Kjeldahl Analysis

Kjeldahl Analysis

Inorganic Analysis

More than 100 years ago Johan Kjeldahl, a research analyst from the Carlsberg Breweries in Denmark, de-velopped a method for the determination of nitrogen in samples of biological origin, i.e. plants and animals. This method is still in use in thousands of laboratories throughout the world, mostly for checking the amount of proteins in food.

The principle of the method: 1-5 g of the sample is placed in a Kjeldahl flask; 20 ml of concentrated sul-furic acid and 5-10 g of a Kjeldahl catalyst are added. Kjeldahl catalysts are 5 g-tablets or powders mainly consisting of potassium or sodium sulphate plus a small percentage of copper sulphate, titanium dioxide, or selenium. A reflux condenser is placed on the flask,

and the mixture is boiled for several hours until a clear solution has been formed. All nitrogen from the sample has now been transformed to ammonium sulphate. By addition of a sufficient quantity of sodium hydroxide, ammonia is released from the latter which is distilled into a flask containing sulfuric acid volumetric solution, thus forming ammonium sulphate again. The excess of sul-furic acid is determined by titration with sodium hydroxide volumetric solution. From the result the analyst can easily calculate the concentration of nitrogen (= proteins) in the sample.

As a variation of this method, the ammonia can also be distilled into a flask con-taining a boric acid solution. The amount of ammonia can directly be deter-mined by titration with hydrochloric acid volumetric solution.

As an alternative to the Kjeldahl method, the nitrogen content can also be ana-lyzed by burning the sample in a quartz rod together with a catalyst to form am-monia. This can be determined by titration, the same way as shown above. For details about this method see section "Elemental Analysis" in Analytical Meth-ods Guide.

Food industry

Governmental laboratories for supervision of food manufacturers

The Kjeldahl method is the world wide official standard for the determination of nitrogen in all kind of food samples, e.g.:

meat and related products, like sausages

grain and flour

cocoa

About this method

Field of application

What is analyzed by this method?

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Kjeldahl Analysis

milk and related products, like cheese, milk powder

baby food

Digestion

Distillation

Titration

Volumetric solutions: sodium hydroxide, hydrochloric acid, sulfuric acid; the user can chose from various concentrations according to his analytical require-ments.

Which reagents are used?

1.00748 Sulfuric acid 98% for the determination of nitrogen

1.10958 Kjeldahl tablets acc. to Wieninger (96.5 % Na2SO4, 1.5 % CuSO4, 2.0 % Se)

1.08030 Selenium reagent mixture (composition acc. to Wieninger)

1.15348 Kjeldahl tablets, mercury and selenium free (47.7 % K2SO4, 47.7 % Na2SO4, 2.8 % TiO2, 1.8 % CuSO4)

1.08076 Kjeldahl catalyst powder, mercury and selenium free

1.16469 Kjeldahl tablets (Missouri catalyst) (48.8 % Na2SO4, 48.9 % K2SO4, 0.3 % CuSO4)

1.07913 Boiling chips

1.07743 Silicon anti-foaming agent

1.05590 Sodium hydroxide solution 32 % GR for the determination of nitrogen

Sulfuric acid volumetric solution

1.00165 Boric acid GR

1.06130 Mixed indicator 5 for ammonia titration

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Kjeldahl Analysis

Cat. No. Designation

111584 Sodium hydroxide solution c(NaOH) = 4 mol/l (4 N)

109136 Sodium hydroxide solution c(NaOH) = 2 mol/l (2 N)

109137 Sodium hydroxide solution c(NaOH) = 1 mol/l (1 N)

109138 Sodium hydroxide solution c(NaOH) = 0,5 mol/l (0,5 N)

105599 Sodium hydroxide solution c(NaOH) = 0,357 mol/l (1/2,8 N)

105595 Sodium hydroxide solution c(NaOH) = 0,33 mol/l (1/3 N)

109139 Sodium hydroxide solution c(NaOH) = 0,25 mol/l (0,25 N)

109140 Sodium hydroxide solution c(NaOH) = 0,2 mol/l (0,2 N)

109141 Sodium hydroxide solution c(NaOH) = 0,1 mol/l (0,1 N)

109142 Sodium hydroxide solution c(NaOH) = 0,02 mol/l (0,02 N)

109913 Sodium hydroxide for the preparation of dilute volumetric solutions or for direct use c(NaOH) = 5 mol/l (5 N) Combi-Titrisol®

109956 Sodium hydroxide solution for 1000 ml c(NaOH) = 1 mol/l (1 N) Titrisol®

109957 Sodium hydroxide solution for 1000 ml c(NaOH) = 0,5 mol/l (0,5 N) Titrisol®

109958 Sodium hydroxide solution for 1000 ml c(NaOH) = 0,25 mol/l (0,25 N) Titrisol®

109959 Sodium hydroxide solution for 1000 ml c(NaOH) = 0,1 mol/l (0,1 N) Titrisol®

109961 Sodium hydroxide solution for 1 l measure solution c(NaOH) = 0,01 mol/l (0,01 N) Titrisol®

109063 Hydrochloric acid c(HCl) = 2 mol/l (2 N)

109057 Hydrochloric acid c(HCl) = 1 mol/l (1 N)

109058 Hydrochloric acid c(HCl) = 0,5 mol/l (0,5 N)

113136 Hydrochloric acid c(HCl) = 0,357 mol/l (1/2,8 N)

109060 Hydrochloric acid c(HCl) = 0,1 mol/l (0,1 N)

109911 Hydrochloric acid 1.000 l for the preparation of dilute volumetric solutions or for direct use c(HCl) = 5 mol/l (5 N) Combi-Titrisol®

109970 Hydrochloric acid for 1000 ml c(HCl) = 1 mol/l (1 N) Titrisol®

109971 Hydrochloric acid for 1000 ml c(HCl) = 0,5 mol/l (0,5 N) Titrisol®

109973 Hydrochloric acid for 1000 ml c(HCl) = 0,1 mol/l (0,1 N) Titrisol®

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Kjeldahl Analysis

109974 Hydrochloric acid for 1000 ml c(HCl) = 0,01 mol/l (0,01 N) Titrisol®

109072 Sulfuric acid c(H2SO4) = 0,5 mol/l (1 N)

109073 Sulfuric acid c(H2SO4) = 0,25 mol/l (0,5 N)

109074 Sulfuric acid c(H2SO4) = 0,05 mol/l (0,1 N)

109912 Sulfuric acid 1.000 l for the preparation of dilute volumetric solutions or for direct use c(H2SO4) = 2,5 mol/l (5 N) Combi-Titrisol®

109981 Sulfuric acid for 1000 ml c(H2SO4) = 0,5 mol/l (1 N) Titrisol®

109982 Sulfuric acid for 1000 ml c(H2SO4) = 0,005 mol/l (0,01 N) Titrisol®

109984 Sulfuric acid for 1000 ml c(H2SO4) = 0,05 mol/l (0,1 N) Titrisol®

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