Introduction

Method of Kjeldahl

The Kjeldahl method was invented by a Danish chemist named Johan Kjeldahl in 1883 and was the first method to be used in the field. In order to determine the nitrogen content of organic and inorganic substances, this method was developed specifically for that purpose.

Currently, Kjeldahl nitrogen determinations are used on a variety of samples, including wastewater and soil, fertilisers and animal and plant feed as well as grains and a variety of other substances. The method is also employed in the estimation of the protein content of food products.

Apparatus and specialised equipment

The Kjeldahl method was developed in the early 1900s and was first used with stone fume hoods and gas mantles. A few years later, the macro-Kjeldahl digestion method, as well as a distillation instrument, were developed and put to use by scientists. In addition to Kjeldahl flasks, the setup included other items. A smaller scale version of the setup, known as micro-Kjeldahl equipment, is available and is made up of smaller-sized units.

In recent years, the equipment has undergone some significant changes, with aluminium or ceramic heating blocks being used as a result of these modifications. This setup is capable of accepting multiple straight digestion tubes at the same time. In addition, “Block digesters” are used in conjunction with benchtop distillation units with steam generators in order to reduce the distillation time by as much as possible. The majority of the equipment is constructed of corrosion-resistant materials.

Process of the Kjeldahl Method.

Since the invention of the Kjeldahl method more than a century ago, the technique and apparatus have been improved and improved upon repeatedly. However, even with the new modifications, the fundamental principles established by Johan Kjeldahl continue to hold true. Organic compounds are oxidised in this procedure, with concentrated sulphuric acid serving as the primary goal or purpose.

The Kjeldahl method is divided into three major steps, which are as follows: The procedure must be carried out in the proper sequence to be effective. Digestion, distillation, and titration are all steps in the process.

1. Digestion: In this method, a specific substance or sample is heated in the presence of sulphuric acid in order to break down the substance or sample. During the oxidation process, the acid degrades the organic substance, releasing reduced nitrogen in the form of ammonium sulphate. Potassium sulphate is commonly used to raise the boiling point of a liquid medium to a higher temperature. In addition to catalysts such as mercury, selenium, copper, or mercury or copper ions, the digestion process makes use of enzymes. When we obtain a clear and colourless solution from the sample, we have completed the decomposition process.

A small amount of sodium hydroxide is added to the solution to convert the ammonium salt to ammonia.

2. Distillation: The solution is now distilled and a small amount of sodium hydroxide is added to convert the ammonium salt to ammonia. A special trapping solution consisting of HCl (hydrochloric acid) and water is used to capture the distilled vapours after they have been captured.

3. Titration: The amount of ammonia or nitrogen present in the sample is determined by back titration, which is a chemical reaction. HCl is neutralised as ammonia dissolves in the acid trapping solution due to the acid trapping solution’s acidity. To determine the amount of acid left behind, use a standard base solution such as NaOH or other bases to perform a back titration on it.

Using a Formula to Calculate

The percentage of nitrogen in a solution can be calculated using the following formula:

Percentage of Nitrogen in the sample =1.4V*N/W

Where,

V is the acid that was used in the titration (ml)

N represents the normality of standard acid.

W denotes the weight of the sample (g)

The Kjeldahl Method has some limitations

As is well known, while the Kjeldahl method of nitrogen analysis has become the worldwide standard, this method is not suitable for compounds that contain nitrogen in the form of azo and nitro groups or rings (quinoline, pyridine, etc.). Following the Kjeldahl method will not work in these situations because the nitrogen cannot be converted to ammonium sulphate.

Conclusion

When used in analytical chemistry, the Kjeldahl method, also known as the Kjeldahl digestion, is a method for the quantitative determination of nitrogen contained in organic substances, as well as nitrogen contained in the inorganic compounds nitrate and nitrate (NH3/NH4+).In order to determine the nitrogen content of organic and inorganic substances, this method was developed specifically for that purpose.Since the invention of the Kjeldahl method more than a century ago, the technique and apparatus have been improved and improved upon repeatedly. However, even with the new modifications, the fundamental principles established by Johan Kjeldahl continue to hold true.while the Kjeldahl method of nitrogen analysis has become the worldwide standard, this method is not suitable for compounds that contain nitrogen in the form of azo and nitro groups or rings (quinoline, pyridine, etc.). Following the Kjeldahl method will not work in these situations because the nitrogen cannot be converted to ammonium sulphate.