Estimation of Carbon and Hydrogen by kjeldahl's method

A known mass (0.3 to 0.5 g) of the given organic compound is digested with concentrated H₂SO₄, in presence of a small quantity of potassium sulphate and copper sulphate in a Kjeldahl's flask. Potassium sulphate raises the boiling point of sulphuric acid and copper sulphate catalyzes the digestion. In 3 to 4 hours, the organic compound is completely decomposed to form ammonium sulphate.

Distillation

The digested reaction mixture, on cooling, is transferred to a round bottomed distillation flask, and distilled with a concentrated alkali solution (NaOH). Ammonia produced is absorbed in a known volume of HCl solution of a known strength.

The un-neutralised HCl is then back-titrated against a standard alkali. From the acid consumed, the amount of ammonia produced and hence the mass of nitrogen is calculated.

Fig: Apparatus for the estimation of nitrogen by Kjeldahl's method

Calculation

Let,

Mass of the organic compound = W g

Volume of the standard acid required for complete neutralization of the evolved ammonia = V mL

Normality of the standard solution of acid = N

From the law of equivalence (normality equation),

1000 mL of 1 N acid = 1000 mL of 1 N NH_3 = 17g NH_3 = 14 g nitrogen

Then,

V mL of N acid = V mL of NH₃

NV milli equivalent of acid = NV milli equivalent of ammonia

Therefore,

Then,

Percentage of nitrogen in the sample =

Estimation of carbon and Hydrogen by Liebig's method (1puc)

Principle A known mass of organic compound is heated in the presence of pure oxygen. The carbon dioxide and water formed are collected and weighed. The percentages of carbon and hydrogen in the compound are calculated from the masses of carbon dioxide and water. Estimation of carbon and hydrogen in an organic compound is based on their conversion to CO, and H₂0 respectively. The percentage of carbon and hydrogen are calculated from the masses of C0₂and H,0.

CH + f x +—lo, ^heat ) xCO, + ^-H₂0 * y a ² 9

Procedure The apparatus consists of a long glass tube. This is called combustion tube. To one end of this tube, a U-tube containing anhydrous calcium chloride and a bottle containing concentrated potassium hydroxide solution (caustic potash) are attached in series. These in turn, are connected to a guard tube containing anhydrous calcium chloride.

The combustion tube is packed with cupric oxide and copper guaze. The other end of the tube has a provision for passing oxygen. The combustion tube is heated in a furnace. The U-tube and the caustic potash bottle are weighed before the start of the experiment. The combustion tube is heated strongly in a current of pure and dry oxygen to remove moisture and C0₂ that may be present.

A known mass of the organic compound taken in a porcelain boat is placed in the combustion tube and strongly heated. The organic compound is oxidized by cupric oxide to C0₂ and H_zO (Nitrogen, if present is also oxidized to oxides of nitrogen. These are reduced back to nitrogen by copper).

The combustion products first pass through the U-tube containing anhydrous calcium chloride (absorption of H₂0) and then through the caustic potash bottle (absorption of C0₂). The U-tube and the caustic potash bottle are weighed after cooling them to laboratory temperature.

Calculations

Mass of organic compound = m g

Increase in mass of U-tube = mass of H_zO = a g

Increase in mass of caustic potash bottle = .mass of CO_z = b g

18 g (1 mole) water contains = 2 g hydrogen

a g water will contain = ^ x a = xgof hydrogen

m g organic compound contains = x g hydrogen

100 g organic compound will contains = — x 100 g of hydrogen

= — x—xlOO 18 m

 2 mass of water x 100

Percentage of hydrogen in the compound = —x-

18 mass of organic compound

44 g (1 mole) carbon dioxide contains = 12 g carbon

12

b g carbon dioxide contains = 77 x b = y g of carbon

o 44

m g organic compound contains = y g of carbon

y

100 g organic compound will contain = — x 100 g of C

12 b _in_ = —x—xlOO 44 m

. 12 x mass of carbon dioxide x 100

Percentage of carbon in the compound = --

44 x mass of organic compound

MIMfe __. _1

Depending upon the composition of organic compound, the following modifications are made in the estimation of carbon and hydrogen by Liebigs method. ''

• Compounds containing nitrogen: During combustion, organic nitrogen is oxidized to its oxides (NO, NO2) which are also absorbed in KOH solution along with CO2. A porcelain boat containing lead dioxide is placed in the combustion tube next to the compound which absorbs only oxides of nitrogen. Alternatively, oxides of nitrogen are reduced to nitrogen by passing over preheated copper gauze placed near the exit of combustion tube.

• Compounds containing sulphur: During combustion, organic sulphur is oxidized to sulphur dioxide which is absorbed in KOH solution. Sulphur dioxide is selectively absorbed by a lead chromate layer placed after CuO layer in the combustion tube.

4PbCr0₄ + 4S0₂ -^^->4PbS0₄ + 2Cr,0₃ + O_z

• Compounds containing halogen: Organic halogens are converted into volatile cupric halides which are absorbed in KOH solution. A roll of silver gauze is placed near the exider of combustion tube. The volatile cupric halides are decomposed, forming stable silver halides.

CuX₂ + 2Ag->2AgX + Cu

X₂ +2Ag——>2AgX