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Distillation and Titration

The distillation converts Nitrogen from Ammonium Sulphate (Solid) to Ammonia (Gas).

The ammonium sulphate present in the digested sample is 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.
VELP Kjeldahl distillation units offer different levels of automation, from entry-level distillation to distillation with automatic reagents addition and residues aspiration, up to fully automated system with integrated colorimetric titration.

The Distillation Steps

The Distillation Steps
Distillation and Titration
Steam Distillation
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.

Ammonia recovery in the receiving solution
Trap the distilled vapors in a dedicated solution of 25-30 ml of VreciverTM 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).

 

Titration

You can 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.

Titration is used 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)2SO+ 2H3BO3  
Distillation and Titration
Automatic Colorimetric Titration  (UDK 159, UDK 169)
VELP Vreceiver™
 is a certified formula composed of Boric Acid powder and a mixture of indicators mentioned by AOAC methods (Bromocresol green and Methyl red) allowing fast and standardized receiving solution preparation for colorimetric titration.
The color is red in the absence of ammonia, turns green in case of a 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).
Distillation and Titration
Titration with an external potentiometric titrator (UDK 149)
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.
 

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