Determination of Ammonium in Soils
Photometric Determination Using the Indophenol Blue Method After Extraction With a Calcium Chloride Solution
Introduction
Ammonium (NH4+) is one of the most important sources of nitrogen for plant nutrition and it is formed with the mineralization of decomposed organic matter.1 Research also has established that ammonium is the naturally preferred source of nitrogen for plant growth.2 With the increasing global demand for organic food products, farmers are resorting to the use of compost and manure. While ready-made ammonium fertilizers have defined concentrations of the cation and related compounds in them, it is important to analyze composts and manure for their accurate ammonium concentration. As the early phases of decomposition account for the loss of nitrogen as ammonia, and there is a natural loss of ammonium as the compost matures.3 Excess ammonium has also been reported to be toxic in higher plants.4
This application note details the photometric determination of ammonium in soils, using the indophenol blue method, after its extraction with calcium chloride solution. Post the sample extraction and in a strongly alkaline solution ammonium nitrogen is present almost entirely as ammonia, which reacts with hypochlorite ions to form monochloramine. This in turn reacts with a phenol compound to form a blue indophenol derivative that is determined photometrically.
Reagents, Instruments, and Materials
Ammonium Test Kit/Reagents
For the measurement one of the following Spectroquant® test kits is necessary:
- Spectroquant® Ammonium Cell Test (1.14739)
- Spectroquant® Ammonium Cell Test (1.14544)*
- Spectroquant® Ammonium Cell Test (1.14558)
- Spectroquant® Ammonium Cell Test (1.14559)
- Spectroquant® Ammonium Test (1.14739)
- Spectroquant® Ammonium Test (1.14752)
- Spectroquant® Ammonium Test (1.00683)
*not compatible with Move 100
Instruments
For the measurement one of the following Spectroquant® photometers is necessary:
- Spectroquant® VIS Spectrophotometer Prove 100 (173016)
- Spectroquant® UV/VIS Spectrophotometer Prove 300 (173017)
- Spectroquant® UV/VIS Spectrophotometer Prove 600 (173018)
Software for Data Transfer
Optional Spectroquant® Prove Connect to LIMS software package (Y.11086) to transfer your data into an existing LIMS system.
Instrument Accessories
- Rectangular cell 10 mm (1.14946) and/or
- Rectangular cells 20 mm (1.14947) and/or
- Rectangular cells 50 mm (1.14944)
Other Reagents and Accessories
- Calcium chloride dihydrate for analysis (1.02382)
- Water for analysis (1.16754)
- Charcoal activated for soil test
- Folded filter
Experimental Procedure
Reagent Preparation
Dissolve 3.68 g of calcium chloride dihydrate for analysis with 1 L of water to prepare a 0.025 mol/L calcium chloride solution.
Sample Preparation
- In a glass bottle, mix 50 g of naturally moist sample, free from coarse stones, with 100 mL of the calcium chloride solution.
- Add 1 spatula - tip full of charcoal activated for soil tests and shake the closed bottle in a shaking machine for 1 hour (alternative: stir in a beaker).
- Let the suspension settle and filter through a folded filter.
- For the determination of the water content dry a similar sample to constant weight in the drying kiln at 105 °C. Drying the sample before the determination is not advisable because of the fast change in the nitrogen - forms.
- The analysis should take place very quickly.
- Storing the sample in plastic bags at room temperature changes the analysis values after a short time already.
Analysis
Determine with the above-mentioned test kits.
Calculation
Ammonium content in mg/kg NH4+ = analysis value in mg/L NH4+ x 2
*Product 1.14544 is not compatible with Move 100.
References
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