When an organic compound containing halogen (Cl, Br or I) is heated in a sealed tube with fuming nitric acid and excess of silver chloride, silver halide is formed from the mass of silver halide obtained, the percentage of the halogen can be calculated.
Procedure: In a hard glass tube (Carius tube), 5ml of fuming HNO3 and 2 to 2.5 g AgNO3 are taken. A small narrow weighing tube, containing a small amount (nearly 0.15 - 0.2g) of accurately weighed organic compound, is introduced in the Carius tube in such a way that nitric acid does not enter the weighing tube. The Carius tube is now sealed and heated in a furnace at 300°C for about six hours.
The tube is then cooled and its narrow end is cut off and the contents are completely transferred to a beaker by washing with water. The precipitate of silver halide formed is filtered through a weighed sintered glass crucible. It is washed, dried and weighed. Observation and calculation:
1. Mass of organic compound taken = W1g
2. Mass of silver halide obtained = W2g
(a) For chlorine :
\(\underset{143.5g}{AgCl}\) ≡ \(\underset{35.5g}{Cl}\)
143.5g of AgCl contains 35.5 g of chlorine
w2g of AgCl Will contain \(\frac{35.5\times w_2}{143.5}g\) of chlorine
This amount of chlorine was present in wig of the compound.
∴ %Cl2 = \(\frac{35.5\times w_2}{143.5}\) x \(\frac{100}{w_1}\)
(b) For bromine:
\(\underset{188g}{AgBr}\) ≡ \(\underset{80g}{Br}\)
188g of AgBr contains 80g of bromine
W2 g of AgBr will contain \(\frac{80\times w_2}{188}g\) of bromine.
∴ %Br2 = \(\frac{80\times w_2}{188}\) x \(\frac{100}{w_1}\)
(c) For Iodine:
\(\underset{235g}{AgI}\) ≡ \(\underset{127g}{I}\)
235g of AgI contains 127g of iodine
W2 g of Agl will contain \(\frac{127\times w_2}{235}g\) of iodine
∴ %I2 = \(\frac{127\times w_2}{235}\) x \(\frac{100}{w_1}\)
