Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : In titratable acidity determination of milk by titration method, the pink colour (phenolphthalein end point) disappears soon, whereas in normal acid-base titration, it persists for long time.
Reason (R) : Due to alkalinity of the medium after the titration, there is a shift in the equilibrium of milk salts releasing more of hydrogen ions.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

To minimize this fading effect and obtain a stable, accurate endpoint:
Add a small amount of neutral potassium oxalate to the milk sample before titration.
This precipitates the soluble calcium as calcium oxalate, preventing the formation of tricalcium phosphate and stabilizing the pink endpoint.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Determining the titratable acidity of milk is a common quality test that uses phenolphthalein as a pH indicator.
During the titration of milk with sodium hydroxide (\(\text{NaOH}\)), the endpoint pink color is unstable and fades quickly.
This is a unique behavior of milk compared to simple, non-buffered acid-base titrations.

Step 2: Detailed Explanation:
Let us analyze Assertion (A):
The assertion states that the pink color at the phenolphthalein endpoint disappears quickly in milk, whereas it remains stable in typical acid-base titrations.
This statement is correct.
When milk is titrated with \(\text{NaOH}\) to a faint pink endpoint (around pH 8.3), the color disappears within 10 to 15 seconds.
Now let us analyze Reason (R):
The reason states that this color change is due to a shift in the milk salt equilibrium under alkaline conditions, which releases more hydrogen ions.
This statement is correct.
Milk contains high concentrations of soluble calcium and phosphate ions in equilibrium.
As the pH increases during titration, these soluble calcium and phosphate ions react to form insoluble tricalcium phosphate:
\[ 3\text{Ca}^{2+} + 2\text{HPO}_4^{2-} \rightarrow \text{Ca}_3(\text{PO}_4)_2 \downarrow + 2\text{H}^+ \] This precipitation reaction releases free hydrogen (\(\text{H}^+\)) ions into the milk.
The released hydrogen ions lower the pH of the milk back below 8.3, causing the phenolphthalein indicator to return to its colorless form and making the pink color disappear.
Because the Reason correctly explains why the endpoint fades, Assertion (A) is correct and Reason (R) is its correct explanation.

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).