Lassaigne method
This is employed to detect halogen, nitrogen and sulphur. Organic compounds are fused along with dry sodium in a fusion-tube and fused mass after extraction with H2O is boiled and filtered. Filtrate that is sodium extract S.E. is employed to detect elements (other than C and H).
1. Organic compounds being covalents generally do not have ionisable groups, therefore direct test is not possible.
2. Fusion along with Na forms soluble salt (such as NaCl, NaCN etc.) that can be easily detected.
3. This type of test fails in case of diazo elements.
4. Sometimes while the amount of nitrogen present is small, the prussian blue is available in colloidal form and the solution looks green.