Total Kjeldahl Nitrogen (TKN) Determination

In general, nitrogen is a limiting factor for biological systems, especially those that rely on bacteria to break down substances. Kjeldahl nitrogen has been widely used for the quantitative determination of nitrogen such as ammonia and ammonium (NH₃ /NH₄⁺) in a variety of samples, including protein content in food and beverages, meat, feed, grains and pasture. In addition, Kjeldahl nitrogen determination can be employed to determine nitrogen in wastewater, soil and other substances. As an official method, Kjeldahl nitrogen determination is described in different specifications such as AOAC, USEPA, ISO, DIN, Pharmacopoeia, etc. At BOC Sciences, combined with our professional wet chemistry knowledge including colorimetric; titrimetric; and potentiometric method, the Kjeldahl method is used to determine the nitrogen content in organic and inorganic samples. Our method is developed based on the wet oxidation of soil organic matter and plant material using sulfuric acid and digestion catalysts and the conversion of organic nitrogen to the ammonium form. In addition, we can determine the TKN according to the kjeldahl method in water and wastewater along with determination of Limit of Detection (LOD) and Limit of Quantification (LOQ).

Figure 1. Nitrogen Testing

Application

• Determination of total Kjeldahl nitrogen in drinking water, surface and brine water, domestic and industrial wastes
• Converts nitrogen components of biological origin, such as amino acids, proteins and peptides, into ammonia

TKN Analysis Methods

The kjeldahl nitrogen in the sample is first converted into ammonia by metal-catalyzed acid digestion, and then the resulting ammonia is separated from the sample by distillation, with the released ammonia being captured in a dilute sulfuric acid solution. The following are the specific steps performed by our chemists:

Digestion

The purpose of the digestion process is to break all nitrogen bonds in the sample and convert all of the organically bound nitrogen into ammonium ions (NH₄⁺). We mix the samples with sulfuric acid, and greatly improve the digestion rate by adding salt, oxidizer and catalyst. During this process, the organic material carbonizes and produces black foam. During the digestion process, the foam decomposes and finally a clear liquid appears, indicating the completion of the chemical reaction. After digestion is complete, the sample is allowed to cool to room temperature, and is diluted with water and transferred to the distillation unit.

Figure 2. The Kjeldahl method of Nitrogen estimation

Distillation

In the distillation step, we add alkali (NaOH) to convert ammonium ions (NH₄⁺) into ammonia gas (NH₃). The receiver vessel of the distillate is filled with an absorption solution to capture the dissolved ammonia gas. Common absorption solutions include aqueous boric acid [B(OH)₃], where ammonia is quantitatively captured by the boric acid solution to form dissolved ammonium ions.

Titration

At BOC Sciences, the concentration of the captured ammonium ions can be determined by using two types of Titration methods, Nesslerization method and potentiometric method:

• Direct Titration

When the boric acid solution is used as the absorbent solution, an acid-base titration is performed using standard solution of sulfuric or hydrochloric acid and a mixture of indicator. In addition, the end point can be determined potentiometrically using a pH-electrode.

• Back Titration

When the sulfuric acid standard solution is used as the absorbing solution, the residual sulfuric acid is titrated against a standard solution of sodium hydroxide, and the amount of ammonia is calculated by the difference.

• Nesslerization

Applicable to concentrations below 1 mg N/liter.

• Potentiometry

Applicable to the range 0.05 to 1400 mg/L.

Equipment

• pH Meter: capable of accepting ion selective electrodes and measuring in millivolt units
• Ammonia probe
• Magnetic Stirring Device
• Digestion apparatus: A Kjeldahl digestion apparatus with appropriately sized digestion flasks and suction to remove SO₃ fumes and water
• Block digester or individual heating elements or hot plates
• Various graduated and volumetric pipets