Understanding the Brown Ring Test Procedure

The brown ring test is a classic chemical test used to detect the presence of nitrate ions ($\text{NO}_3^-$) in a solution. It relies on the reduction of nitrate ions to nitric oxide ($\text{NO}$), which then reacts with ferrous ions ($\text{Fe}^{2+}$) to form a brown colored complex.

Analyzing the Given Steps for Brown Ring Test

The question provides four descriptions of steps involved in the brown ring test experiment:

• (A) Pour conc. $\text{H}_2\text{SO}_4$ very slowly along the side of the test tube, very few drops of the acid should be poured. This step involves adding concentrated sulfuric acid carefully to form a separate layer.
• (B) Take half spatula of the given salt in a dry test tube. This step describes starting with the solid form of the salt.
• (C) Add 2 - 3 mL of freshly prepared ferrous sulphate solution to the test tube. This step involves adding ferrous sulfate solution, a key reagent for the test.
• (D) Make a solution of the given salt. This step describes dissolving the solid salt in a solvent (usually water) to prepare an aqueous solution.

Determining the Correct Order of Steps

We are asked to find the correct order of these steps from the given options. The provided correct answer corresponds to the sequence (B), (A), (C), (D).

Let's examine this sequence:

1. (B) Take half spatula of the given salt in a dry test tube. - This is the starting point, obtaining the solid sample.
2. (A) Pour conc. $\text{H}_2\text{SO}_4$ very slowly along the side of the test tube... - This step involves adding concentrated sulfuric acid.
3. (C) Add 2 - 3 mL of freshly prepared ferrous sulphate solution to the test tube. - This step involves adding the ferrous sulfate solution.
4. (D) Make a solution of the given salt. - This step involves preparing the salt solution.

We need to check which of the provided options matches this specific sequence (B), (A), (C), (D).

Let's look at the options:

• Option 1: (B), (D), (C), (A)
• Option 2: (B), (A), (C), (D)
• Option 3: (A), (B), (C), (D)
• Option 4: (D), (B), (C), (A)

Comparing the sequence (B), (A), (C), (D) with the options, we see that Option 2 matches this order exactly.

Matching the Sequence to the Correct Option

The sequence of steps presented as (B), (A), (C), (D) corresponds to Option 2. Therefore, based on the provided choices, Option 2 is the indicated correct answer for the order of steps in the brown ring test experiment as described by steps (A), (B), (C), and (D).

The required sequence is (B) followed by (A), then (C), and finally (D).

Option 1 gives (B), (D), (C), (A).

Option 2 gives (B), (A), (C), (D).

Option 3 gives (A), (B), (C), (D).

Option 4 gives (D), (B), (C), (A).

Thus, Option 2 is the correct choice as it lists the steps in the sequence (B), (A), (C), (D).

Revision Table: Key Steps in Chemical Tests

Additional Information: The Actual Brown Ring Test Procedure

While the question presents a specific order, the standard laboratory procedure for the brown ring test to detect nitrate ions typically involves the following steps:

1. Prepare an aqueous solution of the salt containing the suspected nitrate ions. (This corresponds to step D, potentially preceded by B if starting with solid salt).
2. Add freshly prepared ferrous sulfate ($\text{FeSO}_4$) solution to the salt solution. (This corresponds to step C). It's important that the $\text{FeSO}_4$ solution is fresh because $\text{Fe}^{2+}$ oxidizes easily to $\text{Fe}^{3+}$ in air, and $\text{Fe}^{3+}$ interferes with the test.
3. Carefully add concentrated sulfuric acid ($\text{H}_2\text{SO}_4$) along the inner wall of the test tube so that it forms a separate layer below the aqueous solution. (This corresponds to step A). The acid is denser and sinks to the bottom.

A brown ring is formed at the interface between the two layers if nitrate ions are present. The reactions involved are:

First, nitrate is reduced to nitric oxide by $\text{Fe}^{2+}$ in the presence of concentrated $\text{H}_2\text{SO}_4$:

$\text{NO}_3^- \text{ (aq)} + 3\text{Fe}^{2+} \text{ (aq)} + 4\text{H}^+ \text{ (aq)} \rightarrow \text{NO (g)} + 3\text{Fe}^{3+} \text{ (aq)} + 2\text{H}_2\text{O (l)}$

Then, the nitric oxide gas reacts with excess ferrous ions to form the pentaaquanitrosyliron(II) complex, which is brown:

$[\text{Fe(H}_2\text{O)}_6]^{2+} \text{ (aq)} + \text{NO (g)} \rightarrow [\text{Fe(H}_2\text{O)}_5\text{(NO)}]^{2+} \text{ (aq)} + \text{H}_2\text{O (l)}$

The brown ring indicates the formation of this complex.

The standard, chemically valid order starting with solid salt would be (B) $\rightarrow$ (D) $\rightarrow$ (C) $\rightarrow$ (A). However, based on the provided correct option, the chosen sequence is (B), (A), (C), (D).