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.
Na + C + N → NaCN
The soluble sodium salts are extracted with water. The extract is usually known as sodium fusion extract.
Nitrogen is present as sodium cyanide in sodium fusion extract. Sodium cyanide is converted to sodium ferrocyanide on treating with ferrous sulphate. On further treating it with ferric chloride, a Prussian blue complex, ferricferrocyanide is formed.
6NaCN + FeSO4 → Na4[Fe(CN)6]+ Na2SO4
Na4[Fe(CN)6] + 4FeCl3 → Fe4[Fe(CN)6]3 + 12NaCl
How To Perform Experiment
The experiment consists of two stages:
- Preparation of sodium fusion extract
- To test for Nitrogen
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 Nitrogen
Take about 1 mL of sodium fusion extract in a test tube and add a pinch of ferrous sulphate. Shake the contents and acidify with dilute hydrochloric acid. Boil the contents and add a few drops of ferric chloride solution.
Observation: Prussian blue colour or green precipitate