Lassaigne’s Test, also known as the sodium fusion test, is a qualitative analytical method used to detect the presence of nitrogen, sulfur, and halogens (chlorine, bromine, iodine) in an organic compound.
In other words, Lassaigne’s Test is a chemical procedure where an organic compound is fused with metallic sodium. This fusion process converts the covalently bonded nitrogen, sulfur, or halogens in the compound into their ionic sodium salts (sodium cyanide, sodium sulfide, sodium halides, respectively). These salts can then be extracted into water and tested for the presence of these elements using specific reagents. The test involves:
- Heating the organic compound with sodium to form sodium salts.
- Extracting these salts in water.
- Performing specific chemical tests for nitrogen, sulfur, and halogens.
Preparation of Sodium Fusion Extract (Lassaigne’s Extract)
Fusion with Sodium: A small piece of metallic sodium is taken in a fusion tube, and a small amount of the organic compound is added. The tube is heated until the mixture reacts and forms a sodium fusion product. This step is crucial because sodium reacts with the elements in the organic compound to form inorganic sodium salts, which are easier to test.
- Nitrogen becomes sodium cyanide (NaCN).
- Sulfur becomes sodium sulfide (Na2S).
- Halogens become sodium halides (NaX, where X is Cl, Br, or I).
Extraction: After cooling, the fused mass is extracted with distilled water, boiled, and filtered. The filtrate is known as Lassaigne’s extract.
Tests for Elements
Test for Nitrogen
Prussian Blue Test
- A portion of the Lassaigne’s extract is taken, and to it, freshly prepared ferrous sulfate solution is added. The mixture is heated to boiling, then cooled, and a drop of ferric chloride is added followed by acidification with hydrochloric acid.
- Observation: If nitrogen is present, the formation of Prussian blue (ferric ferrocyanide) indicates a positive result.
FeSO4+6NaCN→Na4[Fe(CN)6]+FeSO4
FeCl3+Na4[Fe(CN)6]→Fe4[Fe(CN)6]3+NaCl
Test for Sulfur
Lead Acetate Test
- The extract is acidified with acetic acid, and lead acetate solution is added.
- Observation: Formation of a black precipitate of lead sulfide (PbS) indicates the presence of sulfur.
Sodium Nitroprusside Test
- To another portion of the extract, a few drops of sodium nitroprusside are added.
- Observation: A deep violet coloration indicates the presence of sulfur.
Test for Halogens
Silver Nitrate Test:
- The extract is boiled with dilute nitric acid to remove cyanide or sulfide if present (which would interfere), then cooled and treated with silver nitrate solution.
Observations
- Chlorine: White precipitate of silver chloride (AgCl) which is soluble in ammonia.
- Bromine: Pale yellow precipitate of silver bromide (AgBr), partially soluble in ammonia.
- Iodine: Yellow precipitate of silver iodide (AgI), insoluble in ammonia.
Layer Test for Bromine and Iodine
- If halogens are suspected, the extract can be treated with chlorine water or diluted nitric acid, then chloroform or carbon tetrachloride is added, and the mixture is shaken.
Observation
- Bromine: Brown layer in the organic solvent.
- Iodine: Violet layer in the organic solvent.
Important Notes
- False Positives: If nitrogen and sulfur are both present in the compound, the formation of sodium thiocyanate (NaSCN) can lead to a red color with ferric chloride, which might be mistaken for blood-red color in the absence of nitrogen, complicating the nitrogen test. Therefore, careful interpretation is needed.
- Safety: Sodium is highly reactive with water, and the reaction must be carried out with caution. Also, cyanide produced in the test is highly toxic, requiring careful handling.