Question

In Kjeldahl’s method of estimation of nitrogen, copper sulphate acts as:
A. An oxidizing agent
B. A reducing agent
C. A catalytic agent
D. A hydrolyzing agent

Answer 1

Kjeldahl’s method is an important method used for the estimation of nitrogen. It is employed largely in the analysis of foodstuffs and fertilizers.
Principle: A known weight of the organic compound is digested with concentrated sulphuric acid. The nitrogen present in it is quantitatively converted into ammonium sulphate. The resultant liquid is heated with concentrated alkali. The ammonium gas evolved is passed through a known excess of a standard acid solution. The volume of unreacted acid is determined by titration with a standard alkali solution. The nitrogen in the given organic compound can be estimated from the amount of ammonia evolved.

Complete step by step answer:
The experiment is carried out in three steps as follows:
Formation of ammonium sulphate: A weighted quantity of the compound is placed within the long-necked flask known as Kjeldahl’s flask. About 25 ml of concentrated sulphuric acid together with a little potassium sulphate and copper sulphate or mercury is added to it. Potassium sulphate raises the boiling point and thus ensures the complete reaction while copper sulphate or mercury acts as a catalyst. The flask is loosely stopped by a glass bulb and is heated gently till the brown color of the liquid is first produced and then disappears. At now, all the nitrogen present within the organic compound gets quantitatively converted into ammonium sulphate.
$C + H + N + {H_2}S{O_4}\xrightarrow{{heat}}C{O_2} + {H_2}O + {\left( {N{H_4}} \right)_2}S{O_4}$
Distillation with alkali: The flask of Kjeldahl is cooled and its contents are diluted with distilled water and the Kjeldahl Used liquid is transferred into a large round-bottomed flask. A couple of pieces of porcelain are added to avoid bumping and a few pieces of zinc are also added which reacts with sodium hydroxide solution to produce hydrogen gas which acts as a carrier for ammonia gas. A round bottomed flask is fitted with a Kjeldahl’s trap and a water condenser. The lower end of the condenser is dipped into a measured volume of standard sulphuric acid solution. The flask is additionally fitted with a dropping funnel and sodium hydroxide solution is added through it. The round bottomed flask is heated and ammonia evolved passes through the known excess of standard sulphuric acid taken. Due to bumping on vigorous boiling, the Kjeldahl trap doesn’t allow any alkali solution to pass into the condenser
${\left( {N{H_4}} \right)_2}S{O_4} + 2NaOH \to N{a_2}S{O_4} + 2{H_2}O + 2N{H_3} \uparrow$
Titration of excess acid: The extra acid left behind is then determined by titration and alkali. Phenolphthalein is used as an indicator.
After discussing we can conclude that copper sulphate acts as a catalytic agent in Kjeldahl’s method

Thus, the correct option for the given question is (C).

Note: This method is simple and quicker than Duma’s method. However, this method does not apply to compounds that contain nitrogen atoms linked to either oxygen or other nitrogen atoms like nitro and azo compounds. It is also not applicable to compounds like pyridine in which a nitrogen atom is present in the ring.