Frequently Asked Question

The Kjeldahl method is a classic chemical analysis technique used to determine the nitrogen content of a sample, typically in food, fertilizer, or wastewater. Here's a basic outline of a manual Kjeldahl procedure, followed by a worked-out example:
Procedure:
1. Digestion:
- Weigh a known amount of your sample into a Kjeldahl flask.
- Add concentrated sulfuric acid (H2SO4), a catalyst (e.g., copper sulfate or selenium), and potassium sulfate (K2SO4) to the flask. The potassium sulfate increases the boiling point of the sulfuric acid, allowing for more efficient digestion.
- Heat the mixture gently at first, then vigorously until the solution clears and a colorless or pale yellow liquid remains. This process breaks down organic matter, converting nitrogen to ammonium ions (NH4+).
2. Distillation:
- After digestion is complete, add a strong base (sodium hydroxide, NaOH) to the flask to make the solution strongly alkaline.
- Distill the ammonia (NH3) produced from the ammonium ions into a solution of boric acid (H3BO3) in a receiving flask.
3. Titration:
- Titrate the solution in the receiving flask with a standardized solution of hydrochloric acid (HCl) to neutralize the ammonia. The endpoint is reached when the solution turns a pale pink color in the presence of an indicator (e.g., methyl red).
4. Calculation:
- The amount of nitrogen in the original sample is calculated based on the volume of HCl used in the titration and the known concentration of the HCl solution.
Worked Example:
Let's say you analyze a 0.500 g sample of fertilizer and, after the distillation and titration steps, you find that 25.00 mL of 0.100 M HCl was required to reach the endpoint. Here's how you would calculate the nitrogen content:
1. Moles of HCl used: 0.02500 L
0.100 mol/L = 0.00250 mol HCl
2. Moles of ammonia (NH3) neutralized: Since the reaction of HCl and ammonia is 1:1, the moles of ammonia are equal to the moles of HCl, which is 0.00250 mol NH3.
3. Mass of nitrogen in the sample: 0.00250 mol NH3
(1 mol N/1 mol NH3)
(14.01 g N/1 mol N) = 0.0350 g N
4. Percentage of nitrogen in the sample: (0.0350 g N / 0.500 g sample)
100% = 7.00% N
Important Notes:
- Safety Precautions: The Kjeldahl method involves strong acids and bases. Always wear appropriate safety gear (gloves, goggles, lab coat) and follow proper laboratory safety procedures.
- This is a simplified outline. The specific details and procedures can vary depending on the type of sample and the desired level of accuracy.