Ion chromatography is the preferred method for determination of nitrate, spectrophotometric determination either the manual or the automatic method can also be used and will give useful results. It should, however, be noted that the Griess method in both versions described in the following gives the sum of nitrate and nitrite.
This method is applicable to the determination of the nitrate content in precipitation with the range 0.02-0.23 mg NO3-N/l (0.1-1.0 mg NO3/l).
Nitrate is reduced to nitrite using cadmium treated with copper sulphate as a reducing agent, in presence of ammonium chloride. Thus, by this method the sum of nitrate and nitrite is determined.
Nitrite and sulphanilamide form a diazo compound which couples with N-(1-naphthyl)-ethylenediamine-dihydrochloride to form a red azo dye. The concentration in the solution is determined spectrophotometrically at 520 nm.
During analysis, use only chemicals of recognized analytical grade. The water used for dilution and rinsing must be double-distilled or deionized and distilled.
Preparation of the calibration curve:
Prepare a calibration curve by plotting the absorbance of each of the standard solutions against its concentration of nitrate.
Transfer 4.0 ml of the precipitation sample to a 25 ml Erlenmeyer flask, using a pipette. Proceed according to Section 4.3.1.6 (2).
Convert the absorbance of the sample to mg NO3/l by means of the calibration curve. The concentration may be expressed as mg N/l by multiplying with 0.226.
Samples containing more than 1 mg NO3/l must be diluted before the analysis.
Do not waste the cadmium used in the analysis. It may be regenerated and used again.
With suitable equipment this method can be made automatic. A detailed description of the automatic method is given in Section 4.3.2.
Morris, A.W. and Riley, J.P. (1963) The determination of nitrate in sea water. Anal. chem. Acta, 29, 272-279.
This method can be used to determine the concentration of
nitrate in precipitation within the range 0.03-1.13 mg NO3-N/l
(0.13-5.0 mg NO3/l).
The method can be extended to include determination of ammonium in solutions,
see Section 4.4.2 and Figure 4.3.1.
Figure 4.3.1: a) Automatic determination of nitrate and ammonium in precipitation samples. b) Reduction column for the determination of nitrate in precipitation samples.
The basis principles are the same as for the manual methods (see Section 4.3.1).
Nitrate is reduced to nitrite using cadmium treated with copper sulphate as reducing agent in the presence of ammonium chloride. Nitrite and sulphanilamide form a diazo compound which couples with N-(a-naphtyl)-ethylenediamine dihydrochloride to give a red azo dye. The concentration of the nitrate in the solution is determined spectrophotometrically at 520 nm. By this method the sum of nitrate and nitrite is determined.
All chemicals must be of recognized analytical grade. The water used for dilution and rinsing must be double-distilled or de-ionized and distilled.
Reagents for the determination of nitrate:
Prepare a series of calibration solutions according to Table
4.3.1.
Table 4.3.1: Calibration solutions for nitrate and ammonium.
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 520 nm (each of the standard solutions against its concentration of nitrate).
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.
Nitrite will interfere with the determination of nitrate.
Henriksen, A. and Selmer-Olsen, A.R. (1970) Automatic methods for determining nitrite in water and soil extracts. Analyst, 95, 514-518.