Determination of Total Nitrogen By Kjeldahl’s Method

1. Determination of Total Nitrogen By Kjeldahl’s Method

2. Determination of Total Nitrogen • NO2- isseldom present in detectable amount • Amount of NO3- in agricultural soil is usually low • In soils with high amount of 2:1 type of clay minerals fixed NH4+ may account for about 10% of total N. Total N ≈ Organic N + NH4+(in soil solution)

3. Methods of total N determination • 1) Dumas method (1831) - sample is combusted with CuO at high temperature to liberate gases (N2O and CO2). - gas evolved is passes through strong alkaline to absorb CO2. - N2O gas is passes over hot Cu to form N2 - N2 gas produced is measured to estimate total N - Dry oxidation process

4. Methods of total N determination 2) Kjeldahl’s Method (1833) - Samples are digested with H2SO4 to convert N in the sample to (NH4)2SO4. - Determination of the NH4+ in the digest by distillation and titration. - Wet oxidation process.

5. Kjeldahl’s Method • Digestion • H2SO4 – Oxidizing agent used to oxidize organic N to (NH4)2SO4. • Na2SO4 – salt used to rise the temperature of the digestion. • Se-CuSO4 - catalyst used to speed up the oxidation rate. (Se-CuSO4) Organic N + H2SO4 (NH4)2SO4 + H2O + CO2 (Na2SO4)

6. Kjeldahl’s Method Digestion procedure • Weigh 10g soil into 500 ml Kjeldahl flask. • Add 10 ml distilled water and leave to stand for 10 mins. • Add the catalyst • Add 2 tablets Na2SO4 • Add 20 ml Conc. H2SO4 • Digest until clear and colourless (1 – 1½ hrs). • Further digest for 30-45 mins. • Allow the flask to cool. • Decant or filter the fluid into a 100 ml volumetric flask washing the sand with altogether 50 ml distilled water, small at a time. • Make up to the mark with distilled water.

7. Kjeldahl’s Method B) Distillation • Free NH4 is liberated from the solution by steam distillation in the presence of excess NaOH: • The NH3 evolved is collected in excess boric acid to form ammonium-borate complex. heat (NH4)2SO4 + 2NaOH 2NH3 Na2SO4 + 2H2O NH3 + H3BO3 NH4+ : H2BO3- + 2H2O

8. Kjeldahl’s Method B) Distillation procedure • Transfer an aliquot of 10ml of the liquid into the Kjeldahl apparatus • Add 15 ml of the strong NaOH. • Distil over steam for 10 minutes into 10 ml of the Boric acid in a 500 ml conical flask.

9. Kjeldahl’s Method C) Titration The amount of NH3 liberated and captured by boric acid is determined by titrating with 0.1N HCl Procedure • Add 8 drops of the mixed indicator. • Titrate with 0.IN HCl till colour changes to grey and then suddenly flashes to pink. • Carry out blank determination with distilled water. NH4+ : H2BO3- + HCl NH4Cl + H3BO3

10. Kjeldahl’s Method Calculation The equivalent weight of NH3 is 17 g/eq; So 14 g of nitrogen are contained in one equivalent weight of NH3. Weight of N in the soil digest (g) = 14 x (A - B) x N 1000 Where A = Vol. of HCl (ml) used in the sample titration B = Vol. of HCl (ml) used in the blank titration N = Normality of the standard acid (eq/l).

11. Kjeldahl’s Method Calculation Mass of soil in 10 ml of digest used for distillation: 100 ml of digest was obtained from 10g of soil 10 ml of digest will be obtained from = 10 ml x 10 g 100ml = 1 g percentage of N in the soil sample = 14 x (A - B) x N x100 1000 x 1

12. Kjeldahl’s Method Calculation i) ppm N = % N x 104 ii) Ib N/acre F.S = ppm N x 2 iii) kg N/ha = ib N/acre F.S x 1.12