Question

The indicator used for estimation of salt content in butter by Volhard's method is

• A. Ferroin
• B. Ferric ammonium sulphate
• C. Potassium dichromate
• D. Potassium chromate

Hint:

Volhard's method is an indirect acidic titration that uses ferric ammonium sulphate as the indicator, whereas Mohr's method is a direct neutral titration that uses potassium chromate.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The salt (sodium chloride, \(\text{NaCl}\)) content in butter is determined quantitatively using argentometric methods.
Volhard's method is an indirect back-titration technique carried out under acidic conditions.

Step 2: Detailed Explanation:
In Volhard's method, chloride ions in the butter sample are precipitated by adding a measured excess of standard silver nitrate (\(\text{AgNO}_3\)) in the presence of nitric acid:
\[ \text{Cl}^- + \text{Ag}^+ (\text{excess}) \rightarrow \text{AgCl} \downarrow \] The unreacted silver ions remaining in solution are then back-titrated against a standard solution of potassium thiocyanate (\(\text{KSCN}\)):
\[ \text{Ag}^+ + \text{SCN}^- \rightarrow \text{AgSCN} \downarrow \] To detect the completion of this back-titration, ferric ammonium sulphate (iron alum) is used as the indicator.
When all silver ions have been precipitated as silver thiocyanate, the first excess of thiocyanate ions reacts with the ferric indicator (\(\text{Fe}^{3+}\)) to form a reddish-brown complex:
\[ \text{Fe}^{3+} + \text{SCN}^- \rightarrow [\text{Fe(SCN)}]^{2+} \] In contrast:
Potassium chromate is the indicator used in Mohr's direct titration method, which is carried out under neutral conditions.

Step 3: Final Answer:
Consequently, ferric ammonium sulphate is the indicator used in Volhard's method for salt estimation in butter.