Fundamentals of Soil Science (1+1)

Exercise 14 - Determination of Total Nitrogen Content of Soils

Nitrogen is one of the major nutrients required for plant growth. Out of the total amount of nitrogen present in soils, nearly 95-99 % is in the organic form and 1- 5% in the inorganic form. Because of the importance of nitrogen in crop growth and for fertility evaluation, the determination of nitrogen in soils hold an important place. Principle: The principle underlying determination of total nitrogen in soil by kjeldhal method involves three steps viz. digestion, distillation and titration. The chemistry involved in these steps is outlined below: Digestion When H2SO4 is heated in the presence of organic matter, the sulphate is reduced to form SO2 and the carbon is oxidized to CO2, whereas H is oxidized to H2O. The nitrogenous compounds are reduced to NH4+. Let us consider glucose, for the sake of explanation. C6H12O6 + 12 H2SO4 = 6 CO2 + 18 H2O + 12 SO2 In the organic matter some nitrates are also present which are accounted for by salicylic acid forming nitrosalicylic acid. The nitrosalicylic acid is then reduced to amino salicylic acid by Na2S2O3, which on digestion with H2SO4 is converted to (NH4)2SO4. The reactions occurring can be summarized as follows: C6H4(OH)COOH + HNO3 = C6H3(OH) NO2COOH + H2O Salicylic acid Nitrosalicylic acid Na2S2O3 + H2SO4 = Na2SO4 + H2S2O3 C6H3(OH) NO2COOH + 3H2S2O3 + H2O = C6H3(OH)NH2COOH + 3H2SO4. Aminosalicylic acid C6H3(OH)NH2 OOH + 27H2SO4 = (NH4)2SO4 + 14CO2 + 26SO2+30H2O Ammonium sulphate In the digestion mixture K2SO4 is added to raise the temperature of digestion. In order to accelerate the speed of digestion, a digestion accelerator containing copper sulphate,mercuricoxide and selenium powder is added. Distillation During distillation of ammonia, pumice stone pieces or glass beads are added to prevent bumping. Additions of zinc cause the liberation of nascent hydrogen in a fine stream and prevent overheating. Zn+2 + 2NaOH = 2H+ + Na2ZnO2 (Sodium zincate) Alkaline conditions by the additions of NaOH prevent escape of NH3 gas. Titration: During titration NH3 reacts with H3BO3 in the following manner: NH3 + HOH = NH4OH 4H3BO3 + 2NH4OH = (NH4)2 B4O7 + 7 H2O. (Ammonium tetra borate) During back titration a replacement titration occurs in which an anion of a strongacid (H2SO4) replaces that of the very weak boric acid: (NH4)2B4O7 + H2SO4 + 5 H2O = 4H3BO3 + (NH4)2SO4 Reagents Concentrated H2SO4 Digestion mixture: K2SO4 : FeSO47H2O : CuSO4 ( 20:2:1) Or 20g CuSO4 5H2O ,3g HgO ,1g Selenium Powder 40% NaOH Boric acid 4% : Approximately 40g of H3BO3 is dissolved in distilled water containing 5 ml. of mixed indicator. The boric acid stock solution is adjusted by dilute H2SO4 or HCl titration until the bluish colour of the indicator weakens towards pink. Mixed indicator solution : Dissolve 0.5g of bromocresol green and 0.1 g methyl red in 100 ml of 95% ethanol and adjust the solution with drops of dilute NaOH or HCl to bluish purple colour at pH 4.5 0.1 N H2SO4 Procedure The procedure for the determination of total nitrogen in soils also involves the three steps i.e. digestion ,distillation and titration Digestion: Weigh 10g soil sample in a 500 ml Kjeldahl flask. Add 30 ml concentration H2SO4 containing 1 g salicylic acid. Keep it for few minutes after shaking and add 5g. Na2S2O3. Heat the material for 5 minutes and cool Add 20 ml of water, 10g K2SO4 and 1 g of the digestion mixture and digest the contents till light blue colour appears. Distillation: Cool the contents and add about 200 ml of water and swirl the flask for two minutes, and take the supernatant liquid into a distillation flask. Add about 50 ml of water to the digestion flask and take the water extract into the distillation flask. Repeat this process for at least 4 times. Add 10 ml of Na2Solution to the distillation flask, and then and 135 ml of 40 per cent KOH slowly from the side of the flask. Add 2 pieces of zinc and distill NH3 into a 25 ml boric acid indicator solution kept in a receiver flask. When no more NH3 is received-test with blue litmus stop the distillation and proceed for titration. Titration: The unreacted boric acid solution in the receiver flask is back titrated with standard H2SO4. The disappearance of blue colour indicates the end point. Run a blank without the soil sample. Calculations: 1 ml. of 0.1N H2SO4 = 0.0014g of N 0.0014 x T x V % N in soil sample = --------------------- x 100 S x W Where: T = ml of 0.1N H2SO4 used for titration V = Total volume of aliquot prepared S = ml.of aliquot taken W = Weight of soil sample taken in gm.