SOP for Determination of Nitrite Nitrogen

Nitrite Nitrogen in Water

Nitrite nitrogen is a chemical parameter that refers to the amount of nitrite ions present in water. Nitrite is a compound that can form when nitrogen compounds, such as ammonia, are broken down by bacteria in the water. It can also be a byproduct of certain industrial processes. Nitrite nitrogen levels in water are measured in parts per million (ppm) or milligrams per liter (mg/L). Elevated levels of nitrite nitrogen in water can be harmful to aquatic life and indicate the presence of other contaminants or pollution. Nitrite nitrogen is also a concern for human health, as it can react with hemoglobin in the blood, reducing the amount of oxygen that can be transported to the body’s tissues. Therefore, regular testing and monitoring of nitrite nitrogen levels in water sources are essential for ensuring safe and healthy drinking water.

Scope

This SOP is applicable for the determination of nitrite nitrogen of the given sample of water.

Principle

The nitrite concentration is determined through the formation of a reddish-purple azo dye produced at pH 2.0–2.5 by the coupling of diazotized sulphanilic acid with N-(1-naphthyl)-ethylenediamine dihydrochloride. The colour obeys Beers’ law up to 180 µg/l with 1 cm path length at 543 nm.

Interference

Nitrogen trichloride (NCI₃) imparts a false red colour when normal order of reagents addition is followed. It can be minimized by adding NED dihydrochloride first and then sulphanalic acid. Ions like Sb³⁺, Au³⁺, Fe³⁺, Bi³⁺, Pb²⁺, Hg²⁺ Ag⁺, PtCl₆^2- interfere. Cupric ions cause low results.

Reagents

Sulphanilamide reagent

Dissolve 5 g sulphanilamide in a mixture of 50 mL conc. HCl and about 300 mL distilled water. Dilute to 500 mL with distilled water.

N-(1-naphthyl)-ethylenediamine dihydrochloride solution 

Dissolve 500 mg dihydrochloride in 500 mL distilled water. Store in a dark bottle.

OR

Color Reagent:

To 800 ml water add 100 ml 85% phosphoric acid and 10 gm sulphanilamide. After dissolving sulphanilamide completely, ad 1 gm N-(1-naphthyl-ethylenediaminedihydrochloride. Mix to dissolve. Then dilute to 1 Lit with water. Solution is stable a month when stored in dark bottle in refrigerator.

Hydrochloric acid: HCl (1+3)

Sodium Oxalate

0.05N Dissolve 3.350 g of sodium oxalate (primary standard grade ) in 1 000 ml of water.

Ferrous Ammonium sulphate

Dissolve 19.607 g of ferrous ammonium sulphate in 20 ml of concentrated sulphuric acid and water and dilute to 1 litre. Standardize with standard dichromate.

Stock nitrite solution

Dissolve 1.232 g NaNO₂ in nitrite free water and dilute to 1 litre. Fresh nitrite from bottle should be taken 1 mL = 250 mg N in the solution. Preserve with 1 ml of chloroform Standardize using sodium oxalate and standard potassium permanganate solution.

Standard nitrite solution

Standardise stock solution. Pipette 50 ml standard 0.05 N KMnO₄, 5 mL conc.H₂SO₄ and 50 mL stock nitrite solution in a glass stoppered flask. Discharge the permanganate colour by ferrous ammonium sulphate solution of 0.05N strength. Carry nitrite free blank through the entire procedure and make necessary corrections. Calculate the nitrite N content of stock solution by the following equation:

A = [(B x C) – (D x E)] x 7/F

Where,

A = mg/mL nitrite N in stock solution,

B = total mL standard KMnO₄ used,

C = normality of KMnO₄ solution,

D = total mL of standard Fe(NH₄)₂(SO₄)₂ used,

E = normality of standard Fe(NH₄)₂(SO₄)₂,

F = mL of stock NaNO₂ solution taken for titration.

Each 1 mL of 0.05 N KMnO₄ consumed by the nitrite corresponds to 1.729 µg NaNO₂ or 350 µg N

Intermediate nitrite solution

Calculate the volume, G, of stock nitrite solution required for intermediate nitrite solution from

G = 12.5/A,

Where A is the stock solution in mg/l.

Dilute the volume G to 250 ml with water (1.00 ml = 50.0 µg N).

Standard nitrite solution:

Dilute 10 ml intermediate nitrite solution to 1000 ml with water; (1 ml = 0.500 µg N)

Calibration Curve:

Make a calibration curve by taking Standard nitrite solution as per following.

Sr. No	ml of Std. nitrite stock (ml)	Concentration in ppm
1	0	0
2	2	0.001
3	4	0.002
4	6	0.003
5	8	0.004
6	10	0.005
7	12	0.006
8	14	0.007
9	16	0.008
10	18	0.009
11	20	0.01

OR

For Higher Range.

Procedure

• If sample contains suspended solid, filter through 0.45µm pore diameter membrane filter.
• Take 50 ml clear sample or to a portion diluted to 50 ml and adjust the pH 5-9, add 2 ml color reagent.
• Measure the absorbance of the solution after 10 minute to 2 h at 543 nm at 1 cm light path.
• By noting the absorbance of an unknown sample, the concentration of nitrate can be determined from the graph in spectrophotometer.

Calculation

Nitrite (mg/l) = µg NO2 N(in 52 ml of Final Volume)/ Volume of Sample

Reference

IS:3025 (Part 34) 1988