Sodium fusion

HOOD NOTE. The fusion reaction is carried out in the hood. Make sure you are wearing safety glasses. All reagents must be of analytical grade, and deionized water 

CAUTION CAUTION Sodium metal can cause serious burns and it reacts violently with water. 

In a small (10 X 75-mm) test tube (soft glass preferred), supported in a transite board (see Fig. 9.2), place about 25-30 mg of clean sodium metal (about one-half the size of a pea). 

CAUTION CAUTION Use forceps to make this transfer never touch sodium metal with your fingers. 

---

The sodium fusion and extraction, if performed strictly in accordance with the above directions, should be safe operations. In crowded laboratories, however, additional safety may be obtained by employing the follow ing modification. Suspend the hard-glass test-tube by the rim through a hole in a piece of stout copper sheet (Fig. 69). Place 1 -2 pellets of sodium in the tube, and heat gently until the sodium melts. Then drop the organic compound, in small quantities at a time, down — =. the tube, allowing the reaction to subside after each addition before the next is made. (If the compound is liquid, allow two or three small drops to fall at intervals from a fine dropping-tube directly on to the molten sodium.) Then heat the complete mixture as before until no further reaction occurs. 

To determine which halogen is present, take 1-2 ml. of the filtrate from the sodium fusion, and add dilute sulphuric acid until just acid to litmus. Add about 1 ml. of benzene and then about 1 ml. of chlorine water and shake. A yellowish-brown colour in the benzene indicates bromine, and a violet colour iodine. If neither colour appears, the halogen is chlorine. The result may be confirmed by testing the solubility of the silver halide (free from cyanide) in dilute ammonia solution silver chloride is readily soluble, whereas the bromide dissolves with difficulty, and the iodide not at all. 

Sulphur. THE LASSAIGNE SODIUM TEST. The sodium fusion will have converted any sulphur present in the original compounds to sodium sulphide. Dissolve a few crystals of sodium nitroprusside, Na8[Fe(CN)5NO],zH20, in water, and add the solution to the third portion of the filtrate obtained from the sodium fusion. A brilliant purple coloration (resembling permanganate) indicates sulphur the coloration slowly fades on standing. Note, (i) Sodium nitroprusside is unstable in aqueous solution and therefore the solution should be freshly prepared on each occasion, (ii) This is a very delicate test for sulphides, and it is essential therefore that all apparatus, particularly test-tubes, should be quite clean. 

Sodium Fusion on Semi mlcro Scale. The Lassaigne test can be readily carried out with as little as 0 01 g. of material, using sodium pellets about 2 mm. in diameter in a tube about 3 x. After fusion, the red-hot tube is plunged into distilled water in a small porcelain crucible or in a boiling tube. The mixture is then heated, filtered and tested as already described. 

The filtered alkaline solution, resulting from the action of water upon the sodium fusion, is treated with ferrous sulphate and thus forms sodium ferro-eyanide ... 

The following alternative procedure is recommended and it possesses the advantage that the same tube may be used for many sodium fusions. Support a Pyrex test tube (150 X 12 mm.) vertically in a clamp lined with asbestos cloth or with sheet cork. Place a cube (ca. 4 mm. side = 0 04 g.) of freshly cut sodium in the tube and heat the latter imtil the sodium vapour rises 4 5 cm. in the test-tube. Drop a small amount (about 0-05 g.) of the substance, preferably portionwise, directly into the sodium vapour CAUTION there may be a slight explosion) then heat the tube to redness for about 1 minute. Allow the test tube to cool, add 3-4 ml. of methyl alcohol to decompose any unreacted sodium, then halffill the tube with distilled water and boil gently for a few minutes. Filter and use the clear, colourless filtrate for the various tests detailed below. Keep the test-tube for sodium fusions it will usually become discoloured and should be cleaned from time to time with a little scouring powder. 

Phosphorus. The presence of phosphorus may be indicated by a smell of phosphine during the sodium fusion. Treat 1 ml. of the fusion solution with 3 ml. of eoneentrated nitric acid and boil for one minute. Cool and add an equal volume of ammonium molybdate reagent. Warm the mixture to 40-50°, and allow to stand. If phosphorus is present, a yellow erystalline precipitate of ammonium phosphomolybdate wUl separate. 

In order to ascertain whether sufficient nickel to complete a given reaction has been used, the liquid phase may be tested for starting material before an attempt is made to isolate a product. In general the sodium fusion test for sulfur is satisfactory for this purpose but in certain individual cases a specific test may be more convenient. Thus, in the desulfurization of thioacetals (mercaptals), unreacted material... 

Amines are easily identified because they re readily soluble in dilute acid. Sodium fusion converts the cimine to the cyanide ion, which is detectable by a Vciriety of methods. The ready formation and decomposition of diazonium salts (discussed in the earlier section Reactions with nitrous acid ) leads to the identification of primary amines. The Hinsberg test (see the nearby sidebcir) is useful in identifying amines. 

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). 

Cyanide ion, and hence nitrogen in the sample, may be detected by the Prussian Blue test. The filtered alkaline solution, resulting from the action of water upon the sodium fusion, is treated with iron(n) sulphate and thus forms sodium hexacyanoferrate(n). Upon boiling the alkaline iron(n) salt solution, some iron(m) ions are inevitably produced by the action of air upon addition of dilute sulphuric add, thus dissolving the iron(n) and (hi) hydroxides, the hexa-cyanoferrate(n) reacts with the iron(m) salt producing iron(m) hexacyano-ferrate(n), Prussian blue ... 

If nitrogen and/or sulphur is present, acidify 3-4 ml of the fusion solution with dilute nitric acid and evaporate to half the original volume in order to expel any HCN and/or H2S which may be present. If nitrogen and sulphur are absent, proceed directly with 2 ml of the sodium fusion filtrate. Render the solution just neutral to litmus by the addition of dilute (5 m) aqueous ammonia solution, then add 5 drops of 5 m acetic acid and 20 mg of lanthanum chloranilate (2,5-dichloro-3,6-dihydroxy-p-benzoquinone, lanthanum salt) and shake intermittently for 10-15 minutes. Filter. A pink-violet coloration of the filtrate is a positive test for fluorine. 

---