Application Note Kjeldahl Nitrogen

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Kjeldatherm Digestion System KBL20S with TZ Controller

Turbosog Fume Scrubber for Kjeldahl and other Digestions

Vapodest 50s with Carousel

Combined Kjeldahl Digestion and Distillation System KI13/26

Kjeldahl is the common name given to the analytical method of determining the amount of Nitrogen in organic substances.

Kjeldahl Helpline

Gerhardt UK operates a Kjeldahl e-mail helpline for Gerhardt equipment users in the UK.

If you need help with your Kjeldahl method – low results, high results or inconsistent results. Then contact us using the information request above stating the problem in the comments box. We’ll get straight back to you by phone or e-mail with technical advice and a useful troubleshooting guide.

History

A Danish chemist, Johan Kjeldahl, 1849-1900, originally developed the method.  Johan Kjeldahl was part of the innovative laboratory team at the Carlsberg Brewery, also famous in microbiology for isolating the famous beer yeast Saccharomyces carlsbergensis, still used today.

Johan Kjeldahl was given the job of finding a way to prevent problems in the brewing process caused by differing amounts of protein in the grain supplied to the brewery.  The lower the amount of protein in the mash the higher the volume of beer produced.

As a start in solving the problem Johan needed a way of calculating the amount of protein in the grain supplied to the brewery quickly and accurately.

Johan developed a method, which reduces the protein by breaking all the bonds in the molecule and converting the nitrogen from the protein into ammonium ions.  Sulphuric acid is used for this purpose.  However, Sulphuric acid alone is too slow in breaking down these bonds and the reaction needed some help.  To speed up the process a combination of heat, salt and catalyst are used.  Heating alone can only get the Sulphuric acid to its boiling point of 338ºC. The addition of salt can raise this to the critical temperature for decomposition of 373ºC.  The addition of a catalyst (Johan used Mercury, today most people use Copper) speeds the reaction even further.  Today the normal operating temperature for Kjeldahl heating systems, KJELDATHERM and TURBOTHERM is between 380 - 420ºC.  Heating above 420ºC does not speed the reaction any more and only wastes energy. Infrared heaters, TURBOTHERM, do however speed the digestion by getting up to temperature very quickly.

Gerhardt are unique in the manufacture of both traditional digestion and modern digestion systems for the digestion of the sample in acid.

Once the ammonium ions had been formed, Johan needed some means of measuring the amount ammonium in the solution.  This resulted in the second phase of Kjeldahl analysis, the distillation.  At this stage, after diluting the acid with water, Sodium Hydroxide is added to the solution.  This frees the ammonium ions from their salt complex and forms free volatile ammonia which with the steam from the distillation is carried out of the solution and condensed into a collection vessel containing Boric acid buffer solution.  The Boric acid captures the ammonia and its pH increases.  An indication in the Boric Acid, usually 4.5 indicator, changes colour as the pH increases.  At the end of the distillation, 40 minutes for the traditional distillation units and about 4 minutes for the rapid distillation unit, the VAPODEST.  The Boric acid solution is then titrated back to its original start point with acid.

The amount of acid used in the titration can be directly compared to the amount of Ammonia, which in turn can be directly linked to the amount of Nitrogen in that Ammonia and finally to the amount of protein in the original sample. The most advanced distillation units Vapodest 50s use automated titration based on pH rather than colour.  External or internal titration systems may be used to calculate the end point rather than relying on subjective colourmetric colour techniques.

The final mathematical equation for the process is as follows.

%Nitrogen = (Titration amount in ml – Blank Titration in ml) x Acid strength in Normality x 1400(Mol Wt.)

                                                                        Sample weight in mg

% Protein = % Nitrogen x Protein Factor

Johan Kjeldahl produced his method for Nitrogen and protein analysis at the Carlsberg brewery over 100 years ago and his method is still the universally accepted method for this analysis. Although other methods, such as Dumas combustion (Gerhardt Dumatherm), are faster, none can cope with the variety of sizes or conditions of samples than Johan Kjeldahl’s original method.  Kjeldahl equipment is used extensively all over the world and provides essential information for laboratories.

For more information on Gerhardt Traditional Kjeldahl Equipment - click here.

For more information on Gerhardt Modern Kjeldahl Digestion Equipment - click here.

For more information on Gerhardt Vapodest Rapid Kjeldahl Distillation Equipment - click here.

For more information on the Dumas Combustion Method - click here.

For more information on the Gerhardt Dumatherm System - click here.