Lassaigne’s Test - Test for Nitrogen, Sulphur, Halogens - Practice Questions & MCQ

Test for Nitrogen
The sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. The formation of Prussian blue colour confirms the presence of nitrogen. Sodium cyanide first reacts with iron(II) sulphate and forms sodium hexacyanidoferrate(II). On heating with concentrated sulphuric acid some iron(II) ions are oxidised to iron(III) ions which react with sodium hexacyanidoferrate(II) to produce iron(III) hexacyanidoferrate(II) (ferriferrocyanide) which is Prussian blue in colour.

                                                                  Prussian blue

Test for Sulphur

• The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulphide indicates the presence of sulphur.
  
• On treating sodium fusion extract with sodium nitroprusside, appearance of a violet colour further indicates the presence of sulphur.
  

In case, nitrogen and sulphur both are present in an organic compound, sodium thiocyanate is formed. It gives blood red colour and no Prussian blue since there are no free cyanide ions.

If sodium fusion is carried out with excess of sodium, the thiocyanate decomposes to yield cyanide and sulphide. These ions give their usual tests.

The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.

If nitrogen or sulphur is also present in the compound, the sodium fusion extract is first boiled with concentrated nitric acid to decompose cyanide or sulphide of sodium formed during Lassaigne’s test. These ions would otherwise interfere with silver nitrate test for halogens.

The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.

                 Ammonium molybdate                           Ammonium phosphomolybdate

A known mass of organic compound is heated in the presence of pure oxygen. The carbon dioxide and water formed are collected and weighed. The percentages of carbon and hydrogen in the compound are calculated from the masses of carbon dioxide and water. Estimation of carbon and hydrogen in an organic compound is based on their conversion to  and  respectively. The percentage of carbon and hydrogen are calculated from the masses of  and .

The apparatus consists of a long glass tube. This is called combustion tube. To one end of this tube, a U-tube containing anhydrous calcium chloride and a bottle containing concentrated potassium hydroxide solution (caustic potash) are attached in series. These in turn, are connected to a guard tube containing anhydrous calcium chloride.

The combustion tube is packed with cupric oxide and copper guaze. The other end of the tube has a provision for passing oxygen. The combustion tube is heated in a furnace. The U-tube and the caustic potash bottle are weighed before the start of the experiment. The combustion tube is heated strongly in a current of pure and dry oxygen to remove moisture and  that may be present.

A known mass of the organic compound taken in a porcelain boat is placed in the combustion tube and strongly heated. The organic compound is oxidized by cupric oxide to  and  (Nitrogen, if present is also oxidized to oxides of nitrogen. These are reduced back to nitrogen by copper).

The combustion products first pass through the U-tube containing anhydrous calcium chloride (absorption of ) and then through the caustic potash bottle (absorption of ). The U-tube and the caustic potash bottle are weighed after cooling them to laboratory temperature.