Question

The alkali not suitable for volumctric dctermination of IICl using phenolphthalcin as an indicator (1) \(\mathrm{NaOH}\) (2) \(\mathrm{Ba}(\mathrm{OH})_{2}\) (3) \(\mathrm{KOH}\) (4) \(\mathrm{NH}_{4} \mathrm{OH}\)

Short answer

NH₄OH

Step-by-step solution

Understand the Question

Identify that the goal is to determine which alkali is not suitable for a volumetric determination of HCl using phenolphthalein as an indicator.

Know the Alkalis

Consider the alkalis given: NaOH, Ba(OH)₂, KOH, and NH₄OH. Understand their properties in acidic and basic titration reactions.

Indicator Properties

Phenolphthalein changes color in a pH range of approximately 8.2 to 10. It is colorless in acidic solutions and turns pink in basic solutions.

Suitability of Each Alkali

Evaluate the suitability of each alkali: 1. NaOH (Strong base, suitable)2. Ba(OH)₂ (Strong base, suitable) 3. KOH (Strong base, suitable) 4. NH₄OH (Weak base, not suitable for phenolphthalein titrations as it doesn't produce a clear endpoint in this pH range)

Conclusion

Determine that NH₄OH (ammonium hydroxide) is not suitable for the volumetric determination of HCl using phenolphthalein.

Key concepts

Alkali Suitability

In a volumetric determination of hydrochloric acid (HCl) using phenolphthalein as an indicator, it is crucial to choose a suitable alkali. Alkalis are substances that accept protons or donate pairs of electrons. For titration purposes, the nature of the alkali—whether it is a strong or weak base—affects the detection of the endpoint.
Common strong bases include:

• Sodium hydroxide (NaOH)
• Barium hydroxide (Ba(OH)₂)
• Potassium hydroxide (KOH)

Ammonium hydroxide (NH₄OH) is an example of a weak base. The suitability of each alkali for titrating HCl needs to be checked, as it affects the accuracy of the results.
Strong bases fully dissociate in water, providing clear and unambiguous color change with phenolphthalein between pH 8.2 and 10. However, weak bases do not dissociate completely, producing less distinct endpoints. Thus, NH₄OH is not suitable for this titration.

Phenolphthalein Indicator

Phenolphthalein is a common indicator used in acid-base titrations. It is known for its distinct color change, which makes it easy to detect the endpoint of the titration.
Phenolphthalein is colorless in acidic solutions. As the pH of the solution increases and enters the range of 8.2 to 10, phenolphthalein turns pink. This color change indicates that the solution is basic.
When titrating HCl (a strong acid), the chosen alkali needs to provide a clear and sharp color change with phenolphthalein. Strong bases like NaOH, Ba(OH)₂, and KOH ensure that the transition from colorless to pink is noticeable and precise. In contrast, weak bases like NH₄OH do not shift the pH sufficiently to produce a clear indicator response, making them less suitable for this purpose.

Titration Endpoint

The titration endpoint is a crucial moment in a titration, indicating the point at which the reactants have been mixed in the exact stoichiometric proportions. For a titration involving HCl and an alkali using phenolphthalein, the endpoint is usually marked by a permanent color change of the indicator.
To achieve an accurate titration endpoint with phenolphthalein:

• A strong base should be used to ensure a sharp endpoint.
• The endpoint is identified when the solution changes from colorless to a faint pink, just as it reaches the pH range of 8.2 to 10.
• A slow and careful addition of the titrant (the alkali) helps in identifying the precise moment when the endpoint is reached.

Using a weak base like NH₄OH can cause difficulties in precisely identifying the endpoint, as the color change may be weak or non-existent due to insufficient pH change. This makes NH₄OH unsuitable for accurate volumetric determination of HCl using phenolphthalein.