Lassaigne’s test

Lassaigne s test is obviously a test also for carbon in the presence of nitrogen. It can be used therefore to detect nitrogen in carbon-free inorganic compounds, e.g., complex nitrites, amino-sulphonic acid derivatives, etc., but such compounds must before fusion with sodium be mixed with some non-volatile nitrogen-free organic compound such as starch... 

In order to detect these elements in organic compounds, it is necessary to convert them Into ionlsable inorganic substances so that the ionic tests of inoiganio qualitative analysis may be applied. This conversion may be accomplish by several methods, but the procedure is to fuse the organic compound with metallio sodium (Lassaigne s test). In this way sodium cyanide, sodium sulphide and sodium halides are formed, which are readily identified. Thus ... 

There are three straightforward tests which will provide useful information as to the general properties of the compound, and the presence of the more important of the elements noted above other than carbon, hydrogen and oxygen these tests are the ignition test heating with soda-lime and the sodium fusion test (Lassaigne s test). 

Procedure for Lassaigne s test. Support a Pyrex test tube (150 x 12 mm) vertically in a clamp lined with sheet cork. Place a cube (c. 4 mm = 0.04 g) of freshly cut sodium in the tube (for precautions in the use of sodium see Section 4.2.68, p. 462). Have in readiness about 0.05 g of the compound (if a solid) on a microspatula or tip of a knife blade, or about 3 drops of the compound (if a liquid) in a dropping pipette. Heat the tube steadily until sodium vapour, which is dark grey in colour, rises 2-3 cm in the test tube and drop the sample, preferably portion-wise, directly on to the molten sodium (CAUTION there may be a slight explosion). Do not allow the sample to fall on to the side of the tube above the vapour layer. Heat the tube to redness for about 2 minutes, and then allow it to cool. Add about 3 ml of methanol to decompose any unreacted sodium and then halffill the tube with distilled water. Boil gently for a few minutes to remove the methanol and extract inorganic salts from the fusion residue gently crush the... 

Test by fusion with alkali metals (Lassaigne s test) ... 

Treat the substance with dilute aqueous sodium hydroxide. Ammonium salts evolve ammonia in the cold. Boil the solution to drive ofif all ammonia, cool, add a slight excess of dilute sulphuric acid, and very carefully evaporate to dryness. Examine the residue for nitrogen by Lassaigne s test. (Part of the residue may be submitted to the test for sulphonic acids, given below.)... 

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