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Le Minh Tam – Food Industry College of HCMC

1. Introduction

2. Kjeldahl Method – Procedure

3. Calculating percent of nitrogen

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Johan Kjeldahl, a Danish chemist, invented a method of determining the amount of protein in samplestaken from a wide variety of organisms.

In 1883 Kjeldahl presented to the Danish Chemical Society.

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The central basis used in this procedure is the oxidation of the organic compound using strong sulfuric acid.

Result:The carbon is converted into carbon dioxide. The hydrogen is converted into water.Especially, the nitrogen from the amine groups found in the peptide bonds of the polypeptide chains, is converted to ammonium ion, which dissolves in the oxidizing solution, and can later be converted to ammonia gas.

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1. The sample is first digested in strong sulfuric acid in the presence of a catalyst, which helps in the conversion of the amine nitrogen to ammonium ions.

2. The ammonium ions are then converted into ammonia gas, heated and distilled. The ammonia gas is led into a trapping solution where it dissolves and becomes an ammonium ion once again.

3. Finally the amount of the ammonia that has been trapped is determined by titration with a standard solution, and a calculation made.

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The purpose: breaking down the bonds that hold the polypeptides together, and convert them to simpler chemicals such as water, carbon dioxide and, of course, ammonia.

Speeding up the reaction by using the catalyst - potassium sulfate (K2SO4) -which raises the boiling point of the digesting acid and thus the temperature of the reaction.

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The purpose: separate the ammonia from the digestion mixture. This is done by:1. Raising the pH of the mixture using sodium

hydroxide (30% NaOH solution). 2. Separating the nitrogen away from the digestion

mixture by distilling the ammonia and then trapping the distilled vapors in a special trapping solution.

3. Removing the trapping flask and rinsing the condenser with water so as to make sure that all the ammonia has been dissolved.

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1. Adding an indicator dye to the acid/ammonia trapping solution.

2. Putting a standard solution of NaOH into the burette

3. Watching for the point at which the dye turns orange, indicating that the "endpoint" has been reached

4. Recording the volume of the neutralizing base (sodium hydroxide solution) that was necessary to reach the end point.

5. Performing a calculation to find the amount of ammonia, and thus nitrogen.

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Claim that:

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3 0.14 100c

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