For detecting the presence of elements in a compound, these have to be converted into ionic forms. This is done by fusing the compound with sodium metal. The elements present in the compound are converted into soluble sodium salts.

2Na + S → Na2S

(i) Sulphur is present in sodium extract as sodium sulphide. It gives a purple colour with sodium nitroprusside due to the formation of sodium thionitroprusside.

Na2S + Na2[Fe(CN)5NO] → Na4[Fe(CN)5NOS]

(ii) A sulphide reacts with lead acetate to yield lead sulphide as a black precipitate.

Na2S + Pb(CH3OO)2 → PbS + 2CH3COONa

(iii) A sulphide reacts with silver nitrate to yield silver sulphide which is black in colour.

Na2S + 2AgNO3 → Ag2S + 2NaNO3

How To Perform Experiment

The experiment consists of two stages:

  1. Preparation of sodium fusion extract
  2. To test for Sulphur

Preparation of Sodium Fusion Extract

Take out a small piece of sodium metal from a petri dish with the help of pair of forceps. Dry the sodium piece between the folds of filter papers. Put a small piece of sodium metal into a clean and dry ignition tube. Hold the tube with a pair of tongs and heat it gently over the flame so that sodium melts and makes a stirring ball. Remove the ignition tube away from the flame and immediately add a small amount of solid organic compound into it.

Heat the tube to red hot. Again add a little of the compound into the tube and heat it again to red hot. Plunge the red hot ignition tube in about 10 mL of distilled water taken in a porcelain dish. Immediately cover the porcelain dish with a wire gauze to avoid bumping of any unreacted sodium piece. Repeat the process of sodium fusion using two more ignition tubes. Break the tubes into small pieces with a clean glass rod. Boil the mixture for about 2-3 minutes and filter.

Test for Sulphur

(i) Take 1 mL of sodium fusion extract in a test tube and add a few drops of sodium nitroprusside to it and shake. Purple colour confirms sulphur present.

(ii) Take 1 mL of sodium fusion extract in a test tube and acidify it with acetic acid. Add a few drops (4-5) of lead acetate solution to it. Black precipitate confirms sulphur present.