Determination of Nitrogen By Kjeldahl’s Method B.Sc. Sneha S. Mule Assistant Professor Department of Chemistry, S. M. Joshi College, Hadapsar

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

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.

Kjeldahl’s Method Digestion Organic N + H2SO4 (NH4)2SO4 + H2O + CO2 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)

Nitrogen Analysis by Kjeldahl’s Method: Titrate unreacted HCL with NaOH This allows the amounts of NH3 that entered the solution to be determined Difference between known excess of HCl and remaining HCl Gives a measure of the organic nitrogen content of the original sample

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.

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

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.

Kjeldahl’s Method Procedure 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

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).

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

Kjeldahl’s Method i) ppm N = % N x 104 ii) Ib N/acre F.S = ppm N x 2 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

THANK YOU