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Kjeldahl distillation

Kjeldahl distillation units offers different levels of automation, from simple distillation, to distillation with automatic reagents addition and residues aspiration, up to fully automated system with integrated colorimetric titration.

The ammonium sulphate present in the digested sample are converted into ammonia gas, heated and distilled. The ammonia gas is led into an acid trapping solution where it dissolves and becomes a trapped ammonium ion once again.
Using the Kjeldahl distillation, nitrites and nitrates are not detected. In order to quantify these elements, a reduction of the sample is necessary (using Devarda alloy) before the Kjeldahl digestion stage.

Kjeldahl distillation

Steam distillation on VELP Scientifica UDK 159, Automatic Kjeldahl Analyzer

Add distilled or deionized water to the test tube containing the digested sample to dilute it (automatically on UDK 139 & UDK 149 Kjeldahl Distillation Units and UDK 159 Automatic Kjeldahl Analyzer). In this way it’s easier to detect all the ammonia.
Separate the nitrogen from the digested mixture by steam distilling (steam output regulation 10-100% on UDK 139 & UDK 149 Kjeldahl Distillation Units and UDK 159 Automatic Kjeldahl Analyzer), in order to extract ammonia from the alkaline solution.
Raise the pH of the digested mixture using sodium hydroxide (35%) (automatically on UDK Kjeldahl Distillation Units) to convert NH4+ (in solid format) into NH3 (gaseous), that will be detected with titration.
Trap the distilled vapors in a dedicated solution of 25-30 ml of boric acid (automatically on UDK 149 Kjeldahl Distillation Unit and UDK 159 Automatic Kjeldahl Analyzer) to trap all the nitrogen, eliminating the risk of loss.

(NH4)2SO4 + 2NaOH → 2NH3 + Na2SO4 + 2H2O
NH3 + H3BO3 → NH4 H2BO3 + H3BO3

Drain the test tube with the digested sample (automatically on UDK 139 & UDK 149 Kjeldahl Distillation Units and UDK 159 Automatic Kjeldahl Analyzer).

Typical process for Kjeldahl distillation

Typical process for Kjeldahl distillation

Now perform the final titration of the ammonia distilled from the sample, considering that if the nitrogen content of the sample is high, a high-concentrated acid for the titration is needed. Another solution is reducing the quantity of the sample used for the analysis, but in some cases it may cause errors giving wrong results.
The following step is the titration, to determine the amount of ammonia distilled off from the digested solution and hence calculate the Kjeldahl nitrogen or protein amount, as %.

2 NH4 H2BO3 + H2SO4 → (NH4)2SO4 + 2H3BO3

With UDK 159 Automatic Kjeldahl Analyzer (distillation and colorimetric titration):

Kjeldahl distillation

Add two indicators to the boric acid (4%) solution, in order to follow the titration process by a color change. The color is red in absence of ammonia, turns green in case of significant amount and grey/pink at the end of the analysis.
Put a standardized solution (titrant) of hydrochloric acid (HCl) or sulfuric acid (H2SO4) in the burette; this solution will be added (automatically) to the colored boric acid containing the ammonia distilled from the sample. The acid reacts with ammonia in order to measure it.
Record the volume of the acid titrant solution that was necessary to reach the endpoint and perform a final calculation to find the amount of nitrogen, expressed as % N or % proteins, in the original sample (automatically).

With an external potentiometric titrator with a pH electrode (connectable to the UDK 149 Kjeldahl Distillation Unit):
The titrator burette adds the acid titrant solution automatically to the boric acid solution containing the distilled ammonia, until reaching the endpoint, corresponding to pH=4.7. In this case we don’t check a color change and we don’t use indicators, but we follow the corresponding change in the pH of the boric acid solution during the titration process using an external potentiometric titrator with a pH electrode.

Today, various scientific associations approve the Kjeldahl method, including the AOAC International (Association of Official Analytical Chemists), AACC (Association of American Cereal Chemists), AOCS (American Oil Chemists Society), EPA (Environmental Protection Agency), ISO (International Standards Organization), and many others. All VELP Scientifica equipment for Kjeldahl nitrogen determination work in accordance with the above-mentioned associations.

Kjeldahl distillation and titration related products