Question

Discuss the principle of estimation of halogens, sulphur and phosphorus in an organic compound.

Answer 1

Hint: Organic compounds are those compounds which contain carbon and hydrogen mainly. Methane contains 1 carbon atom and 4 hydrogen atoms. Organic compounds also contain other elements like O, N, P, S. These are known as heteroatoms. Their amount in the compound can be estimated by various methods.

Complete answer:
Let’s look at the various methods involved in the estimation of halogens, sulphur and phosphorus.
Let mass of organic compound taken= m
And, mass of precipitate for analysis = $m_1$

Estimation of halogens:
Halogens are estimated by the process known as Carius method. In this method a known quantity of the organic compound is reacted with fuming nitric acid in the presence of silver nitrate. Carbon and hydrogen get oxidized and the halogen is converted to silver halide. It is then collected, filtered, washed and dried.
$$Percentageofhalogen={\displaystyle \frac{X_{atomicmass}\times m_1\times 100}{Ag{X}_{molecularmass}\times m}}$$

Estimation of sulphur:
A small amount of organic compound is reacted with fuming nitric acid. The sulphur present is oxidized to sulphuric acid. An excess amount of barium chloride is added to precipitate out the sulphur present as barium sulphate. It is then filtered, washed and dried.
$$Percentageofsulphur={\displaystyle \frac{32\times m_1\times 100}{233\times m}}$$
Where 233g is the molar mass of barium sulphate.

Estimation of phosphorus:
A small amount of organic acid is reacted with fuming nitric acid. The phosphorus present is oxidized to phosphoric acid. Ammonia and ammonium molybdate are then added to get the phosphorus as ammonium phosphomolybdate as a precipitate. It is then filtered, washed and dried.
$$Percentageofphosphorous={\displaystyle \frac{31\times m_1\times 100}{1877\times m}}$$
Where 1877 is the molar mass of ammonium phosphomolybdate.
Hence, we have defined all the principles of estimation.

Note: Students should note that the above tests are used for the quantitative analysis of halogens, sulphur and phosphorus. The Lassaigne’s test is used for the detection of these three heteroatoms (qualitative Analysis).