Lassaigne method

For the qualitative determination of the elements nitrogen, sulfur, and chlorine, the Lassaigne method is usually used. Combine approximately 50-100 mg of a finely divided sample with a pea-sized piece of sodium or potassium in a pyrolysis tube. Heat this carefully in a Bunsen flame until the metal melts. (Caution Wear safety glasses and keep the opening of the tube directed away from the eyes.) The sample must be free of water, which would react explosively with the metal. Sodium and potassium must be stored in oil or immersed in a similar inert hydrocarbon. When used, a small piece of the metal is held with tweezers and the required amount is cut off with a knife or a spatula onto a piece of filter paper. Then carefully blot it dry with the filter paper. Use it immediately and return the remainder to the oil-containing bottle. The remainder should not be destroyed by throwing it into water. 

This method, due to Middleton (Analyst, 1935, 6o, 154), has the advantage over Lassaigne s method (pp. 321-326) that the use of metallic sodium is avoided it has the disadvantage, however, that the reagents are not so readily obtained pure, and the method requires rather more time. 

When an organic compound is heated with a mixture of zinc powder and sodium carbonate, the nitrogen and halogens are converted into sodium cyanide and sodium hahdes respectively, and the sulphur into zinc sulphide (insoluble in water). The sodium cyanide and sodium hahdes are extracted with water and detected as in Lassaigne s method, whilst the zinc sulphide in the residue is decomposed with dilute acid and the hydrogen sulphide is identified with sodium plumbite or lead acetate paper. The test for nitrogen is thus not affected by the presence of sulphur this constitutes an advantage of the method. 

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 ... 

The action of arsine on silver and mercury salts has attracted much attention owing to the important application to analytical methods for arsenic (p. 319). The action of arsine on a dilute aqueous solution of silver nitrate has long been known to yield metallic silver, arsenious acid and nitric acid.9 With more concentrated solutions the introduction of a few bubbles of arsine produces a deep lemon-yellow coloration, the liquid also acquiring an acid reaction. The coloration disappears after one or two days, silver is precipitated and the colourless solution contains arsenious and arsenic acids.10 If a rapid stream of arsine be passed into a concentrated solution of silver nitrate at 0° C. the whole liquid solidifies to a yellow crystalline mass which rapidly blackens with separation of silver. Lassaigne represented the reaction with the dilute solution by the equation... 

Elemental analyses for nitrogen by the methods of Kjeldahl and Lassaigne were negative. None of the initiator, triethylamine, could be detected after the poly(oxymethylene) chain had been depolymerized. 

Simple methods of chemical analysis are useful in certain cases. Qualitative chemical tests such as the Beilstein and Lassaigne tests for the presence of particular elements (e.g. halogens, nitrogen, sulphur) are of use in the identification of an unknown polymer. Combustion analysis for quantitative determination of elemental composition can be used to confirm the purity of a homopolymer and to determine the average... 

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