4.3  Determination of nitrate in precipitation

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.

4.3.1  The manual spectrophotometric Griess method

4.3.1.1  Field of application

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).

4.3.1.2  Principle

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.

4.3.1.3  Instrumentation

4.3.1.4  Chemicals

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

4.3.1.5  Reagents

  1. 5% ammonium chloride solution:
    Dissolve 5 g ammonium chloride in water in a 100 ml volumetric flask. Adjust the pH of the solution to 8.6 using diluted ammonia. Dilute with water to the mark.
  2. 1.2 M hydrochloric acid:
    Dilute 10 ml concentrated hydrochloric acid to 100 ml with water in a volumetric flask.
  3. 2 M hydrochloric acid:
    Dilute 16.7 ml concentrated hydrochloric acid to 100 ml with water in a volumetric flask.
  4. 1% sulphanilamide solution:
    Dissolve 1.0 g of sulphanilamide in some 1.2 M hydrochloric acid (2) in a 100 ml volumetric flask. Dilute with 1.2 M hydrochloric acid (2) to the mark.
  5. 0.1% N-(1-naphthyl)-ethylenediamine dihydrochloride solution:
    Dissolve 0.1 g N-(1-naphthyl)-ethylenediamine dihydrochloride in some water, in a 100 ml volumetric flask. When all is dissolved, dilute to the mark.
  6. 2% copper sulphate solution:
    Dissolve 2.0 g copper sulphate in water in a 100 ml volumetric flask, and dilute to the mark.
  7. Reducing agent for nitrate:
    Transfer 10 g of cadmium to a beaker, add 2 M hydrochloric acid (3) to cover the cadmium and stir. Rinse well with water. Add immediately 100 ml of the 2% copper  sulphate solution (6), and mix well. Pour off excess of solution. Rinse with water until there is no more precipitated copper in the washing water. The reducing agent must not be exposed to the air.
  8. Standard nitrate solution I, 1000 mg NO3/l:
    Dissolve exactly 1.6305 g potassium nitrate in water in a 1000 ml volumetric flask. Dilute to the mark.
  9. Standard nitrate solution II, 100 mg NO3/l:
    Dilute 10 ml of standard nitrate solution I with water to 100 ml in a volumetric flask.

4.3.1.6  Calibration

Preparation of the calibration curve:

  1. Transfer to 100 ml volumetric flasks 0, 100, 250, 500 and 1000 µl of standard nitrate solution II. Dilute with water to the mark and mix well. The concentration of         nitrate in the five flasks are 0.0, 0.1, 0.25, 0.50 and 1.00 mg NO3/l.
  2. By means of a pipette, transfer 4.0 ml of each of these standard solutions to a 25 ml Erlenmeyer flask. Add 6.0 ml 5% ammonium chloride solution using a pipette, and approximately 0.5 g of the nitrate reducing agent to the Erlenmeyer flask. Shake vigorously for 10 minutes. Transfer 8.0 ml of this solution by means of a pipette  to a test tube. Add 2.0 ml 1% sulphanilamide solution and 2.0 ml of 0.1% N-(1-naphthyl)-ethylenediamide dihydro­chloride solution using pipettes. Mix well, and leave for 10 minutes for the colour to develop. Transfer this solution to a 20 mm cell. Measure the absorbance of the solution at 520 mn.

Prepare a calibration curve by plotting the absorbance of each of the standard solutions against its concentration of nitrate.

4.3.1.7  Analytical procedure

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.

4.3.1.8  References

Morris, A.W. and Riley, J.P. (1963) The determination of nitrate in sea water. Anal. chem. Acta, 29, 272-279.


4.3.2  Automatic spectrophotometric Griess method

4.3.2.1  Field of application

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.

 

4.3.2.2  Principle

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.

4.3.2.3  Instrumentation

4.3.2.4  Chemicals

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

4.3.2.5  Reagents

Reagents for the determination of nitrate:

  1. Buffer solution:
    Dissolve 100 g ammonium chloride in ca. 700 ml water in a 1000 ml volumetric flask. Adjust pH to 8.6 with diluted ammonia. Dilute with water to the mark.
  2. Sulphanilamide solution:
    Dissolve 10 g sulphanilamide in a 10% HCl solution in a 1000 ml volumetric flask. Dilute with the acid solution to the mark.
  3. 0.1% N-(1-naphthyl)-ethylenediamine dihydrochloride solution:
    Dissolve 1 g N-(1-naphthyl)-ethylenediamine in some water in a 1000 ml volumetric flask. When all is dissolved, dilute to the mark with water.
  4. Reducing agent:
    Prepare a 2% copper sulphate solution. Wash the cadmium with 0.1 M hydrochloric acid and water. Add some of the 2% copper sulphate solution to the freshly washed cadmium and stir for 1–2 min. Pour off the solution and wash the reducing agent with water. Repeat until the washing water is clear. Fill the column as shown in Figure 1b. The reducing agent must not be exposed to air.
    The reducing agent may be recovered in the following way:
    Remove the cadmium from the column and wash with 0.1 M hydrochloric acid. Add some of the 2% copper sulphate solution and stir until the blue colour of the solution has disappeared.
  5. Standard solutions (for nitrate and ammonium):
    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.
  6. Standard solution II, 5.0 mg NO3/l and 2.0 mg NH4/l:
    Dilute 5 ml of standard solution I (5) to 500 ml with water in a volumetric flask.

4.3.2.6  Calibration and analytical procedure

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).

4.3.2.7  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.

4.3.2.8  Interferences

Nitrite will interfere with the determination of nitrate.

4.3.2.9  References

Henriksen, A. and Selmer-Olsen, A.R. (1970) Automatic methods for determining nitrite in water and soil extracts. Analyst, 95, 514-518.


Last revision: November 2001