Ocreport.docx

Lassaigne’s test is used to detect the presence of elements: Nitrogen (N), Sulfur (S), Chlorine (Cl), Bromine (Br) and Iodine (I). This test involves following two steps. i) preparation of sodium fusion extract (SFE). ii) Detection of elements using SFE.

i) Preparation of SFE A small amount of organic substance is fused with small quantity of sodium metal in a fusion tube. The red hot fusion tube is then plunged into distilled water and the contents are boiled for a few minutes, then cooled and filtered. The filtrate obtained is called sodium fusion extract (SFE) or Lassaigne’s extract. It is usually alkaline. If it is not alkaline, a few drops of NaOH solution may be added to make it alkaline.

ii) Detection of elements using SFE Thus obtained SFE is used to detect the presence of elements like N,S,Cl, Br & I. The elements in the organic compound react with sodium during fusion reaction as follows:

used)

Na + C + N ------------> NaCN

(if N is present)

2Na + S ----------------> Na2S

(if S is present)

Na + S + C + N --------> NaSCN

(if both N & S are present & insufficient amount of Na is

Na + X -------------------> NaX

(If halogens are present)

Where X = Cl/Br/I Hence SFE may contain any of or all of ionic forms of respective elements.

To a portion of SFE, freshy prepared ferrous sulphate, FeSO4 solution is added and warmed. Then about 2 to 3 drops of FeCl3 solution are added and acidified with conc. HCl. The appearance of a Prussian blue color indicates the presence of nitrogen.

Note: Conc. HCl is added to convert Ferrous hydroxide, a green precipitate to ferrous chloride, which issoluble in water. Otherwise the green precipitate may interfere with Prussian blue color. Fe(OH)2 + 2HCl --------------> FeCl2 + 2H2O

When both N & S are present: However if both N & S are present in the organic compound and SFE is prepared with insufficient amount of Na metal, the thiocyanate (SCN-) is formed instead of cyanide ion (CN-). The thiocyanate, SCN- reacts with Fe3+ to give a blood red coloration due to formation of [Fe(SCN)]2+. Fe3+ + SCN– -------------> [Fe(SCN)]2+ Note: However, if sodium fusion is carried out with excess of sodium, the thiocyanate, SCN - is decomposed to give CN– & S2–. Hence in this case, sulfur and nitrogen are to be identified in separate tests.

i) The appearance of a deep violet color upon addition of a few drops of sodium nitroprusside to SFE indicates the presence of sulfur.

ii) Sulphur can also be detected by adding lead acetate to SFE, acidified with acetic acid. The formation of a black precipitate (PbS) indicates the presence of sulfur.

Test for Halogen

Using silver nitrate solution Carrying out the test This test has to be done in solution. If you start from a solid, it must first be dissolved in pure water. The solution is acidified by adding dilute nitric acid. (Remember: silver nitrate + dilute nitric acid.) The nitric acid reacts with, and removes, other ions that might also give a confusing precipitate with silver nitrate.

Confirming the precipitate using ammonia solution Carrying out the confirmation Ammonia solution is added to the precipitates. original precipitate AgCl

observation precipitate dissolves to give a colourless solution

What is the ammonia doing? The ammonia combines with silver ions to produce a complex ion called the diamminesilver(I) ion, [Ag(NH3)2]+. This is a reversible reaction, but the complex is very stable, and the position of equilibrium lies well to the right.

A solution in contact with one of the silver halide precipitates will contain a very small concentration of dissolved silver ions. The effect of adding the ammonia is to lower this concentration still further.

What happens if you multiply this new silver ion concentration by the halide ion concentration? If the answer is less than the solubility product, the precipitate will dissolve. That happens with the silver chloride, and with the silver bromide if concentrated ammonia is used. The more concentrated ammonia tips the equilibrium even further to the right, lowering the silver ion concentration even more. The silver iodide is so insoluble that the ammonia won't lower the silver ion concentration enough for the precipitate to dissolve. KMnO4 Test for chlorides, violet color is the result for this test because we added a potassium permanganate to the solution and also we added a silver nitrate so the color as the result is violet color.