Nitrogen in organic manure / oil cake

1. ESTIMATION OF NITROGEN IN ORGANIC MANURE / OIL CAKE

Aim 

         To estimate the N content in organic manure / oil cake.

Principle

         Nitrogen present in organic form in the organic manure / oil cake is digested with sulphuric acid containing salicylic acid in the presence of sodium thiosulphate and converted to ammonium sulphate. Then the ammoniacal form of N is distilled and estimated.

Reactions

2NH4NO3 + H2SO4                           → (NH4)2SO4 + 2HNO3

C6H4OH.COOH + HNO3                  → C6H3OH.COOH.NO2 + H2O

                                                                     ( Nitro salicylic acid)

Na2S2O3 + H2SO4                             → Na2SO4 + H2SO3 + S

C6H3OHCOOHNO2 + 3H2SO3 + H2O   → C6H3NH2OHCOOH + 3H2SO4

2C6H3NH2OHCOOH + 27H2SO4     → (NH4)2SO4 + 26SO2 + 14 CO2 + 30 H2O

Reagents required

1.                  Conc. H2SO4

2.                  Salicylic acid

3.                  Sodium thiosulphate

4.                  Potassium sulphate

5.                  Copper sulphate

6.                  40% NaOH

7.                  Sodium sulphide

8.                  N/ 10 H2SO4

9.                  N / 10 KOH

10.              Methyl red indicator

Procedure

a)      Digestion

Procedure

         Weigh accurately 1 g of FYM / compost / oil cake / green manure and transfer into a macro Kjeldahl flask.  Add 30 ml of conc. H2SO4 containing 1 g of salicylic acid.  Mix the contents and allow at least half an hour with frequent shaking.  Add about 1.5 g of sodium thiosulphate (Na2S2O3.5H2O) and mix well.  Digest over a low flame until frothing ceases.  Then add 10 g of K2SO4 and 1 g of CuSO4.5H2O and digest the contents until the liquid in the flask turns apple green.

b) Distillation

         Transfer the manure digest from the Kjeldahl flask into the distillation flask. Rinse the Kjeldahl flask with distilled water 2-3 times and transfer the washings to the distillation flask.  Add 300 ml distilled water and a few pieces of metal zinc and porcelain bits.  Add 10 ml of 10% sodium sulphide and 120 ml of 40% NaOH.  Stopper the flask connecting with the distillation unit and distil the contents.  Collect the distillate in a 250 ml ice tumbler / tall form beaker containing 25 ml of 0.1 N H2SO4 with methyl red indicator. When all the ammonia is evolved as tested by red litmus paper at the end of delivery tube, back titrate the excess acid against 0.1 N KOH.  Calculate the percentage of nitrogen in the organic manure /oil cake from the volume of 0.1 N H2SO4 utilized to absorb the evolved ammonia.

Calculations

Weight of manure / oil cake taken:1.0 g

Volume of 0.1 N H2SO4 taken in the ice-tumbler:x ml

Volume of 0.1 N KOH used on back titration:y ml

Volume of 0.1 N H2SO4 utilized by the evolved ammonia:(x-y) = z ml

1 ml of 0.1 N H2SO4:0.0014 g of N

Z ml of 0.1 N H2SO4:0.0014 x z g of N

This is present in 1 g of manure / oil cake

In 100 gm of manure/oilcake i.e. % of N:

Things to Learn

i)        Indicate the purpose of addition of Salicylic acid.

ii)  What is the purpose of digesting the organic manure?

iii) What is meant by back titration? Give an example.