## 4500-NO 2  

## NITROGEN (NITRITE)*

4500-NO 2  

## 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 NO 2  -N/L (See ¶ B.1 a ). Nitrite values can be obtained by the automated method given in Section 4500- NO 3  .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-NO 3  .I).



## 4500-NO 2  

## B. Colorimetric Method

1. General Discussion

a. Principle: Nitrite (NO 2  ) 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 NO 2  -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 NO 2  concentrations can be determined by diluting a sample.

b. Interferences: Chemical incompatibility makes it unlikely that NO 2  , free chlorine, and nitrogen trichloride (NCl 3 ) will coexist. NCl 3 imparts a false red color when color reagent is added. The following ions interfere because of precipitation under test conditions and should be absent: Sb 3  , Au 3  , Bi 3 

, Fe 3  , Pb 2  , Hg 2  , Ag  , chloroplatinate (PtCl 6 2

 ), and meta

vanadate (VO 3 2

 ). Cupric ion may cause low results by cata

lyzing 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 NO 2  . Make the determination promptly on fresh samples to prevent bacterial conversion of NO 2  to NO 3  or NH 3 . 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 NO 2  , use either of the fol

lowing procedures to prepare nitrite-free water:

1) Add to 1 L distilled water one small crystal each of KMnO 4 and either Ba(OH) 2 or Ca(OH) 2 . Redistill in an all-borosilicate

glass 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 H 2 SO 4 and 0.2 mL MnSO 4 solution (36.4 g MnSO 4  H 2 O/100 mL distilled water) to each 1 L distilled water, and make pink with 1 to 3 mL KMnO 4 solution (400 mg KMnO 4/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 Na2C 2 O 4 , 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 (SO 4 ) 2  6H 2 O plus 20 mL conc H 2 SO 4 in water and dilute to 1000 mL. Standardize as in Section 5220B.3 d.

e. Stock nitrite solution: Commercial reagent-grade NaNO 2 assays at less than 99%. Because NO 2  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 NaNO 2 content, add a known excess of standard 0.05N KMnO 4 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 Na2C 2 O 4 or 0.05M Fe(NH4 ) 2 (SO 4 ) 2 , and back-titrate with standard permanganate solution.

1) Preparation of stock solution—Dissolve 1.232 g NaNO 2 in water and dilute to 1000 mL; 1.00 mL 250 g N. Preserve with 1 mL CHCl 3 .

2) Standardization of stock nitrite solution—Pipet, in order, 50.00 mL standard 0.05N KMnO 4 , 5 mL conc H 2 SO 4 , and 50.00 mL stock NO 2  solution into a glass-stoppered flask or bottle. Submerge pipet tip well below surface of permanganate-acid solution while adding stock NO 2  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 Na2C 2 O 4 . Titrate excess Na2C 2 O 4 with 0.05N KMnO 4 to the faint pink end point. Carry a water blank through the entireprocedure 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 Na2C 2 O 4 , omit heating and extend reaction period between KMnO 4 and Fe 2  to 5 min before making final KMnO 4 titration.

Calculate NO 2  -N content of stock solution by the following equation:

A [(B  C )

 (D  E )]  7
F

where: A mg NO 2  -N/mL in stock NaNO 2 solution, B total mL standard KMnO 4 used, C normality of standard KMnO 4 , D total mL standard reductant added, E normality of standard reductant, and F mL stock NaNO 2 solution taken for titration.

Each 1.00 mL 0.05N KMnO 4 consumed by the NaNO 2 solu

tion corresponds to 1725 g NaNO 2 or 350 g NO 2  -N. f. Intermediate nitrite solution: Calculate the volume, G , of stock NO 2  solution required for the intermediate NO 2  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 NO 2  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 KMnO 4 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 Na2C 2 O 4 into 400-mL beakers. To each beaker, in turn, add 100 mL distilled water and stir to dissolve. Add 10 mL 1  1 H 2 SO 4 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 H 2 SO 4 . Normality of KMnO 4 g Na2 C 2 O 4 ( 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 1 N HCl or NH4 OH 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 afteradding color reagent to samples and standards, measure absorbance at 543 nm. As a guide use the following light paths for the indicated NO 2  -N concentrations:

Light Path Length

cm NO 2  -N
g/L

1 2–25 5 2–6 10  2

5. Calculation

Prepare a standard curve by plotting absorbance of standards against NO 2  -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  NO 2  -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 NO 3   NO 2  -N/L, the recoveries were 100%, 102%, and 100%, respec

tively. 1

7. Reference

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

8. Bibliography

B OLTZ , 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-NO 2  )/Colorimetric Method 4-119