Total kjeldahl nitrogen TKN determination method

The Total Kjeldahl Nitrogen (TKN) is the combination of the organic nitrogen, and the ammonia nitrogen forms in a wastewater sample. Moreover, the TKN experiment helps to determine the organic nitrogen amount in the sample, and that is by subtraction of ammonia nitrogen from the TKN value.

Wastewater treatment plant

For TKN values less than 5 mg/l the colorimetric method should be used, but for samples of unknown, and more than 5 mg/l concentrations, the titrimetric method should be used.

Objective

The objective of this experiment is to determine the Total Kjeldahl Nitrogen (TKN) value, using the titrimetric method for a wastewater sample.

Theory

The Total Kjeldahl nitrogen determination is composed of three steps, which are digestion, distillation, and titration.

1. Digestion: it is accomplished by boiling the sample in sulfuric acid, which results in ammonium sulfate solution. Potassium sulfate (K2SO4) catalyst is added; it raises the boiling point of the digesting acid, and mercuric oxide is added as well.

H₂SO₄ + Sample → CO₂(g) + H₂O(g) + SO₂(g) + (NH₄)₂SO₄(aq)

2. Distillation: It involves separation of ammonia – nitrogen. This is accomplished by adding sodium hydroxide-thiosulfate reagent (NaOH).

2NaOH + (NH₄)₂SO₄ → 2NH₃ + Na₂SO₄ + 2H₂O

The ammonia gas converts into ammonium ion because of the low pH of the solution , and then converts the boric acid to the borate ion.

H₃BO₃ + NH₃ →  NH₄⁺ + H₂BO₃- 

3. Titration: The amount of ammonia present, and thus the amount of nitrogen present in the sample, is determined by titration.

H₂BO₃ +H⁺ → H₃BO₃

The concentration of ammonia that is measured from TKN method is calculated by the following equation :

 

Where:
A = volume of sulfuric acid titrated for the wastewater sample (mL).
B = volume of sulfuric acid that titrated (mL).
V= volume of digestion reagent added (mL)

Equipments and materias

Equipments

1. Kjeldahl digestion apparatus.
2. Graduated cylinder.
3. Flasks.
4. Distillation apparatus.
5. Mixer.
6. Burette and stand for titrating.

Materials: 

1. Kjeldahl digestion reagent which consists of sulfuric acid, Potassium Sulfate and Mercuric Oxide.
2. Boric acid.
3. Sodium Hydroxide (NaOH).
4. Waste water sample.

Procedure

Most of the procedure steps were done by the lab instructor; however he told us the steps he did.

➧ 50 ml of wastewater water was added to a digestion tube.
➧ 50 ml of digestion reagent was added to the sample and was shacked until it reached to a homogeneous sample.
➧ The tube was put in the digestion apparatus for 2 hours (350-450 ˚C) then it was cooled.
➧ 25 ml of sodium hydroxide (NaOH) reagent was added to the sample.
➧ 25ml of Boric acid was added to a flask, and then the flask was put in the distillation apparatus (for about 15 min).
➧ After finishing the distillation, H₂SO₄ was added to the Boric acid in the titration process until the color was transferred from green to purple.
➧ The volume of H₂SO₄ was measured.

Data & Calculation:

Table-1: Data obtained from the experiment

#	Sample volume (ml)	Initial volume H₂SO₄ V1 (ml)	Final volume H₂SO₄ V2 (ml)
Wastewater sample	189	5.98	10.25

The amount of H₂SO₄ needed for making titration for blank sample (B) =0 ml

TKN-N = (A – B) *280/ Sample volume

Where:
A = amount of H2SO4 needed for making titration for wastewater sample.
B= amount of H2SO4 needed for making titration for blank sample
Sample Calculation:

The amount of H2SO4 needed for wastewater sample (A) =V2 –V1
=10.52 – 5.98 = 4.54ml

B=0.0 ml

TKN-N = [(4.54 – 0.0)*280]/189 = 6.727 mg/L.

Table-2: Calculation table

#	Initial volume H₂SO₄ V1 (ml)	Final volume H₂SO₄ V2 (ml)	H₂SO₄ needed ml	TKN-N mg/l
Wastewater sample	5.98	10.25	4.54	6.727

Conclusion

This experiment aim to determine the amount of organic nitrogen and ammonia nitrogen. Noting that it is one of the most challenging, dangerous, labor-intensive tests performed in wastewater treatment plant laboratories

The amount of ammonia gives us an indication about the treatment of wastewater, since the decrease of ammonia’s amount in the wastewater means that the wastewater is better treated.

Moreover, the organic nitrogen which may be dissolved or suspended can be calculated by:

Organic Nitrogen = TKN - Ammonia nitrogen

The amount of ammonia in a sample gives an idea about the efficiency of treatment, so when the amount of ammonia decreases in the wastewater, the wastewater is better treatment, thus It is important to measure the TKN (ammonia and organic nitrogen).

The amount of TKN in our experiment is 6.727 mg/l which is expected according to the table below form the lab manual ml, and because the value is low, this means treatment of wastewater is good.

Table-3

Expected TKN concentration (mg/L)	ML of sample to use
0-1	500
1-10	250
10-20	100
20-50	50
50-100	25

Some of the disadvantages of Total Kjeldahl Nitrogen test:

1) Long testing time, and dangerous experiment.
2) No measure of true protein content, because it only measures non protein nitrogen in addition to the nitrogen in proteins
3) High temperature is needed while using the concentrated sulfuric acid.

 

Sources of errors:

➧ Dirty containers.
➧ Systematic errors in the Instruments calibrations and no precision of volumes reading.
➧ Loss of the original material when adding sodium hydroxide because of the strong reaction.
➧ Losses of material may occur through the digestion apparatus.

Engineering Use

Application of this experiment:
1) Wastewater Treatment for removal and testing nitrogen.
2) Beverage and Food production analysis for protein.
3) Agriculture
4) Kjeldahl has been adapted to serve as the standard method of nitrogen analysis in environmental applications and the pharmaceutical and chemical industries.