Nitrogen in urea

ESTIMATION OF NITROGEN IN UREA

Aim

To estimate nitrogen content in the given fertilizer sample - urea

Principle

         The amide form of N present in urea is converted to ammoniacal nitrogen (ammonium sulphate) when it is digested with concentrated sulphuric acid. The ammoniacal N in the digested material is liberated when it is distilled with a strong alkali. The liberated ammonia is absorbed in a known excess standard acid and the actual volume of standard acid used is determined by back titrating against standard alkali. From the actual volume of standard acid consumed, the N content of the urea is determined.

Reaction During digestion:

        CO (NH2)2 + H2SO4 + H2O                (NH4)2SO4 + CO2 + H2O

  Reaction During distillation:

       (NH4)2SO4 + 2NaOH                         Na2SO4 + 2H2O

        NH3 + H2O                                NH4OH

        NH4OH + H2SO4                           (NH4)2SO4 + 2H2O

  Reaction During titration:  

        2KOH + H2SO4*                              K2SO4 + 2H2O

                      * Excess H2SO4 not used to absorb NH3

Materials required

(i) Concentrated sulphuric acid (ii) N/10 Sulphuric acid (iii) N/10 KOH (iv) Potassium sulphate (v) Copper sulphate (vi) 40% NaOH (vii) 10% Sodium sulphide (viii) Zinc granules and porcelain bits (ix) Kjeldahl flask  (x) Volumetric flask 500 ml and (xi) Funnel.

Procedure

·         Weigh exactly 1 g of the fertilizer sample and transfer into a Kjeldahl digesting flask taking care to see that no fertilizer granule sticks to the neck of the flask.

·         Add about 30ml of concentrated sulphuric acid mix well.

·         Add the digestion mixture – about 2g of potassium sulphate and about 0.2g of copper sulphate(10:1 ratio)

[Addition of potassium sulphate increases the boiling point of the acid and thereby the digestion time is reduced. Copper sulphate acts as a catalyst to hasten the reaction and also it serves as an indicator (appearance of apple green colour indicates the completion of digestion)]

·         Digest the material till it turns into apple green.

·         Transfer the whole digested material, after cooling into a 500 ml volumetric flask with the help of a funnel without spilling. Rinse the Kjeldahl flask with about 50ml of water 3 or 4 times and transfer this into the volumetric flask. Then make up the volume and shake the contents thoroughly so as to get a homogeneous solution.

·         Pipette out 50 ml of this solution into a distillation flask and add about 500- 600ml of water. Add few zinc and porcelain bits (Addition of zinc keeps the contents of the flask in a reduced condition and also maintains the pressure at a higher level).

·         Add about 20 – 30 ml of 40% NaOH till the contents become distinctly alkaline as tested with a litmus paper. Then add about 10 ml of sodium sulphate (Copper or mercury will adsorb, if present, the ammonia liberated during distillation. Copper forms cupra ammonium complex. The ammonia bound in this way will not be released during distillation. When sodium sulphate is added , the copper is converted to its sulphate. Sulphides of copper will not bind ammonia and hence the addition of sodium sulphide along with 40% NaOH).

·         Start distillation after placing a known excess of N/10 sulphuric acid at the delivery end and determine the actual volume of N/10 sulphuric acid consumed and calculate the percentage  N.

Note: Do not remove or lower the flame during distillation. This leads to back suction of the standard acid kept at the delivery end.

Calculations

Weight of fertilizer sample taken =1 g

Volume made up after digestion=500ml

Volume of aliquot pipetted out for distillation=50ml

Volume of N/10 H2SO4 taken in the ice tumbler=25ml

Volume of N/10 KOH used for back titration=X ml

Actual volume of N/10 H2SO4 consumed=(25-X) ml

1 ml of N/10 H2SO4=0.0014 g N

Therefore (25- X)ml of N/10 H2SO4=0.0014 x (25-X)g  N

This is from 50ml of aliquot 

Therefore in 500ml

=

This is present in 1 g of fertilizer sample

Therefore percentage of total N in the given sample

=

Things to Learn

i)        What is the form of N in urea?

ii)      Why should we digest the urea before distillation?

iii)    What is the purpose of adding 10% Na2S while distillation?

iv)    Give the reason for the addition of the following

a)       H2SO4                            b)   K2SO4                    c)  CuSO4

v)         How will you prepare 1000 ml of 0.1 N KOH?