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4500-NH3 NITROGEN (AMMONIA)*

* Approved by Standard Methods Committee, 1997.

 

4500-NH3 F. Phenate Method

1. General Discussion

 

    a. Principle: An intensely blue compound, indophenol, is formed by the reaction of ammonia, hypochlorite, and phenol catalyzed by sodium nitroprusside.

    b. Interferences: Complexing magnesium and calcium with citrate eliminates interference produced by precipitation of these ions at high pH. There is no interference from other trivalent forms of nitrogen. Remove interfering turbidity by distillation or filtration. If hydrogen sulfide is present, remove by acidifying samples to pH 3 with dilute HCI and aerating vigorously until sulfide odor no longer can be detected.

 

2. Apparatus

 

    Spectrophotometer for use at 640 nm with a light path of 1 cm or greater.

 

3. Reagents

 

    a. Phenol solution: Mix 11.1 mL liquified phenol (>89%) with 95% v/v ethyl alcohol to a final volume of 100 mL. Prepare weekly. CAUTION: Wear gloves and eye protection when handling phenol; use good ventilation to minimize all personnel exposure to this toxic volatile substance.

    b. Sodium nitroprusside, 0.5% w/v: Dissolve 0.5 g sodium nitroprusside in 100 mL deionized water. Store in amber bottle for up to 1 month.

    c. Alkaline citrate: Dissolve 200 g trisodium citrate and 10 g sodium hydroxide in deionized water. Dilute to 1000 mL.

    d. Sodium hypochlorite, commercial solution, about 5%. This solution slowly decomposes once the seal on the bottle cap is broken. Replace about every 2 months.

    e. Oxidizing solution: Mix 100 mL alkaline citrate solution with 25 mL sodium hypochlorite. Prepare fresh daily.

    f. Stock ammonium solution: See Section 4500-NH3.D.3d.

    g. Standard ammonium solution: Use stock ammonium solution and water to prepare a calibration curve in a range appropriate for the concentrations of the samples.

 

4. Procedure

 

    To a 25-mL sample in a 50-mL erlenmeyer flask, add, with thorough mixing after each addition, 1 mL phenol solution, 1 mL sodium nitroprusside solution, and 2.5 mL oxidizing solution. Cover samples with plastic wrap or paraffin wrapper film. Let color develop at room temperature (22 to 27oC) in subdued light for at least 1 h. Color is stable for 24 h. Measure absorbance at 640 nm. Prepare a blank and at least two other standards by diluting stock ammonia solution into the sample concentration range. Treat standards the same as samples.

 

5. Calculations

 

    Prepare a standard curve by plotting absorbance readings of standards against ammonia concentrations of standards. Compute sample concentration by comparing sample absorbance with the standard curve.

 

6. Precision and Bias

 

    For the manual phenate method, reagent water solutions of ammonium sulfate were prepared and analyzed by two analysts in each of three laboratories. Results are summarized in Table 4500-NH3:III.

 

                        TABLE 4500-NH3:III. PRECISION DATA FOR MANUAL PHENATE METHOD

                                       BASED ON TRIPLICATE ANALYSES OF AMMONIUM SULFATE                      

Relative
NH3-N Standard
Lab/ Concentration Optical     Deviation
Analyst mg/L Density    %

1/1

 0.1

0.129

 1.55

1/2

 0.1

0.114

 9.66

2/1

 0.1

0.100

10.2   

2/2

 0.1

0.122

 2.36

3/1

 0.1

0.112

 3.61

3/2

 0.1

0.107

 1.94

1/1

 0.3

0.393

 0.39

1/2

 0.3

0.364

 0.32

2/1

 0.3

0.372

 2.64

2/2

 0.3

0.339

 0.90

3/1

 0.3

0.370

 0.31

3/2

 0.3

0.373

 0.46

1/1

 0.5

0.637

 0.77

1/2

 0.5

0.630

 0.56

2/1

 0.5

0.624

 1.65

2/2

 0.5

0.618

 0.86

3/1

 0.5

0.561

 0.27

3/2

 0.5

0.569

   0.91  

 

 

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7. Bibliography

 

SOLORZANO, L. 1969. Determination of ammonia in natural waters by the phenolhypochlorite method. Limnol. Oceanogr. 14:799.

 

PARSONS, T.R., Y. MAITA & C.M. LALLI. 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Elmsford, N.Y.

 

         

Standard Methods for the Examination of Water and Wastewater. 20th Ed. American Public Health Association, American Water Works Association, Water Environment Federation.