Question

Prussian blue obtained in detection of nitrogen is (A) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (B) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (C) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]\) (D) \(\mathrm{Na}_{2}[\mathrm{Fe}(\mathrm{SCN})]\)

Short answer

The correct answer is (B) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\), which represents Prussian blue, also known as ferric ferrocyanide.

Step-by-step solution

Option A

(A) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) This formula represents sodium hexacyanoferrate(II), which is different from Prussian blue. Therefore, this option is incorrect.

Option B

(B) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) This formula represents ferric ferrocyanide, which is in fact the chemical formula for Prussian blue. The correct option is (B).

Option C

(C) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]\) This formula represents sodium pentacyanonitrosylferrate(II). This compound is not Prussian blue, so this option is incorrect.

Option D

(D) \(\mathrm{Na}_{2}[\mathrm{Fe}(\mathrm{SCN})]\) This formula represents sodium thiocyanatoiron(II) complex. This compound is not Prussian blue, so this option is incorrect. The correct answer is (B) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\), which represents Prussian blue, also known as ferric ferrocyanide.

Key concepts

Detection of Nitrogen in Organic Compounds

Identifying elements in organic compounds is a fundamental aspect of chemistry, and nitrogen is no exception. The presence of nitrogen is commonly detected using the Lassaigne's test. This test involves heating the organic compound with sodium metal to convert nitrogen to sodium cyanide, which then reacts with iron(II) sulfate to form the deep blue colored compound known as Prussian blue.

Here's the process in a nutshell:

• The organic compound is fused with sodium metal, leading to the formation of sodium cyanide if nitrogen is present.
• The fusion mixture is then treated with iron(II) sulfate, which allows the iron(II) ions to react with the cyanide ions.
• Upon acidification, iron(II) ions are oxidized to iron(III) ions forming the ferric ferrocyanide complex.
• The appearance of a Prussian blue precipitate confirms the presence of nitrogen in the original organic compound.

This qualitative test is a cornerstone in organic chemistry for establishing the presence of nitrogen, often before proceeding to quantitative analysis.

Sodium Hexacyanoferrate(II)

Sodium hexacyanoferrate(II), also known as sodium ferrocyanide, is denoted by the formula \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\). It's an important compound in chemistry used both in the Prussian blue detection method and as a sequestrant in the food industry.

Though it contains the cyanide group, it's noteworthy that this compound is not toxic due to the strong bond between the cyanide ions and the central iron ion. In the context of detecting nitrogen in organic compounds, it's the precursor to forming Prussian blue under the right conditions - when exposed to acidic conditions and iron(III) ions.

Ferric Ferrocyanide

Ferric ferrocyanide is the chemical name for Prussian blue and is represented by the formula \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\). This deep blue pigment has a rich history in art dating back to the early 18th century and has been deployed in medical treatments for certain types of heavy metal poisoning.

In a laboratory context, the formation of Prussian blue signifies the Lassaigne's test's completion and thus the presence of nitrogen in an organic sample. This occurs through a reaction with ferric ions to form the characteristic blue precipitate, affirming Prussian blue's profound utility not only as a pigment but also as a chemical indicator.