Method for Ammonia Measurement in Ambient Air (Indophenol Blue Method)

Principle:

  NH₃ in air is collected by scrubbing a known volume of air in a  dilute solution of  sulphuric acid. The  ammonium sulphate formed in the sample is analyzed colorimtrically by reaction with phenol and alkaline sod.hypochlorite to produce indophenol. The reaction is accelerated by the addition of sodium nitropruside as catalyst.

Range & Sensitivity:

  The range of the method is 20 – 700ug/m3. The analytical range is 0.2ug/ml

Interferences 

  interfering particulate matter in the air can be removed by filtration of the air sample.

Preparation of Reagent

A. For Sampling:

  Absorbing Reagent: 0.1N Sulphuric Acid

  Caution: Sulphuric Acid is highly toxic ;avoid contact with skin and especially with eyes. Wash hands after handling it.

B. For Analysis:

  1. Sodium Nitropruside Solution: Dissolve 2gm Sodium Nitropruside  in 100ml distilled water.

  2. Buffer Solution: Dissolve 50 gm of Sodium phosphate and 74ml of 6.75 M NaOH in 1 litre of distilled water .

  3. Sodium Hypochlorite Solution:  Mix 30ml of 0.1N sodium hypochlorite and 30 ml of 6.75 M Sodium hydroxide and dilute to 100 ml with distilled water.

4. Phenol Solution : Dilute 20ml of 45% phenol  solution with 1ml of 2% Sodium Nitropruside Solution  and  dilute to 100ml with distilled water.

C. For Calibration Curve:

  1. Ammonia Stock Solution :1000 µg NH₃/ml

  2. Standard Ammonia Solution :10 µg NH₃/ml(Caution: Prepare fresh)

Sampling:

1. Place 10 ml of absorbing reagent in an impinger

2. Sample for desired time at flow rate 0.5 – 2.0 lpm

3. After Sampling measure the volume of absorbing reagent

4. Makeup the evaporating absorbing reagent with distilled water as initial volume

5. Transfer sampled absorbing reagent in sample storage bottle

6. Go to lab for analysis

Analysis (NH₃)

1. Mix thoroughly the sample of bottles by shaking several times.

2. Take two 25 ml  glass stopper graduated cylinder. (blank and sample)

3. Pipette 10 ml unexposed absorbing reagent in Blank   and 10 ml exposed absorbing reagent (Sampled solution) in glass stopper graduated cylinder.

4. Add 2 ml Buffer solution .

5. Add  5 ml of working phenol solution .

6.Mix thoroughly and add 3 ml distilled water .

7. Add 2.5 ml of working hypochlorite solution and  Mix thoroughly .

8. Dilute to 25ml with distilled water ,mix and store in the dark for 30 minutes to develop colour.

9. Measure the absorbance at 630 nm  against the field blank reagent.