Animated Solution for Chemistry - Organic Chemistry: In Carius method, halogen containing organic compound is heated with fuming nitric acid in the presence of

When chemists synthesize or discover a new organic compound, one of the first tasks is to figure out exactly what elements are inside it and in what quantities. This is where quantitative analysis comes into play. The Carius method is a classical, highly reliable technique specifically designed for the quantitative estimation of halogens—namely chlorine, bromine, and iodine—within an organic compound.

Inside the Carius Tube

Imagine you have a small sample of an unknown organic compound, and you suspect it contains a halogen. To find out how much, you place a carefully weighed amount of this compound into a specialized piece of glassware called a Carius tube. This is a thick-walled glass tube designed to withstand high pressures.

Along with your organic compound, you add two crucial reagents: fuming nitric acid (${\mathrm{HNO}}_3$) and silver nitrate (${\mathrm{AgNO}}_3$). Once everything is inside, the top of the glass tube is melted and sealed completely shut. This ensures that no gases can escape during the vigorous reaction that is about to take place.

Oxidation and Precipitation

The sealed Carius tube is then placed into a furnace and heated strongly. Inside this high-temperature, high-pressure environment, a fierce chemical battle occurs.

The fuming nitric acid acts as an incredibly powerful oxidizing agent. Its job is to completely destroy the organic framework of the compound. It attacks the carbon and hydrogen atoms, oxidizing them entirely into carbon dioxide (${\mathrm{CO}}_2$) gas and water (${\mathrm{H}}_2\mathrm{O}$) vapor.

But what happens to the halogen? As the organic molecule breaks apart, the halogen atoms are liberated. Because we cleverly added silver nitrate (${\mathrm{AgNO}}_3$) to the mix, the free halogen immediately reacts with the silver ions. This reaction forms a solid, insoluble precipitate of silver halide ($\mathrm{AgX}$).

For example, if the compound contained chlorine, you would see a white precipitate of silver chloride ($\mathrm{AgCl}$). If it contained bromine, a pale yellow precipitate of silver bromide ($\mathrm{AgBr}$) would form.

After the reaction is complete and the tube has cooled, it is carefully broken open. The silver halide precipitate is filtered out, washed, dried, and precisely weighed. By knowing the mass of this precipitate, chemists can use simple stoichiometry to calculate the exact percentage of the halogen that was originally present in the organic compound.

Returning to our question, it asks which reagent is heated with the organic compound and fuming nitric acid. The answer is unequivocally silver nitrate (${\mathrm{AgNO}}_3$), as it is the essential trapping agent that captures the halogen as a weighable solid.