Question

The function of boiling the sodium extract with conc. \(\mathrm{HNO}_{3}\) before testing for halogens is: (a) To make the solution acidic (b) To make the solution clear (c) To convert \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}^{3+}\) (d) To destroy \(\mathrm{CN}\) and \(\mathrm{S}^{2-}\) ions which will otherwise give precipitate

Short answer

(d) To destroy \(\mathrm{CN}\) and \(\mathrm{S}^{2-}\) ions.

Step-by-step solution

Understanding the Problem

We need to determine the purpose of boiling the sodium extract with concentrated nitric acid (\(\mathrm{HNO}_3\)) before testing for halogens in a qualitative analysis procedure.

Review the Options

We have four options to consider: (a) making the solution acidic, (b) making the solution clear, (c) converting \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}^{3+}\), and (d) destroying \(\mathrm{CN}\) and \(\mathrm{S}^{2-}\) ions.

Analyze Each Option

(a) While \(\mathrm{HNO}_3\) is an acid, the main goal in this context is not just to acidify the solution. (b) There is no specific need to make the solution clear before testing for halogens. (c) Conversion between iron oxidation states is not relevant to halogen detection. (d) The purpose of boiling with \(\mathrm{HNO}_3\) is indeed to destroy \(\mathrm{CN}\) and \(\mathrm{S}^{2-}\), as these ions can interfere with the detection of halogens by giving false-positive results.

Determine the Correct Answer

The most appropriate reason for using \(\mathrm{HNO}_3\) in this reaction is to oxidize and destroy \(\mathrm{CN}\) and \(\mathrm{S}^{2-}\) ions which might form precipitates that affect the test results for halogens.

Key concepts

Halogen Detection

In chemistry, qualitative analysis is a method used to identify the chemical constituents of a substance or mixture. One important aspect is halogen detection.
Halogens are a group of elements found in Group 17 of the periodic table, which includes chlorine, bromine, and iodine, among others. Detecting these elements is crucial in various chemical analyses. The characteristic reactions of halogens often involve the formation of precipitates with silver ions, which can be identified visually.

For instance:

• Chlorine reacts with silver nitrate to form white silver chloride.
• Bromine produces a pale yellow silver bromide precipitate.
• Iodine results in a yellow silver iodide precipitate.

Knowing these reactions helps in identifying the presence of specific halogens in a sample. Halogens play a pivotal role due to their reactive nature, hence special methods like sodium fusion are used to prepare samples for analysis.

Sodium Extract

The sodium fusion extract, also known as the Lassaigne's extract, is an essential step in the qualitative analysis of substances to test for elements like halogens, nitrogen, and sulfur. This involves fusing the organic compound with metallic sodium.
The fusion process converts these covalently bonded elements into ionic forms, which makes them easier to detect in subsequent tests. For example:

• Carbon and nitrogen in the compound form cyanide ions (\( m{CN^{-}}\)).
• Sulfur forms sulfide ions (\( m{S^{2-}}\)).
• Halogens convert into corresponding halide ions.

After the fusion, the mixture is treated with water to extract these ions. The resulting solution, known as the sodium fusion extract, allows for easy identification of these elements by various qualitative tests, such as precipitating with silver nitrate for halogens.

Interference Removal

In qualitative analysis, interference from other ions can often lead to inaccurate results. Therefore, it is crucial to remove or neutralize these interferences to achieve reliable outcomes. One of the common interferences during halogen detection is by cyanide and sulfide ions.
These ions can react with silver ions to form similar precipitates, such as silver cyanide and silver sulfide, that might be mistaken for halogen precipitates. To prevent this, the sodium extract is boiled with concentrated nitric acid (*HNO₃*).

This acid serves a dual purpose:

• It oxidizes both cyanide (\( m{CN^{-}}\)) and sulfide (\( m{S^{2-}}\)) ions, effectively destroying them by breaking their bonds.
• This eliminates potential false-positive results in halogen testing, ensuring that only actual halogen ions react with the silver ions.

This step is pivotal in achieving accurate qualitative analysis in halogen detection.