Kjeldahl Nitrogen Detection method

This procedure involves digestion and distillation. The soil is digested in concentrated H2SO4 with a
catalyst mixture to raise the boiling temperature and promote the conversion from organic-N to
NH4-N. The NH4-N from the digest is obtained by steam distillation, using excess NaOH to raise the
pH.

The distillate is collected in saturated H3BO3 and then titrated with dilute H2SO4 to pH 5.0.The
the method determines ammonium-N, most of the organic-N forms, and a variable fraction of nitrate-N in soil. For most soils, the Kjeldahl procedure is a good estimate of total soil N content. If desired, nitrate-N can be included through the reduced iron or salicylic acid modifications of the Kjeldahl procedure (see the following section).

Apparatus

• Distillation unit
• Block-digester
• Automatic titrator connected to a pH-meter
• Vortex tube stirrer

Reagents

A. Catalyst Mixture (K2SO4 – CuSO4.5H2O – Se), 100:10:1 w/ w ratio Grind reagent-grade chemicals separately and mixed. If caked, grind the mixture in a porcelain pestle and mortar to pass a 60-mesh screen (0.250 mm), taking care not to breathe Se dust or allow Se to come in contact with skin.

B. Sulfuric Acid (H2SO4), concentrated (98 %, sp. gr. 1.84)

C. Sodium Hydroxide Solution (NaOH), 10 N Dissolve 400 g NaOH in DI water, transfer the solution to a 1-L heavy-walled Pyrex flask, let it cool, and bring it to volume.

D. Boric Acid Solution (H3BO3), saturated
1. Add 500 g H3BO3 to a 5-L flask.
2. Add 3 L DI water, and swirl vigorously.
3. Leave overnight.

Note There should always be solid H3BO3 on the bottom of the flask.

E. Tris Solution [hydroxymethyl aminomethane] (C4H11NO3), 0.01 N
1. Dry reagent-grade Tris in an oven at 80 °C for 3 hours, cool in a desiccator, and store in a
tightly stoppered bottle.
2. Dissolve 1.2114 g Tris in DI water, transfer to a 1-L flask, and bring to volume.

F. Sulfuric Acid Solution (H2SO4), 0.01 N
1. Add 28 mL concentrated H2SO4 to about 600 – 800 mL DI water in a 1-L volume, mix well, let
it cool, and bring it to volume. This solution contains 1 N H2SO4 solution (Stock Solution).
2. Pipette 10 mL Stock Solution to the 1-L flask, and bring to volume with DI water. This solution
contains 0.01 N H2SO4.

G. Standard Stock Solution
1. Dry reagent-grade ammonium sulfate (NH4)2SO4 in an oven at 100 °C for 2 hours, cool in a
desiccator, and store in a tightly stoppered bottle.
2. Dissolve 5.6605 g dried (NH4)2SO4 in DI water, and bring to 1-L volume. This solution contains
1.2 g NH4-N / L (Stock Solution).

Procedure

A. Digestion

1. Weigh 1 g air-dry soil (0.15 mm) into a 100-mL calibrated digestion tube.
2. Add about 5.0-5.5 g catalyst mixture, a few pumice boiling granules, 15 mL concentrated
   H2SO4 (in the fume hood), and swirl carefully. Place a glass funnel in the neck of the tube,
   then place tubes in the rack, and leave overnight.
3. Place the tube rack in the block digester and slowly increase the temperature setting to about 370°C. The H2SO4 should condense about halfway up the tube neck, and when the solution clears, continue heating for about 3 hours.
4. Lift the tubes rack out of the block digester, carefully place it on a rack holder, and let the tubes

   cool to room temperature.

5. Slowly add about 15 mL DI water to the tubes, cool, and bring to volume. If tube contents are

   solidified and do not dissolve, heat the tubes again until the precipitate (gypsum) dissolves,
   then cool with tap water.

6. Each batch of samples for digestion should contain at least one reagent blank (no soil), and

   one chemical standard (no soil, 1 mL of the Stock Solution).

B. Distillation

Before starting a batch for distillation, the distillation unit should be steamed out for at least

10 minutes. Adjust the steam rate to 7 – 8 mL distillate per minute. Water should flow through the condenser jacket at a rate sufficient to keep the distillate temperature below 22 °C.

Calibrate the pH meter with buffer solutions of pH 7.0 (buffer), and 4.0 (sensitivity), after setting the temperature. Then standardize the 0.01 N H2SO4 in the Auto-Titrator by titrating three separate 10-mL aliquots of the primary standard, 0.01 N Tris solution, to pH 5.0. The
titrations should agree within 0.03 mL, if not; titrate further aliquots until an agreement is

found.

Carry out distillations as follows:

1. Dispense 1 mL saturated H3BO3 solution and 1 mL DI water into a 100-mL Pyrex
evaporating dish, placed underneath the condenser tip, with the tip touching the solution surface.
2. Pipette 10-mL aliquot into a 100-mL distillation flask, and add 10 mL 10 N NaOH solution.
3. Immediately attach the flask to the distillation unit with a clamp, start distillation, and continue for 3 minutes, lower the dish to allow distillate to drain freely into the dish.
4. After 4 minutes when about 35-mL distillate is collected, turn off the steam supply, and
wash the tip of the condenser into the evaporating dish with a small amount of DI water.
5. Titrate the distillate to pH 5.0 with standardized 0.01 N H2SO4 using an Auto-Titrator.
6. Ea ch distillation should contain at least two standards and two blanks (reagent blanks).

Recovery of NH4-N should be at least 96 %.

Notes

1. After finishing titration, wash the Teflon-coated magnetic stirring bar, the burette tip, and
   the combined electrode into the dish.
2. Between different samples, steam out the distillations. Disconnect distillation flasks containing the digested sample and NaOH, and attach a 100-mL empty distillation flask to the distillation unit. Place a 100-mL empty beaker underneath the condenser tip, turn off
   cooling water supply (drain the water from the condenser jacket), and steam out for 90
   second.

Calculations

Technical Remarks

1. The block digester may be insulated with an asbestos shield to obtain a more uniform
   temperature distribution.
2. Add 3 mL concentrated H2SO4 to DI water in the sound bottom flask in the heating mantle to

   trap any NH3 present. Also, add Teflon Boiling Chips to ensure smooth boiling.

3. The precision of the method depends upon the complete conversion of organic N into NH4- N,

   the digestion temperature and time, solid: acid ratio and the type of catalyst used.

4. The ideal temperature for digestion is 320-370 oC. At lower temperatures, digestion may

   not be complete, while above 410 o C, loss of NH3 may occur.

5. The catalysts (the mixture of CuSO4 or Se) are used to hasten the digestion process. Potassium sulfate is added to raise the boiling point of the acid so that loss of acid by volatilization is prevented.