DETERMINATION OF NITRITE-N IN WATER
APPARATUS:
Spectrophotometer for use at 543 nm, providing a light path of I cm or longer
REAGENTS:
Nitrite free water: Use nitrite free water in making all reagents and dilutions
Color reagent: To 800 ml water, add 100 ml 85 % phosphoric acid and 10 g
sulfanilamide. After dissolving sulfanilamide completely, add 1 g N-(1-naphthyl)ethylenediamine dihydrochloride. Mix to dissolve, then dilute to 1 liter with water.
Solution is stable for about a month when stored in a dark bottle in refrigerator.
Sodium oxalate, 0.025M (0.05N):
Dissolve 3.350 g Na2C2O4, primary standard grade, in water and dilute to 1000 ml.
Ferrous ammonium sulfate, 0.05M (0.05N):
Dissolve 19.607 g Fe(NH4)2 (SO4)2 .6H2O plus 20 ml of conc. H2SO4 in water and
dilute to 1000 ml.
Stock nitrite solution:
Dissolve 1.232 g NaNO2 in water and dilute to 1000 ml; 1.00 ml = 250 µg N.
Preserve with 1 ml CHCl3 .
Intermediate nitrite solution:
Prepare an intermediate nitrite solution having 50 ppm concentration of NO2-N from
stock nitrite solution.
Standard nitrite solution:
Dilute 10.0 ml of the intermediate nitrite solution (50 ppm) to 1000 ml with water;
I ml = 0.50 µg N = 0.50 ppm = 50ppb
Standard potassium permanganate titrant, 0.01 M (0.05 N):
Dissolve 1.6 g KMNO4 in 1 liter distilled water. Keep in a brown glass-stoppered
bottle and age for at least one week. Standardize this solution frequently.
PROCEDURE:
If sample contains suspended solids, filter through a 0.45 µm pore diameter
membrane filter. If sample pH is not between 5 and 9, adjust to that range with 1N
HCl or NH4OH as required. To 25 ml sample, or to a portion diluted to 25 ml, add 2
ml color reagent and mix.
Between 10 min. and 2 hr after adding color reagent, measure absorbance at 543 nm.
Preparation of calibration standards from working standard of 50 ppb
Standard Solutions
Volume (ml) used of the
working standard, diluted
to 25 ml.
Standard Conc.
( ppb NO2-N )
ZERO
5.0
10.0
25.0
BLANK
10
20
50
Calculation: Prepare a standard curve by plotting absorbance of standards against
NO2-N concentration. Compute sample concentration directly from curve.
DETERMINATION OF NITRATE-N IN WATER
APPARATUS:
pH meter; Nitrate combination electrode; Stirrer; 120 ml beakers
REAGENTS:
Stock Nitrate Solution:
Dry potassium nitrate (KNO3) in an oven at 1050 C for 24 hours. Dissolve 0.7218 g
in water and dilute to 1000 ml. This makes 100 mg/l NO3-N
Intermediate Nitrate Solution:
Dissolve 10 ml of 1000 ppm NO3-N solution to 100 ml; 1 ml = 100 µg/ml = 100 ppm
Buffer Solution
Dissolve 17.32 g Al2(SO4)3.18H2O, 3.43 g Ag2SO4, 1.28 g H3BO4, and 2.52 g
sulfamic acid in about 800 ml water. Adjust to pH 3.0 by slowly adding 0.01 N
NaOH. Dilute to 1000 ml and store in a dark glass bottle.
Sodium Hydroxide Solution NaOH 0.1 N
Reference Electrode filling solution
Dissolve 0.53 g (NH4)2SO4 in water and dilute to 100 ml.
PROCEDURE:
Preparation of calibration standards from 100 ppm stock NO3-N solution:
Standard Solutions
Volume (ml) used of the
working standard, diluted
to 100 ml.
Standard Conc.
( ppm NO3-N )
ZERO
0.2
0.5
1.0
5.0
BLANK
0.2
0.5
1.0
5.0
Preparation of calibration curve: Transfer 0.2, 0.5, 1.0, and 5 ml of intermediate NO3N solution to 100 ml beaker and make volume to 100 ml. Add 1 ml buffer solution,
add 0.5 g of Ag2SO4 solid and stir. Lower the electrodes and take the mV reading
when it is stable. These will give 0.2, 0.5, 1.0, and 5.0 of NO3-N respectively. Plot
potential measurements (Rel mV) against NO3-N concentrations on semilogrithmic
graph paper, with NO3-N concentrations on the logarithmic axis (abscissa) and
potential (in millivolts) on the linear axis (ordinate). A straight line with a slope of
+57 ±3 mV/decade at 250C should result.
Measurement of samples: Transfer 100 ml sample to a 100 ml beaker, add 1 ml buffer
solution and stir. Measure standards and samples at about the same temperature.
Immerse electrode tips in sample and record potential reading when stable. Read
concentration from calibration curve and multiply with dilution factor if any.
*Note. Add 0.5 g of silver sulfate to all samples and standards if the chloride
concentration is above 200 ppm.