Chapter 10. The Determination of Nitrite in Sea Water 
1.0 Scope and field of application
This procedure describes a method for the determination of reactive nitrite in seawater.This method is suitable for the assay of oceanic concentrations between 0.01 mmol l -1 to 2.5 mmol l -1 . This method is a modification of Strickland and Parsons (1968).

2.0 Definition
The reactive nitrite concentration is given in units of mmol kg -1 in seawater.

3.0 Principle of Analysis
The determination of nitrite is based on the method of Strickland and Parsons (1968).Nitrite reacts with sulfanilamide in an acid solution resulting in a diazonium compound.This is then coupled with N-(1-Naphthyl)-ethylenediamine dihydrochloride to form a colored azo dye, the extinction of which can be measured spectrophotometrically.

4.0 Apparatus
Spectrophotometer

5.0 Reagents
5.1 Sulfanilamide solution:
Dissolve 5 g of reagent grade sulfanilamide in a mixture of 50 ml concentrated hydrochloric acid and 300 ml of deionized water. This solution is diluted to 500 ml with deionized water, stored in a glass bottle, and is stable for many months.

5.2 N-(1-Naphthyl) - ethylenediamine dihydrochloride solution:
Dissolve 0.50 g of the dihydrochloride in 500 ml of deionized water. Store in a dark bottle and replace monthly or immediately if a brown coloration develops.

6.0 Sampling
6.1 Samples are collected on deeper casts after the oxygen, CO2 and salinity samples have been drawn. Shallow samples (upper 250 m) are collected on the gases cast after oxygen, CO2 , DOC and salinity samples are drawn. An in-line filter (0.8 mm Nuclepore filter) is connected to the Niskin bottle. The spigot is opened and three sets of samples are collected from the water that passes through the filter by gravity
filtration. Each bottle is rinsed three times and then filled just below the shoulder. Care must be taken to avoid overfilling of samples to be frozen. These bottles are transferred to a freezer (- 20°C) and kept frozen until analysed.

6.2 Contamination is a major problem with nutrient samples, especially in the upper ocean where the ambient concentrations are low. All the nutrient bottles are rigorously cleaned before use. The cleaning begins by a wash with a detergent (Aquet) followed by a rinse with 10% HCl, three rinses with deionized water and a final rinse with de-ionized water.

6.3 Prolonged storage of samples is not advisable, even if frozen. Sufficient water should be sampled for washing purposes.

7.0 Procedures
7.1 Sample analysis

7.1.1 Samples should be at a temperature between 15° C and 30° C.

7.1.2 The 125 ml Erlenmeyer flasks and 50 ml measuring cylinder to be used in this analysis should be rinsed twice with the sample seawater and shaken dry.

7.1.3 Measure 50 ml of the sample into a 125 ml Erlenmeyer flask.

7.1.4 Add 1.0 ml of the sulfanilamide solution to each flask, mix, and permit the reagent to react for between 2 and 8 minutes.

7.1.5 Add 1.0 ml of the N-(1-Naphthyl) - ethylenediamine dihydrochloride solution and mix immediately.

7.1.6 Measure the extinction of the samples at 543 nm between 10 minutes and 2 hours after the addition of the  naphthylethylenediamine reagent. Extinctions less than 0.1 in a 1 cm cell should be reread in a 10 cm cell.

7.2 Reagent blank determination

7.2.1 The reagent blank is determined using de-ionized water as sample instead of seawater, exactly following the procedure outline in section 7.1. This should be done in duplicate.

7.2.2 A reagent blank should not exceed 0.03 and should be determined for each batch of samples.

7.3 Standardization

7.3.1 Primary nitrite standard: Dissolve 0.345 g of dried anhydrous reagent grade sodium nitrite in 1000 ml of deionized water. 1 ml = 5 mmol N. This solution is stored in a dark bottle with 1 ml of chloroform as a preservative and is stable for 1-2 months.

7.3.2 Working nitrite standard: Dilute 10.0 ml of the primary standard solution to 1000 ml with deionized water (1 ml = 0.05 mmol).

7.3.3 Prepare four standard solutions by diluting 2.0 ml of working nitrite standard up to 50 ml in de-ionized water. Carry out the nitrite determination to each standard as described above in section 7.2.

8.0 Calculation and expression of results
8.1 A standardization factor F can be calculated as:

        20 mmol/kg = concentration of the standard
        E s = mean absorbance of the standards
        E b = mean absorbance of the blanks
 

8.2 The nitrite concentration is calculated by:
        mM NO2 = corrected absorbance *  F
where:
        F = standardization factor
        corrected absorbance = sample absorbance - reagent blank

9.0 References
Strickland, J.D.H., and Parsons, T.R. (1968). Determination of reactive nitrite. In: A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Bulletin 167, 71–75.