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.
This method is applicable to the determination of the ammonium content in precipitation within the range 0.04 to 2.0 mg NH4/l.
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.
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.
Preparation of calibration curve:
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.
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.
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.
This method can be used to determine the concentration of ammonium within the range 0.05 to 2.0 mg NH4 /l.
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.
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.
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)
Reagents for the determination of ammonium:
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 |
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.
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.
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.
Harwood, J.E. and Huysen, D.J. (1970) Automated analysis of ammonia in water. Water Res., 4, 695-704.