4500-NO2

 NITROGEN (NITRITE)*

4500-NO2

 A. Introduction

1. Occurrence and Significance

For a discussion of the chemical characteristics, sources, and

effects of nitrite nitrogen, see Section 4500-N.

2. Selection of Method

The colorimetric method (B) is suitable for concentrations of

5 to 1000 g NO2

-N/L (See ¶ B.1a). Nitrite values can be

obtained by the automated method given in Section 4500-

NO3

.E with the Cu-Cd reduction step omitted. Additionally,

nitrite nitrogen can be determined by ion chromatography (Section 4110), and by flow injection analysis (see Sections 4130 and

4500-NO3

 .I).

4500-NO2

 B. Colorimetric Method

1. General Discussion

a. Principle: Nitrite (NO2

) is determined through formation

of a reddish purple azo dye produced at pH 2.0 to 2.5 by

coupling diazotized sulfanilamide with N-(1-naphthyl)-ethylenediamine dihydrochloride (NED dihydrochloride). The applicable range of the method for spectrophotometric measurements

is 10 to 1000 g NO2

-N/L. Photometric measurements can be

made in the range 5 to 50 g N/L if a 5-cm light path and a green

color filter are used. The color system obeys Beer’s law up to

180 g N/ L with a 1-cm light path at 543 nm. Higher NO2



concentrations can be determined by diluting a sample.

b. Interferences: Chemical incompatibility makes it unlikely

that NO2

, free chlorine, and nitrogen trichloride (NCl3) will

coexist. NCl3 imparts a false red color when color reagent is

added. The following ions interfere because of precipitation

under test conditions and should be absent: Sb3, Au3, Bi3,

Fe3, Pb2, Hg2, Ag, chloroplatinate (PtCl6

2), and metavanadate (VO3

2). Cupric ion may cause low results by catalyzing decomposition of the diazonium salt. Colored ions that

alter the color system also should be absent. Remove suspended

solids by filtration.

c. Storage of sample: Never use acid preservation for samples

to be analyzed for NO2

. Make the determination promptly on

fresh samples to prevent bacterial conversion of NO2

 to NO3

or NH3. For short-term preservation for 1 to 2 d, freeze at 20°C

or store at 4°C.

2. Apparatus

Colorimetric equipment: One of the following is required:

a. Spectrophotometer, for use at 543 nm, providing a light

path of 1 cm or longer.

b. Filter photometer, providing a light path of 1 cm or longer

and equipped with a green filter having maximum transmittance

near 540 nm.

3. Reagents

a. Nitrite-free water: If it is not known that the distilled or

demineralized water is free from NO2

, use either of the following procedures to prepare nitrite-free water:

1) Add to 1 L distilled water one small crystal each of KMnO4

and either Ba(OH)2 or Ca(OH)2. Redistill in an all-borosilicateglass apparatus and discard the initial 50 mL of distillate. Collect

the distillate fraction that is free of permanganate; a red color

with DPD reagent (Section 4500-Cl.F.2b) indicates the presence

of permanganate.

2) Add 1 mL conc H2SO4 and 0.2 mL MnSO4 solution (36.4

g MnSO4  H2O/100 mL distilled water) to each 1 L distilled

water, and make pink with 1 to 3 mL KMnO4 solution (400 mg

KMnO4/L distilled water). Redistill as described in the preceding

paragraph.

Use nitrite-free water in making all reagents and dilutions.

b. 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 L with water.

Solution is stable for about a month when stored in a dark bottle

in refrigerator.

c. Sodium oxalate, 0.025M (0.05N): Dissolve 3.350 g

Na2C2O4, primary standard grade, in water and dilute to 1000

mL.

d. Ferrous ammonium sulfate, 0.05M (0.05N): Dissolve

19.607 g Fe(NH4)2 (SO4)2  6H2O plus 20 mL conc H2SO4 in

water and dilute to 1000 mL. Standardize as in Section

5220B.3d.

e. Stock nitrite solution: Commercial reagent-grade NaNO2

assays at less than 99%. Because NO2

 is oxidized readily in the

presence of moisture, use a fresh bottle of reagent for preparing

the stock solution and keep bottles tightly stoppered against the

free access of air when not in use. To determine NaNO2 content,

add a known excess of standard 0.05N KMnO4 solution (see ¶ h

below), discharge permanganate color with a known quantity of

* Approved by Standard Methods Committee, 2000.

4-118 INORGANIC NONMETALS (4000)
standard reductant such as 0.025M Na2C2O4 or 0.05M

Fe(NH4)2(SO4)2, and back-titrate with standard permanganate

solution.

1) Preparation of stock solution—Dissolve 1.232 g NaNO2 in

water and dilute to 1000 mL; 1.00 mL 250 g N. Preserve

with 1 mL CHCl3.

2) Standardization of stock nitrite solution—Pipet, in order,

50.00 mL standard 0.05N KMnO4, 5 mL conc H2SO4, and 50.00

mL stock NO2

 solution into a glass-stoppered flask or bottle.

Submerge pipet tip well below surface of permanganate-acid

solution while adding stock NO2

 solution. Shake gently and

warm to 70 to 80°C on a hot plate. Discharge permanganate

color by adding sufficient 10-mL portions of standard 0.025M

Na2C2O4. Titrate excess Na2C2O4 with 0.05N 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.

If standard 0.05M ferrous ammonium sulfate solution is substituted for Na2C2O4, omit heating and extend reaction period

between KMnO4 and Fe2 to 5 min before making final KMnO4

titration.

Calculate NO2

-N content of stock solution by the following

equation:

A [(B  C)  (D  E)]  7

F

where:

A mg NO2

-N/mL in stock NaNO2 solution,

B total mL standard KMnO4 used,

C normality of standard KMnO4,

D total mL standard reductant added,

E normality of standard reductant, and

F mL stock NaNO2 solution taken for titration.

Each 1.00 mL 0.05N KMnO4 consumed by the NaNO2 solution corresponds to 1725 g NaNO2 or 350 g NO2

-N.

f. Intermediate nitrite solution: Calculate the volume, G, of

stock NO2

 solution required for the intermediate NO2 solution

from G 12.5/A. Dilute the volume G (approximately 50 mL)

to 250 mL with water; 1.00 mL 50.0 g N. Prepare daily.

g. Standard nitrite solution: Dilute 10.00 mL intermediate

NO2

 solution to 1000 mL with water; 1.00 mL 0.500 g N.

Prepare daily.

h. Standard potassium permanganate titrant, 0.05N: Dissolve

1.6 g KMnO4 in 1 L distilled water. Keep in a brown glassstoppered bottle and age for at least 1 week. Carefully decant or

pipet supernate without stirring up any sediment. Standardize

this solution frequently by the following procedure:

Weigh to the nearest 0.1 mg several 100- to 200-mg samples

of anhydrous Na2C2O4 into 400-mL beakers. To each beaker, in

turn, add 100 mL distilled water and stir to dissolve. Add 10 mL

1  1 H2SO4 and heat rapidly to 90 to 95°C. Titrate rapidly with

permanganate solution to be standardized, while stirring, to a

slight pink end-point color that persists for at least 1 min. Do not

let temperature fall below 85°C. If necessary, warm beaker

contents during titration; 100 mg will consume about 6 mL

solution. Run a blank on distilled water and H2SO4.

Normality of KMnO4 g Na2C2O4

(A  B)  0.067

where:

A mL titrant for sample and

B mL titrant for blank.

Average the results of several titrations.

4. Procedure

a. Removal of suspended solids: If sample contains suspended

solids, filter through a 0.45-m-pore-diam membrane filter.

b. Color development: If sample pH is not between 5 and 9,

adjust to that range with 1N HCl or NH4OH as required. To 50.0

mL sample, or to a portion diluted to 50.0 mL, add 2 mL color

reagent and mix.

c. Photometric measurement: Between 10 min and 2 h after

adding color reagent to samples and standards, measure absorbance at 543 nm. As a guide use the following light paths for the

indicated NO2

-N concentrations:

Light Path Length

cm

NO2

-N

g/L

1 2–25

5 2–6

10 2

5. Calculation

Prepare a standard curve by plotting absorbance of standards

against NO2

-N concentration. Compute sample concentration

directly from curve.

6. Precision and Bias

In a single laboratory using wastewater samples at concentrations of 0.04, 0.24, 0.55, and 1.04 mg NO

3  NO2

-N/L, the

standard deviations were 0.005,  0.004,  0.005, and 

0.01, respectively. In a single laboratory using wastewater samples at concentrations of 0.24, 0.55, and 1.05 mg NO3

 

NO2

-N/L, the recoveries were 100%, 102%, and 100%, respectively.1

7. Reference

1. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1979. Methods for Chemical Analysis of Water and Wastes. Method 353. 3. U.S. Environmental Protection Agency, Washington, D.C.

8. Bibliography

BOLTZ, D.F., ed. 1958. Colorimetric Determination of Nonmetals. Interscience Publishers, New York, N.Y.

NYDAHL, F. 1976. On the optimum conditions for the reduction of nitrate

by cadmium. Talanta 23:349.

NITROGEN (NITRITE) (4500-NO2

)/Colorimetric Method 4-119