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:
Statement I is correct but Statement II is incorrect
Statement I is incorrect but Statement II is correct
Both Statement I and Statement II are incorrect
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The Correct Option isA
Solution and Explanation
This question asks us to evaluate two statements related to the titration of sodium hydroxide (NaOH) with potassium hydrogen phthalate (KHP).
Concept Used:
Primary Standard: A primary standard is a highly pure and stable compound used to prepare a solution of an accurately known concentration, which can then be used to standardize other solutions. A substance must meet several criteria to be a primary standard:
1. High level of purity (typically >99.9%). 2. Stable and unreactive with the atmosphere (non-hygroscopic, does not absorb CO₂). 3. High molar mass to minimize relative error during weighing. 4. Readily soluble in the chosen solvent (usually water). 5. Reacts stoichiometrically and rapidly with the substance being standardized.
Acid-Base Indicators: An acid-base indicator is a weak acid or weak base that changes color over a specific pH range. The choice of a suitable indicator for a titration depends on the pH at the equivalence point. The indicator's working pH range must overlap with the sharp pH change that occurs around the equivalence point. For a weak acid-strong base titration, the solution at the equivalence point is basic due to the hydrolysis of the conjugate base of the weak acid. Therefore, an indicator that changes color in the basic pH range is required.
Step-by-Step Solution:
Step 1: Evaluate Statement (I).
Statement (I) says: "Potassium hydrogen phthalate is a primary standard for standardisation of sodium hydroxide solution."
We need to check if potassium hydrogen phthalate (KHP, \( \text{KHC}_8\text{H}_4\text{O}_4 \)) meets the criteria for a primary standard:
- Purity: KHP can be obtained in a very high state of purity. - Stability: It is a solid that is stable upon drying and is not hygroscopic (does not absorb water from the air). It also doesn't react with components of the atmosphere like CO₂. - Molar Mass: The molar mass of KHP is approximately 204.22 g/mol. This is a high value, which means that a relatively large mass is needed to make a solution, thus minimizing percentage errors in weighing. - Solubility: It is soluble in water. - Reaction: It is a weak acid and reacts completely and stoichiometrically with strong bases like sodium hydroxide.
Since KHP fulfills all the essential requirements, it is widely used as a primary standard for standardizing strong base solutions like NaOH. Therefore, Statement (I) is correct.
Step 2: Evaluate Statement (II).
Statement (II) says: "In this titration phenolphthalein can be used as indicator."
The titration involves the reaction between a weak acid (KHP) and a strong base (NaOH). The net ionic equation is:
At the equivalence point, all the weak acid has been converted to its conjugate base, the phthalate ion (\( \text{C}_8\text{H}_4\text{O}_4^{2-} \)). This conjugate base will hydrolyze in water to produce hydroxide ions, making the solution basic.
Due to the production of \( \text{OH}^- \) ions, the pH at the equivalence point of a weak acid-strong base titration is greater than 7 (typically in the range of 8 to 10).
Phenolphthalein is an indicator that changes color from colorless to pink in the pH range of approximately 8.2 to 10.0. Since this range coincides with the sharp pH increase at the equivalence point of the KHP-NaOH titration, phenolphthalein is an excellent choice for this titration. Therefore, Statement (II) is also correct.
Final Result:
Both Statement (I) and Statement (II) are correct. KHP is an ideal primary standard for NaOH, and phenolphthalein is the appropriate indicator for the titration because the equivalence point lies in its pH transition range.
Thus, both Statement (I) and Statement (II) are true.
