Question

In Mohr's method of salt estimation of table butter, the type of titration employed is

• A. Argentometric
• B. Iodometric
• C. Acid-base
• D. Potentiometric

Hint:

To ensure accurate results with Mohr's method, keep the sample pH neutral (between 6.5 and 9.0).
At low pH (acidic conditions), the chromate indicator is converted into dichromate, delaying the endpoint.
At high pH (alkaline conditions), silver ions precipitate as silver hydroxide, interfering with the titration.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Table butter contains added salt (sodium chloride, $\text{NaCl}$) as a preservative and flavor enhancer, with concentrations typically ranging from $1.5\%$ to $2.5\%$.
Mohr's method is the standard analytical technique used to determine the chloride ion ($\text{Cl}^-$) concentration in food products, which is then used to calculate the salt content.

Step 2: Detailed Explanation:
Mohr's method is a precipitation titration that specifically uses silver ions ($\text{Ag}^+$) as the titrant, classifying it as an argentometric titration.
In this procedure, a butter sample is melted and mixed with hot water to extract the salt into the aqueous phase.
An aliquot of this aqueous phase is then titrated with a standard solution of silver nitrate ($\text{AgNO}_3$) in the presence of a potassium chromate ($\text{K}_2\text{CrO}_4$) indicator.
During the titration, the added silver ions react with the chloride ions in the sample to form a white precipitate of silver chloride ($\text{AgCl}$):
\[ \text{Ag}^+ (\text{aq}) + \text{Cl}^- (\text{aq}) \rightarrow \text{AgCl} (\text{s}) \downarrow \quad (\text{White Precipitate}) \] Because silver chloride is highly insoluble, this reaction continues until all available chloride ions are precipitated.
The first excess drop of silver nitrate then reacts with the chromate indicator ions ($\text{CrO}_4^{2-}$) to form a reddish-brown precipitate of silver chromate ($\text{Ag}_2\text{CrO}_4$):
\[ 2\text{Ag}^+ (\text{aq}) + \text{CrO}_4^{2-} (\text{aq}) \rightarrow \text{Ag}_2\text{CrO}_4 (\text{s}) \downarrow \quad (\text{Red-Brown Precipitate}) \] This distinct color change from yellow to reddish-brown indicates the endpoint of the titration.
Since the titrant used is silver nitrate ($\text{Ag}^+$), this method is classified as an argentometric titration.

Step 3: Final Answer:
The type of titration employed in Mohr's method is argentometric titration.