Question

Mixed indicator used in the titration of the distillate in MicroKjeldahl method is made up of

• A. Methyl orange and methylene blue
• B. Methyl orange and methyl red
• C. Methylene blue and bromocresol red
• D. Methyl red and methylene blue

Hint:

Remember the color transitions of Tashiro's mixed indicator:
- Alkaline (before titration starts) $\rightarrow$ Green.
- Endpoint (neutralization) $\rightarrow$ Grey.
- Acidic (after the endpoint is passed) $\rightarrow$ Reddish-violet.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The MicroKjeldahl method is a standard analytical procedure used to determine nitrogen and protein content in dairy products.
The final step of this method involves titrating distilled ammonia, which is trapped in a boric acid solution, using a standard mineral acid.
A mixed pH indicator is used to detect the endpoint of this neutralization reaction.

Step 2: Detailed Explanation:
During the distillation step of the Kjeldahl method, ammonia gas (\(\text{NH}_3\)) is distilled into a receiving solution of boric acid (\(\text{H}_3\text{BO}_3\)).
The ammonia reacts with the boric acid to form ammonium borate, which increases the pH of the receiving solution.
This solution is then titrated with standard hydrochloric acid (\(\text{HCl}\)) or sulfuric acid (\(\text{H}_2\text{SO}_4\)) to neutralize the borate ions and restore the pH of the pure boric acid solution.
The endpoint of this titration occurs at approximately pH 4.8.
To detect this endpoint clearly, a mixed indicator containing methyl red and methylene blue (often known as Tashiro's indicator) is used.
- Methyl red is a pH indicator that changes from red (acidic) to yellow (basic) across a pH range of 4.4 to 6.2.
- Methylene blue acts as a background color contrast agent and does not change color with pH in this range.
The combination of these two compounds produces a sharp color transition:
- In the basic ammonium borate solution, the mixed indicator is green.
- At the endpoint (pH 4.8), the color transitions through a neutral grey.
- Once a slight excess of acid is added, the solution turns reddish-violet (pink).
This distinct transition makes the endpoint much easier to identify than using methyl red alone.

Step 3: Final Answer:
The mixed indicator used in the titration is made of methyl red and methylene blue.