aniline if the standardization of \(\mathrm{HClO}_{4}\) is conducted at a temperature that is lower than that for the analysis of the unknown? (c) The procedure calls for a sample that contains \(3-4\) mmoles of aniline. Why is this requirement necessary?

Short Answer

Expert verified
(a) Glacial acetic acid increases aniline protonation. (b) A higher analysis temperature decreases reported concentration. (c) 3-4 mmol ensures accurate titration.

Step by step solution

01

Explaining the Choice of Solvent

Aniline is a weak base with a low base dissociation constant, \( K_b = 3.94 \times 10^{-10} \). In water, aniline would be poorly dissociated, making it difficult to conduct a precise titration. Glacial acetic acid is used as the solvent because it acts as a better ionizing solvent for amines like aniline, thereby increasing the degree of protonation of aniline and allowing for a more accurate titration with \( \mathrm{HClO}_4 \).
02

Effect of Temperature on Aniline Concentration

Glacial acetic acid expands with temperature. If the standardization of \( \mathrm{HClO}_4 \) is conducted at a lower temperature and the analysis at a higher temperature, the volume of the solvent would be greater during the analysis. This results in a greater volume of solvent per mole of aniline, thereby decreasing the reported concentration of aniline when the temperature is higher during analysis compared to standardization.
03

Importance of Required Millimole Range

Using 3-4 mmol of aniline ensures that the titration is conducted within the optimal range of the indicator's color change. This range ensures that the volume of titrant used is large enough to minimize percentage errors in volume measurement and small enough to avoid excessive dilution of the solution, which can affect the accuracy of the titration endpoint.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Glacial Acetic Acid
Glacial acetic acid is often used as a solvent in acid-base titrations, especially when dealing with substances like aniline, which is a weak base. Here's why glacial acetic acid is favored in these scenarios:
  • Unlike water, which is a strong polar solvent, glacial acetic acid offers a medium where aniline can show better dissociation.
  • It helps in protonating weak bases like aniline more effectively than water, thus enhancing their reactivity when titrated with a strong acid like \( \mathrm{HClO}_4 \).
By using glacial acetic acid, the precision of the titration process is improved, resulting in more reliable analytical outcomes.
Ionizing Solvent
An ionizing solvent plays a crucial role in titration by facilitating the dissociation of acids or bases into ions. In the context of titrating aniline, glacial acetic acid serves this purpose well:
  • Aniline has a weak base dissociation constant, making it poorly dissociated in polar solvents like water.
  • Glacial acetic acid changes this dynamic by creating a medium that allows aniline to ionize more effectively.
  • This increased ionization leads to a higher degree of reaction with the titrant, \( \mathrm{HClO}_4 \), which is vital for the accuracy of the titration.
Thus, understanding the choice of ionizing solvent is key to conducting a successful titration of compounds like aniline.
Weak Base Dissociation Constant
The dissociation constant of a base, especially a weak one like aniline, is critical in determining its behavior in a titration process:
  • Aniline has a very small dissociation constant \( K_b = 3.94 \times 10^{-10} \), indicating low dissociation in water.
  • This small value implies that in an aqueous solution, only a tiny fraction of aniline molecules contribute hydroxide ions.
  • Using an appropriate solvent such as glacial acetic acid helps facilitate the measurement of these minimal ion concentrations, enhancing the reaction with the titrant.
The dissociation constant is an essential piece of information, guiding the choice of solvent and the titration method.
Thermal Expansion
Thermal expansion is a physical property of materials that causes them to change volume with temperature. With glacial acetic acid, this property can affect titration:
  • Glacial acetic acid has a high coefficient of thermal expansion, meaning its volume changes significantly with temperature fluctuations.
  • For instance, at 25掳C, 100 mL of glacial acetic acid might expand to 100.22 mL at 27掳C.
  • This expansion can dilute the concentration of a solution, impacting the concentration calculations of the analyte as temperature varies during analysis.
Careful temperature control is necessary to ensure consistency and accuracy when performing titrations with glacial acetic acid as the solvent.
Titration Endpoint
The titration endpoint is the point at which the reaction between the titrant and the analyte is complete and is usually indicated by a color change:
  • A methyl violet indicator is used to signal the titration endpoint by changing color when the reaction reaches completion.
  • This endpoint is crucial for determining the amount of titrant needed to react completely with the analyte, thus providing a measure of its concentration.
  • The requirement of having 3-4 mmol of aniline ensures the endpoint is reached within a measurable range, reducing errors in the titration process.
Understanding the titration endpoint is vital for accurate chemical analysis, helping to determine the precise amount of a substance within a solution.

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Most popular questions from this chapter

Explain why it is not possible for a sample of water to simultaneously have \(\mathrm{OH}^{-}\) and \(\mathrm{HCO}_{3}^{-}\) as sources of alkalinity.

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