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Explain Kjeldahl's method. |
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Answer» Solution :Principle: This method is based on the fact that an organic compound containing nitrogen is heated with cone, `H_(2)SO_(4)` , the nitrogen is CONVERTED to ammonium sulphate. The resultant liquid is heated with excess of alkali and then LIBERATED ammonia gas is absorbed in excess of standard acid. The amount of ammonia (nitrogen) is determined by finding the amount of acid neutralised by back titration with same standard alkali. Procedure: A weighed quantity of the substance 0.3 to 0.5g is placed in a special long necked Kjeldahl flask made of pyrex glass. About 25 ml of conc. `H_(2)SO_(4)` together with a little `K_(2)SO_(4)` and `CuSO_(4)` (catalyst) are added to it, the flask is loosely stoppered by a glass bulb and heated gently in an inclined position. The heating is continued till the brown colour of the liquid disappears leaving the content clear as before. At this point all the nitrogen is converted to ammonium sulphate. The kjeldahl flask is cooled and its contents are diluted with distilled water and carefully transferred into a 1 litre rund bottom flask. An excess NaOH is poured down the SIDE of the flask and it is fired with a kjeldhals trap and a water condenser. The lower end of the condenser dips in a measured volume of excess of `(N)/(20)H_(2)SO_(4)` solution. The liquid in the round bottom flask is heated and 20 liberated ammonia is distilled to sulphuric acid. When no more ammonia passes over test the distillate with red litmus) the receiver is removed. The excess of acid is then determined by titration with alkali, using phenolphthalein as the indicator.  Calculation Weight of the substance = Wg Volume of `H_(2)SO_(4)` required for the complete neutralisation of evolved `NH_(3)` = V ml Strength `H_(2)SO_(4)` used to neutralise `NH_(3)` = N. LET the volume and strength of `NH_(3)` formed are `V_(1)` and `N_(1)` respectively. `V_(1)N_(1) = VN` `"The amount of nitrogen present in W g of organic compound"=(14 xx NV)/(1 xx 1000 xx w)` `"Percentage of nitrogen"=(14 xx NV)/(1000 xx w) xx 100=(1.4NV)/(w)%` |
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