Experiment No. 6: TO DETECT SULFUR PRESENT IN GIVEN ORGANIC COMPOUND BY LASSAIGNE TEST
OBJECTIVE:
TO DETECT THE FOREIGN ELEMENT PRESENT IN GIVEN ORGANIC COMPOUND
APPARATUS REQUIRED
- Fusion tube
- Tong
- Burner
- Porcelain basin
- Beaker
- Test tube
CHEMICALS REQUIRED
- Sodium metal
- Given organic compound
- NaOH
- FeSO4
- FeCl3
- HCl
- Lead acetate
- Sodium nitroprusside
- AgNO3
THEORY
Organic compounds are covalent compounds and generally insoluble in abater and carry out molecular reactions which are very slow. So, the given organic compound is fused with highly reactive metal such as sodium to give a water-soluble ionic compound which carries out ionic reactions which are fast. This test is known as the Lassaigne test. Nitrogen, sulphur and halogens are known as hetero elements or foreign elements.
Foreign elements are those elements other than carbon, hydrogen and oxygen which form the basic constituent of organic compound. Foreign elements are covalently bonded with the carbon of organic compounds.
- If nitrogen is present, it forms sodium cyanide
Na + C + N → NaCN → Na+ + CN–
- If sulphur is present, it forms sodium sulphide
Na + S → Na2S → 2Na+ + S—
- If halogen is present, it forms sodium halide
Na + Cl2 → 2NaCl → Na+ + Cl–
Sometimes, when both nitrogen and sulphur are present in the same organic compound then sodium sulphocyanide is formed.
Na + C + N + S → NaCNS → Na+ + CNS–
PROCEDURE
A freshly cut pea-shaped sodium metal was taken out from the kerosene oil and dried by pressing it between the folds of filter paper. The metal is placed in a fusion tube with a tong and was heated gently until the sodium melted to a shiny globule. Then a pinch of organic compound was added to it and heated strongly until the lower end of the tube became red hot. The hot tube was then plunged into the porcelain basin containing 10 ml of water. The particles were grinded with the pestle. The solution was foiled and filtered. The filtrate is called sodium extract solution (Lassaigne solution).
OBSERVATION TABLE
| Experiment | Observation | Inference |
| 1. Detection of nitrogen: 2 ml of sodium extract solution was taken in a clean and dry test tube and a few drops of NaOH solution was added. Then freshly prepared FeSO4 solution was added and boiled and cooled. Finally, FeCl3 solution was added and acidified with conc. HCl. | Prussian blue colour was not obtained | Absence of nitrogen |
| 2. Detection of sulphur: i. 2 ml of sodium extract solution was taken in a clean and dry test tube and a few drops of acetic acid solution were added followed by a few drops of lead acetate. | Black ppt. was obtained | Presence of sulphur |
| ii. 2 ml of sodium extract solution was taken in a clean and dry test tube and few drops of sodium nitroprusside solution was added. | Violet color was obtained | Presence of sulphur |
| 3. Detection of nitrogen and sulphur together: 2 ml of sodium extract solution was taken in a clean and dry test tube and a few drops of ferric chloride solution was added. | No any ppt. was obtained | Absence of nitrogen and sulphur together |
| 4. Detection of halogen: 2 ml of sodium extract solution was taken in a clean and dry test tube and acidified with nitric acid and a few drops of AgNO3 solution was added to it. | No any ppt. was obtained | Absence of halogen |
Reactions involved
- Lead acetate test
Na_{2}S + (CH_{3}COO)_{2}Pb \rightarrow \underset{Black\ ppt.}{PbS} + 2CH_{3}COONa- Nitroprusside test
Na_{2}S + Na_{2}[Fe(CN) NO] \rightarrow \underset{\underset{(Violet)}{Sodium\ sulphonitroprusside} }{Na_{4}[Fe(CN)_{5}NOS]}
RESULT
The given sample of the organic compound contains sulphur as a foreign element.
CONCLUSION
We can detect the foreign element present in the given organic compound by the Lassaigne test.
PRECAUTION
- A large amount of sodium metal was not taken.
- The contents of the basin were boiled only after the completion of unreacted sodium.
- If a dark colour was formed, the compound did not decompose completely and the process was repeated.
