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
