Nitrogen, quantitative analysis

Nitrogen is present in nutrient solutions in various forms, including:

• Ammonium (NH₄⁺) – Plant-available, often as NH₄Cl or (NH₄)₂SO₄.
• Nitrate (NO₃⁻) – The most important nitrogen source for plants.
• Organically bound nitrogen – In proteins, amino acids or urea.

The determination is carried out using various methods:

• Kjeldahl method: Digestion of organic nitrogen compounds and titration of ammonium.
• Ion chromatography: separation of NH₄⁺ and NO₃⁻.
• Spectrophotometry: Determination of NO₃⁻ via the Griess reaction.
• Titration: Direct determination of NH₄⁺ with formaldehyde or back titration.

Detailed titration of ammonium with formaldehyde

1. Principle of the method

The titration is based on the reaction of ammonium ions (NH₄⁺) with formaldehyde (HCHO) , which produces methyleneimine (CH₂=NH) and water:

The resulting CH₂=NH increases the pH value as H⁺ ions are consumed. The solution is then back-titrated with hydrochloric acid (HCl) .

2. Chemicals

• Formaldehyde solution (37%)
• Hydrochloric acid (HCl), c = 0.05 mol/L
• Indicator: Methyl red or bromocresol green

3. Experimental setup

Required equipment:

• Burette (25 mL, division 0.1 mL)
• Erlenmeyer flask (100 mL)
• Pipette (10 mL)
• pH meter or indicator paper

4. Implementation

1. Use a pipette to transfer 10 mL of the nutrient solution into a 100 mL Erlenmeyer flask.
2. Add 5 mL of formaldehyde solution and mix well.
3. Add indicator (e.g. methyl red).
4. Titrate with 0.05 mol/L HCl until the color changes from yellow to red.

5. Calculation of the ammonium concentration

The concentration of NH₄⁺ is calculated from the consumption of the HCl solution: 

6. Example calculation:

• HCl concentration: 0.05 mol/L
• Consumed volume: 7.2 mL (0.0072 L)
• Sample volume: 50 mL (0.050 L)
• $c({\mathrm{NH}}_4⁺)=\frac{0.0072\cdot 0.05}{0.050}=0.0072\text{mol/L}=7.2\text{mmol/L}$

Advantages of titration:

• Simple and cost-effective experimental setup.
• Relatively quick implementation.
• Enables precise determination at medium concentrations.

Disadvantages of titration:

• Influence by other nitrogen compounds.
• Requires careful pH control.

Comparison to other methods

Conclusion

The choice of method for nitrogen analysis depends on the specific analytical requirements, the required sensitivity, and the available equipment. Titration is a suitable method for the rapid and cost-effective determination of ammonium, while other methods such as the Kjeldahl method or ion chromatography may be superior in certain contexts.