Detection of Nitrogen, Sulphur, Halogens and Phosphorus -- Lassaigne's Test

Lassaigne's sodium fusion test is the standard, general-purpose method for detecting nitrogen (in any class of nitrogen-containing compound), sulphur and the halogens all from one preparation. Its underlying trick is chemical conversion: fusing the organic compound with metallic sodium converts whatever nitrogen, sulphur or halogen it contains from a covalently-bonded, water-insoluble form into simple, water-SOLUBLE sodium salts (sodium cyanide, sodium sulphide, sodium halide, and so on), which can then be tested for in solution using ordinary inorganic qualitative-analysis reactions.

Preparing the extract: a small piece of sodium, first dried by pressing it between folds of filter paper (to remove any surface kerosene/moisture), is placed in a fusion tube and gently heated until it melts into a shining globule. A pinch of the organic compound under test is then dropped onto that molten sodium, and the tube is heated further until the resulting reaction subsides and the tube itself glows red-hot -- ensuring the fusion has gone to completion. The still-hot tube is then plunged directly into roughly 50 mL of distilled water held in a china dish, and its bottom is deliberately broken by striking it against the dish, releasing its fused contents into the water. The resulting mixture is boiled for about ten minutes and then filtered; the clear filtrate obtained is the Lassaigne's extract (or sodium fusion extract), and it is this single extract that every test below is run on.

Test for nitrogen: if the original compound contained nitrogen, fusion converts it to sodium cyanide (Na + C + N -> NaCN) in the extract. Treating a portion of the extract with freshly prepared ferrous sulphate solution, then with a little ferric ion and concentrated HCl, produces a Prussian blue (sometimes described as green) precipitate -- confirming nitrogen was present. The role of the added HCl is specifically to dissolve away a greenish precipitate of ferrous hydroxide that forms from the excess sodium hydroxide reacting with the ferrous sulphate; left undissolved, that ferrous hydroxide would mask the genuine Prussian-blue colour. If the compound contained BOTH nitrogen and sulphur together, the fusion instead produces sodium sulphocyanide (thiocyanate), and testing that with ferric chloride gives a distinctive blood-red colouration (ferric sulphocyanide) rather than blue -- so a blood-red result signals N and S together, not N alone.

Test for sulphur (three independent confirmatory tests are given): (a) treating a portion of the extract with freshly prepared sodium nitroprusside solution gives a deep violet or purple colouration (this same reagent is also the standard test for sulphide ion, S2-, in ordinary inorganic salt analysis); (b) acidifying a separate portion of the extract with acetic acid and adding lead acetate solution gives a black precipitate of lead sulphide (PbS); (c) an oxidation-based test, in which the original organic substance is fused with a mixture of potassium nitrate and sodium carbonate, oxidising any sulphur present all the way to sulphate; extracting the fused mass with water, acidifying with HCl, and adding barium chloride solution then gives a white precipitate of barium sulphate, confirming sulphur was present. …