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.
If a compound contains both nitrogen and sulphur, sodium thiocyanate is formed on fusing with sodium.
Na + C + N + S → NaCNS
Sodium thiocyanate gives blood red colour with ferric chloride.
NaCNS + FeCl3 → [Fe(CNS)]2+ + Na+ + 3Cl-
How To Perform Experiment
The experiment consists of two stages:
- Preparation of sodium fusion extract
- To test for Nitrogen & 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 Nitrogen & Sulphur
Take about 1 mL of sodium fusion extract in a test tube and acidify it with dilute hydrochloric acid. Add a few drops of ferric chloride solution. Blood red colour indicates presence of nitrogen and sulphur.