Question

Given below are two statements :
Statement (I) : Potassium hydrogen phthalate is a primary standard for standardisation of sodium hydroxide solution.
Statement (II) : In this titration phenolphthalein can be used as indicator.
In the light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement I and Statement II are correct
• B. Statement I is correct but Statement II is incorrect
• C. Statement I is incorrect but Statement II is correct
• D. Both Statement I and Statement II are incorrect

Answer 1

The Correct Option is A

This question assesses the understanding of two statements regarding the titration of sodium hydroxide (NaOH) using potassium hydrogen phthalate (KHP).

Concept Used:

Primary Standard: A primary standard is a compound of high purity and stability, used to establish the precise concentration of a titrant solution. Key requirements for a primary standard include:

1. Exceptional purity (typically exceeding 99.9%).
2. Atmospheric stability (non-hygroscopic, resistant to CO₂ absorption).
3. High molar mass to minimize weighing errors.
4. Good solubility in the solvent (usually water).
5. Stoichiometric and rapid reaction with the substance being standardized.

Acid-Base Indicators: Acid-base indicators are weak acids or bases that exhibit distinct color changes within specific pH ranges. An appropriate indicator for a titration must have a color transition range that encompasses the pH at the equivalence point. For the titration of a weak acid with a strong base, the equivalence point is basic due to the hydrolysis of the conjugate base of the weak acid. Consequently, an indicator functioning in the basic pH range is necessary.

Step-by-Step Solution:

Step 1: Evaluate Statement (I).

Statement (I) posits: "Potassium hydrogen phthalate is a primary standard for standardisation of sodium hydroxide solution."

We verify if potassium hydrogen phthalate (KHP, \( \text{KHC}_8\text{H}_4\text{O}_4 \)) meets primary standard criteria:

- Purity: KHP is available in very high purity.
- Stability: It is a stable solid, unreactive with atmospheric moisture (non-hygroscopic) and CO₂.
- Molar Mass: Its molar mass is approximately 204.22 g/mol, a high value that reduces relative weighing errors.
- Solubility: It is soluble in water.
- Reaction: As a weak acid, it reacts completely and stoichiometrically with strong bases like NaOH.

Given its adherence to these requirements, KHP is commonly employed as a primary standard for standardizing strong base solutions like NaOH. Thus, Statement (I) is correct.

Step 2: Evaluate Statement (II).

Statement (II) states: "In this titration phenolphthalein can be used as indicator."

This titration involves a weak acid (KHP) and a strong base (NaOH). The relevant net ionic equation is:

\[ \text{HC}_8\text{H}_4\text{O}_4^- (aq) + \text{OH}^- (aq) \rightarrow \text{C}_8\text{H}_4\text{O}_4^{2-} (aq) + \text{H}_2\text{O}(l) \]

At the equivalence point, the phthalate ion (\( \text{C}_8\text{H}_4\text{O}_4^{2-} \)), the conjugate base of KHP, undergoes hydrolysis, generating hydroxide ions and making the solution basic:

\[ \text{C}_8\text{H}_4\text{O}_4^{2-} (aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{HC}_8\text{H}_4\text{O}_4^- (aq) + \text{OH}^- (aq) \]

Consequently, the pH at the equivalence point for a weak acid-strong base titration exceeds 7, typically falling between 8 and 10.

Phenolphthalein transitions from colorless to pink within the pH range of approximately 8.2 to 10.0. This range aligns perfectly with the steep pH increase at the equivalence point of the KHP-NaOH titration, making phenolphthalein a suitable indicator. Therefore, Statement (II) is also correct.

Final Result:

Both Statement (I) and Statement (II) are accurate. KHP is an ideal primary standard for NaOH, and phenolphthalein is an appropriate indicator for this titration as its pH transition range matches the equivalence point.

Hence, both Statement (I) and Statement (II) are true.