Kjeldahl Method

Summary

The Kjeldahl method, developed by Johan Kjeldahl in 1883, is a standard procedure for determining nitrogen content in samples, particularly ammonia. It involves adding copper sulfate and potassium sulfate to a sample, followed by heating with sulfuric acid to break down organic matter. The ammonia produced is captured and distilled with a sodium hydroxide solution, then titrated with hydrochloric acid. The method is applicable for analyzing nitrogen in fertilizers but not for nitrogen in nitro or azide groups. This method remains widely used in analytical chemistry for nitrogen determination.

Takeaways

• 😀 Johan Kerala developed a model in 1883 for the quantitative determination of nitrogen chemicals, such as ammonia.
• 😀 The process begins by weighing 0.8 grams of copper sulfate and 7 grams of potassium sulfate.
• 😀 A 1-gram sample is mixed with potassium sulfate and copper sulfate and placed into a Kjeldahl flask.
• 😀 The sample undergoes destruction with sulfuric acid at temperatures between 420°C for 60 to 120 minutes.
• 😀 The reaction causes the solution to go from dark to transparent, with no smoke produced.
• 😀 Copper sulfate acts as a catalyst while potassium sulfate increases the boiling point of sulfuric acid.
• 😀 After the destruction, 75 milliliters of Aquarius and 16 milliliters of 40% sodium hydroxide are added.
• 😀 During distillation, a chemical reaction between sodium hydroxide and ammonium sulfate produces ammonia gas.
• 😀 The ammonia gas is captured by a 25-milliliter solution of 1% boric acid, with a green or metal red bromocresol indicator.
• 😀 The solution is titrated with 0.1 N hydrochloric acid, and a color change from green to pink indicates the end point.
• 😀 The moles of hydrochloric acid used are equivalent to the moles of nitrogen in the sample, allowing for determination.
• 😀 This method is applicable to samples such as fertilizers but does not work for nitrogen in the mitral and azure groups.

Q & A

Who developed the method for determining nitrogen content in ammonia?

The method for determining nitrogen content in ammonia was developed by Johan Kerala in 1883.

What role does copper sulfate play in the nitrogen determination method?

Copper sulfate acts as a catalyst in the destruction process during the determination of nitrogen content.

Why is potassium sulfate added to the sample in the nitrogen determination method?

Potassium sulfate is added to increase the boiling point of sulfuric acid, which helps to facilitate the reaction during the destruction process.

What is the purpose of sulfuric acid in the nitrogen determination process?

Sulfuric acid is used in the destruction step to break down the sample and release nitrogen in the form of ammonia gas.

What temperature is required for the destruction step in this method?

The destruction step requires heating at 420°C for 60 to 120 minutes.

What happens during the distillation process?

During the distillation process, ammonia gas is released as a result of the chemical reaction between sodium hydroxide and ammonium sulfate.

How is ammonia captured during the nitrogen determination method?

Ammonia is captured by a 25 milliliter solution of 1% boric acid, which contains a bromocresol indicator.

What is the role of the bromocresol indicator in this method?

The bromocresol indicator helps to visually detect the endpoint of the titration, changing color from green to pink when the reaction is complete.

How is the endpoint of the titration identified?

The endpoint is identified by a color change from green to pink, which occurs when the solution changes from colorless to pink.

What does the moles of hydrochloric acid used in the titration represent?

The moles of hydrochloric acid used in the titration are equivalent to the moles of nitrogen in the sample being analyzed.

What are the limitations of this method for nitrogen determination?

This method is not suitable for nitrogen in nitro or azide groups, and is specifically designed for nitrogen in ammonium compounds, such as those found in fertilizers.