SOP for ESTIMATION OF NITRITE NITROGEN

PRINCIPLE:

Nitrite Nitrogen is determined through formation of a reddish purple azo-dye produced at              pH 2 – 2.5 by coupling diazotized sulphanalic acid with NED dihydrochloride.

MINIMUM DETECTION LIMIT: 0.01 mg/l

REAGENTS:

1.    Sulphanilamide reagent: Dissolve 5gm of the material in a mixture of 50ml of conc. hydrochloric acid & 300ml of water. Dilute to 500ml with water. The reagent is stable for several months.

2.    Sodium oxalate (0.05N): Dissolve 3.35 g Sodium oxalate, primary standard grade in water and dilute to 1000ml.

3.    Ferrous Ammonium Sulphate (0.05N): 19.607 g FAS plus 20 ml Conc. Sulfuric acid dilute to 1000 ml

4.    Stock nitrite solution: Dissolve 1.232 g Sodium nitrite in 1000 ml (250 ppm). Preserve it with 1 ml of chloroform.

5.    Standardization of stock nitrite solution:

Pipette out 50 ml standard (0.05M) KmnO4, 5 ml Conc. Sulfuric acid and 50 ml stock nitrite solution into a glass stopper flask or bottle. Submerge pipette tip well below permanganate acid solution while adding stock nitrite solution. Shake gently. Discharge permanganate colour by adding sufficient 10 ml portion of standard 0.05M FAS. Wait for 5 min.

Titrate excess FAS with 0.05 M KmnO₄ to the faint pink end point. Carry a water blank through the entire procedure and make the necessary corrections in the final calculation as shown in the equation below.

Calculate nitrite content of stock solution

            [ (B x C) – (D x E)] x 7

 A =  ----------------------------------------

                                               F

           

Where,

                        A = mg nitrite nitrogen /L in stock solution

                        B = Total ml of standard KMnO₄ used

                        C = Normality of standard KMnO₄

                        E = Normality of standard reductant,

                        F = ml stock sodium nitrite solution taken for titration.

Each one ml potassium permanganate consume by sodium nitrite solution corresponds to 1750 g nitrite nitrogen.

           

6.    Intermediate nitrite solution: Calculate the volume G of stock nitrite solution required for the intermediate nitrite solution from G = 12.5/A. Dilute the volume G (approximately 50 ml) to 250 ml with water. (50 ppm). Prepared daily

7.    Standard nitrite solution: Dilute 10 ml intermediate nitrite solution to 1000 ml with water (0.5 ppm)

PROCEDURE:

Removal of suspended solids: If sample contains suspended solids remove it by passing sample through 0.45 mm membrane filter.

Colour development: If sample pH is not between 5 – 9, adjust to that range with 1 N HCl or NH₄OH as required. To 50 ml sample or to a portion diluted to 50 ml add 2 ml colour reagent and mix.

Photometric measurement: Bet. 10 min and 2 hr. after adding colour reagent to samples and standards, measure absorbance at 543 nm.

CALCULATION:

Prepare a standard curve by plotting absorbance of standards against nitrite nitrogen concentration. Compute the sample concentration directly from curve.

Nitrite Nitrogen (as NO2-N) /l  =  µg NO2-N (in 52 ml of Final volume)

                                                                                       ml of Sample

REFERENCE: IS : 3025   ( Part 34 ) – 1988