Kjedhal's Method

Kjedhal's Method is use to estimate the nitrogen in fertilizers,drugs, food stuffs and other organic compounds. This method is simpler and more convenient than Duma's method

Fig 1 Heating Kjedhal's Flask

 Fig 2Distillation in Kjedhal's Flask

 

Principle :

In this method, our sample (an organic compound whose mass is known) is heated (360–410 °C ) with concentrated sulphuric acid(H₂SO₄) which will convert nitrogen present quantitatively in our organic compound sample into ammonium sulphate. Now the solution obtained after this process we again heated with the sodium hydroxide. The ammonia gas evolved is passed into a known but excess volume of standard acid (HCl, H₂SO₄). To calculate the amount of acid left unused we use titrating the solution with standard alkali. After calculating the amount of acid left unused we use that acid for the neutralisation of ammonia. With the help of this percentage of nitrogen is calculated

C ,H,S ----------------------→ CO₂ + SO₂ +  H₂O

                 (conc. H₂SO₄)

N  ----------------------→ (NH₄)₂SO₄

        (conc. H₂SO₄)

For organic compound

(NH₄)₂SO₄  +2NaOH ----------------> Na₂SO₄ + 2NH₃ 2H₂O

2NH₃ + H₂SO₄ ----------------------> (NH₄)₂SO₄  

NH₃ +HCl ---------------------------> NH₄Cl

Procedure:

The apparatus used in the experiment are shown in the figure. Let assume we have 1g of the organic compound for which we want to calculate the nitrogen percentage. First, we will heat our organic compound with concentrated H₂SO₄  in Kjeldhal's Flask which have a long neck with a loose glass stopper. Then we will add a small amount of potassium sulphate and copper sulphate the potassium sulphate will increase the boiling point of  H₂SO₄ and copper sulphate will act as a catalyst. Heating the solution for 2 to 3 hours so that carbon and hydrogen present in our organic compound are oxidised to carbon dioxide and water vapours respectively while nitrogen is converted into ammonium sulphate.

Gases like carbon dioxide and water vapour escape through the loose stopper whereas ammonium sulphate remains in the flask. Now the content of the flask is cooled and transferred to another round-bottomed flask. These are diluted with water and a few drops of indicator of methyl orange are added. The indicator will show pink colour showing the acidic nature of the solution. The flask is fitted with a dropping funnel. About 40% solution of NaOH is added with the help of a dropping funnel and the flask is heated.During heating ammonia is liberated which passes through the trap which retains water.The ammonia vapour condensed and the liquid ammonia is formed is absorbed in  a known excess solution of sulphuric acid.The unreacted (or excess) acid left after absorption of ammonia is estimated by titration with standard alkali solution. The difference between the initial amount of acid taken and that left after the reaction gives the amount of acid reacted with ammonia

Formula

$$\frac{}{1.4XMX2(V-\frac{V_1}{2})g}$$

w = mass of organic compound

V (mL) = Volume of standard H₂SO₄ of Molarity M

V1 (mL)= NaOH  of molarity M