4.4  Determination of ammonium in precipitation

Ammonium may be determined together with the other major cations in precipitation if ion chromatograph equipment for cations is available. If not, the method described below is a good alternative.

4.4.1  Spectrophotometric by the indophenol blue method

4.4.1.1  Field of application

This method is applicable to the determination of the ammonium content in precipitation within the range 0.04 to 2.0 mg NH4/l.

4.4.1.2  Principle

In an alkaline solution (pH 10.4-11.5) ammonium ions react with hypochlorite to form monochloramine. In the presence of phenol and an excess of hypochlorite, the monochloramine will form a blue coloured compound, indophenol, when nitroprusside is used as catalyst. The concentration of ammonium is determined spectrophotometrically at 630 mn.

4.4.1.3  Instrumentation

4.4.1.4  Chemicals

During analysis, use only chemicals of recognized analytical grade. The water used for dilution and rinsing should be double-distilled or de-ionized and distilled.

4.4.1.5  Reagents

  1. Reagent A:
    Dissolve 3.5 g phenol and 0.040 g sodium nitroprusside in 100 ml water. Store the solution refrigerated in the dark. If the colour of the solution turns greenish, it must be discarded, and a fresh solution prepared.
  2. Reagent B:
    Dissolve 1.8 g sodium hydroxide in some water in a 100 ml volumetric flask. Add 4.0 ml 1 M sodium hypochlorite solution, and dilute with water to the mark. Store the solution refrigerated in the dark. If the solution is stored for weeks, the concentration should be checked by titration with a sodium thiosulphate solution.
  3. Standard ammonium solution I, 100 mg NH4/l:
    Ammonium chloride must be dried for one hour at 100 °C.
    Dissolve 0.2965 g of the dried salt in water in a 1000 ml volumetric flask. Dilute to the mark with water. The solution is stable for six months when stored in a refrigerator.
  4. Standard ammonium solution II, 4 mg NH4/l:
    By means of a pipette, transfer 20.0 ml of standard ammonium solution I to a 500 ml volumetric flask. Dilute with water to the mark. This standard ammonium solution, and the ammonium solutions made for preparing the calibration curve, must be freshly made.

4.4.1.6  Calibration

Preparation of calibration curve:

  1. Transfer to 100 ml volumetric flask 0.0, 1.0, 2.0, 5.0, 10.0, 25.0 and 50.0 ml of standard ammonium solution II. Dilute to the mark with water. The concentrations of these solutions are 0.00, 0.04, 0.08, 0.2, 0.4, 1.0 and 2.0 mg NH4/l. Transfer 5.0 ml of each of these standard solutions and 5.0 ml of water to a 30 ml test tube.
  2. Add to the test tube 250 µl reagent A using a micro pipette, and mix well. Add then 250 µl reagent B using a micro-pipette and mix well. Cover the opening of the tube with some inert material. Place the tube in the water bath at 50 °C for two hours.
  3. Cool the solution to room temperature, and transfer it to a 10 mm cell. Measure the absorbance at 630 nm.
  4. Prepare a calibration curve by plotting the absorbance of each of the standard solutions against its concentration of ammonium. Prepare a new calibration curve for each series of samples.
  5. In order to check for ammonium in the reagents, take a photometric reading of the blank (0.00 mg NH4/l) against water. The absorbance should not exceed 0.020.

4.4.1.7  Analytical procedure

Transfer 5.0 ml of the sample and 5.0 ml of water to a 30 ml test tube. Proceed according to Section 4.4.1.6 (2) and (3). Convert the spectrophotometric readings of the sample to mg NH4/l by means of the calibration curve. The concentration may be expressed as mg N/l by multiplying with 0.778. Samples containing more than 2.0 mg NH4/l must be diluted. With suitable equipment the “Indophenol method” can be made automatic. A detailed description is given in Section 4.4.2.

4.4.1.8  Interferences

Iron (III) may interfere if the concentration is more than 2 mg/l. This concentration of iron (III) does not occur very often in precipitation samples.

If the pH-value of the sample is lower than 3, the sample should be neutralized.

If the sample is turbid, both the sample and the blank should be filtered through a white band filter.

4.4.1.9  References

Koroleff, F. (1970) Direct determination of ammonia in natural waters as indophenol blue. In: Information on Techniques and Methods for Seawater Analysis. Charlottenlund, Internat. Counc. Exploration of the sea (Interlab. Rept. 3). pp. 19-22.

4.4.2  Automatic spectrophotometric determination of ammonium by the indophenol blue method

4.4.2.1  Field of application

This method can be used to determine the concentration of ammonium within the range 0.05 to 2.0 mg NH4 /l.

4.4.2.2  Principle

The basic principles are the same as for the manual methods (see Section 4.4.1).

The reaction between ammonium and hypochlorite in an alkaline solution (pH: 10.5 to 11.5) gives monochloramine. In the presence of phenol and an excess of hypochlorite, the monochloramine will form a blue coloured compound, indo­phenol, when nitroprusside is used as catalyst. The concentration of ammonium in the solution is determined spectrophotometrically at 630 nm.

4.4.2.3  Instrumentation

 

 

 

 

 

 

 

 

 

 

 

Figure 4.4.1: a) Automatic determination of nitrate and ammonium in precipitation samples. b) Reduction column for the determination of nitrate in precipitation samples.


4.4.2.4  Chemicals

All chemicals must be of recognized analytical grade. The water used for dilution and rinsing must be double-distilled or deionized.

Potassium nitrate (KNO3)
Phenol (C6H5OH)
Sodium nitroprusside (Na2Fe(NO) (CN)5 · 2H2O)
Sodium hydroxide (NaOH)
Sodium hypochlorite solution (NaOCl) 1M: Use a solution containing approximately 3.5% active chlorine (35g/l) in 0.1M NaOH (e.g. British Drug House, no. 23039)
Ammonium sulphate ((NH4)2 SO4)
Sodium thiosulphate (Na2S2O3)

4.4.2.5  Reagents

Reagents for the determination of ammonium:

  1. Dissolve 3.5 g phenol and 0.040 g sodium nitroprusside in 100 ml water. Store the solution refrigerated in the dark. If the colour of the solution turns greenish, a fresh solution must be prepared.
  2. Dissolve 1.8 g sodium hydroxide in some water in a 100 ml volumetric flask. Add 4.0 ml hypochlorite solution and dilute with water to the mark. Store the solution refrigerated in the dark. If the solution is stored for weeks, it should be checked by titration with a sodium thiosulphate solution.
    Dilute reagent (1) and reagent (2) with water 1:4 before use. These solutions must be prepared daily.
    Standard solutions (for nitrate and ammonium):
  3. Standard solution I, 500 mg NO3/l and 200 mg NH4/l:
    Dry potassium nitrate and ammonium sulphate for 1 hour at 105 °C, and then cool for 20 minutes in a desiccator. Dissolve exactly 0.815 g potassium nitrate and exactly 0.735 g ammonium sulphate in water in a 1000 ml volu­metric flask. Dilute to the mark with water. Store the solution refrigerated in the dark.
  4. Standard solution II, 5.0 mg NO3/l and 2.0 mg NH4/l:
    Dilute 5 ml of standard solution I (3) to 500 ml with water in a volumetric flask.

4.4.2.6  Calibration and analytical procedure

Prepare a series of calibration solutions according to Table 4.4.1.


Table 4.4.1: Calibration solutions for ammonium and nitrate.

Calibration solution No.

mg NO3/l

mg NH4/l

 

1

5.0

2.0

Standard solution II

2

2.5

1.0

Dilute 100 ml of standard solution II to 200 ml with water

3

0.5

0.2

Dilute 20 ml of standard solution II to 200 ml with water

4

0.25

0.10

Dilute 10 ml of standard solution II to 200 ml with water

5

0.125

0.05

Dilute 5 ml of standard solution II
to 200 ml with water

6

0.0

0.0

Water

 

These solutions may be stored in the refrigerator for a few days.

Start the pump and check the flow, all connections, tubings and debubblers with water running through the instrument. Turn on the photometers and the recorders (paper speed 10 mm/min.). Connect the tubings to the reagents and check that the baseline is stable.

Avoid air in the column containing the reducing agent. Therefore, do not connect the column to the pump before the apparatus is filled with liquids.

Fill the cups of the automatic sampler with samples and standard solutions. Sampling time is 90 seconds and rinsing time with water after each sample is 105 seconds. Start with the calibration solutions and run the calibration solution no. 1, 3, 5 and 6 between every tenth sample.

After analyses, run water through the system until all reagents are rinsed out. Turn off the recorder, photometer, sampler and pump, and loosen the tubings in the pump so they are not stretched.

Prepare a calibration curve by plotting the absorbances at 630 nm of each of the standard solutions against its concentration of ammonium.

4.4.2.7  Expression of results

Convert the recorder response (absorbance) of the sample to mg N/l by means of the calibration curves obtained just before or after the sample.

4.4.2.8  Interferences

Iron (III) ions may interfere with the determination of ammonium if the concentration is higher than 2 mg/l. This does not often occur in precipitation samples.

4.4.2.9  References

Harwood, J.E. and Huysen, D.J. (1970) Automated analysis of ammonia in water. Water Res., 4, 695-704.


Last revision: November 2001