This document describes a method for determining ammonia in the atmosphere using an indophenol method. Air is bubbled through a dilute sulfuric acid solution to collect ammonia as ammonium sulfate. The ammonium sulfate is then analyzed colorimetrically by reacting it with phenol and sodium hypochlorite to produce a blue indophenol dye, the concentration of which is measured spectrophotometrically. The method can detect ammonia concentrations of 20-700 μg/m3 in air samples of 1 hour or more. Various reagents, equipment, calibration procedures and calculations are described to carry out the analysis.

1. Determination of Ammonia in the Atmosphere
(Indophenol Method)
1. Principle of the Method
1.1 Ammonia in the atmosphere is collected by bubbling a measured volume
of air through a dilute solution of Sulfuric Acid to form ammonium
sulfate.
1.2 The ammonium sulfate formed in the sample is analyzed colorimetrically
by reaction with phenol and alkaline sodium hypochlorite to produce
indophenol, a blue dye. The reaction is accelerated by the addition of
sodium nitroprusside as catalyst.
2. Range and Sensitivity
2.1 With a sampling rate of 1 to 2 L/min a concentration range of 20 to
700µg/m3
(0.025 to 1 ppm) in air may be determined with a sampling time
of 1h.
2.2 The limit of detection of the analysis is 0.2 µg NH3/mL.
3. Interferences
3.1 Ammonium compounds in suspended particulate matter will be
determined if they are not removed by prefiltration.
3.2 Prefilters may remove some gaseous ammonia. See section 7.1.1
3.3 Ferrous, chromous, and manganousions if present in mg amounts cause
positive interference in the analytical procedure because of precipitation.
Copper ions inhibit color development strongly and therefore cause
negative interference. Addition of Ethylene Diamine Tetraacetic
Acid(EDTA) prevents these effects. Nitrite and sulfite interfere if present
in 100-fold excess. Based on tests with solutions, formaldehyde causes a
negative interference of 10 to 15%. Interfering particulate matter in the air
can be removed by filteration of the air sample.
4. Precision and accuracy
4.1 Replicate samples, collected with glass prefilters and analyzed manually
showed a relative coefficient of variation of 30% in the 0.7 to 21µg/m3
(1
to 30ppb) range . This coefficient varies with concentration of atmospheric
Ammonia and decreases to 5%in the 700µg/m3
(1 ppm) range.
4.2 No accuracy data are available.
5. Apparatus
5.1 Vacuum Pump. Any vacuum pump that will maintain a vacuum of 60kPa
with an air flow of 5L/min
5.2 Flow Measuring Device. A calibrated flow meter or a critical orifice to
measure or control the air flow from 1 to 2 L/min. The flow meter should
be calibrated under conditions of use.
5.3. Absorber. A fritted bubbler, midget impinger, or other gas scrubber
designed for a flow rate of 1 to 2L/min with 10mL or more absorbing
solution is suitable for sampling.

2. 5.4 Prefilter Holder. An open-face Teflon, Laxan, or similar filter holder
which can be connected in line before the bubbler may be used as
perobjective of monitoring
5.5 Prefilters. Organic free glass fiber filter use for air sampling for
suspended particulate matter are satisfactory.
5.6 Spectrophotometer. Capable of measuring the absorbance at 630 nm.
5.7 Glassware. Low-actinic glassware or vessels must be used for analysis.
The glassware must be rinsed with 1.2 N HCL,and washed at least 6 times
with reagent water before immediate.
6. Regents.
6.1 Purity. All chemicals should meet specifications of ACS Analytical
Regent grade.
6.2 Water. Water must meet specification of ASTM Regent water type II, and
be ammonia free.
6.3 Absorbing solution. Dilute 3.0 mL of concentrated H2SO4(18M) to 1L
with water to obtain 0.1 N H2SO4
6.4 Analytical Regents.
Sodium nitroprusside [Sodium nitrosylpentacyanoferrate(iii)] dissolve 2g
Sodium nitroprusside in 100mL of water. The solution keeps well in the
refrigerator for 2 months.
6.75 M Sodium Hydroxide: Dissolve 270g hydroxide in about 1L of
distilled water. Boil down to 600mL in order to volatilize the ammonia
contained in the reagent. Cool and fill to 1L. Store in polyethylene bottle.
Caution: this solution is extremely caustic. Prevent contact with skin or
eyes.
Sodium hypochlorite solution: Dilute 5 to 6% analytical reagent sodium
hypochlorite with distilled water to give a 0.1N solution (3.7%) Strength is
determined before dilution by iodimetric titration or colorimetry after
appropriate dilution. The solution keeps well for 2 months in a
refrigerator.
Phenol solution 45% v/v, Melt phenol by immersing a bottle containing
the material in a water bath at 60ºC, Pour 45mL(50g) in to a 100mL
warmed cylinder and fill to mark with methanol.
Buffer: Dissolve 50g of Na3,PO4, 12H2O and 74mL of 6.75M NaOH in 1
L of distilled water.
Working hypochlorite solution: Mix 30mL of 0.1 N sodium hypochlorite
and 30mL of 6.75 M Sodium hydroxide and dilute to 100mL with distilled
water. Prepare fresh daily
Working phenol solution: Mix 20mL of the 45% Phenol solution with 1
mL of 2% Sodium nitroprusside and dilute to 100mL with distilled water.
Prepare fresh every 4 hr

3. 6.5 Ammonia Standard solution:
Ammonia stock solution. Dissolve 3.18 g of NH4CL or 3.88 g of
(NH4)2SO4 in 1 L of distilled water (1 mL equal to 1mg NH3). Add a drop
of CHCl3 for better preservation. The solution is stable for 2 months.
Ammonia working solution: Dilute 10ml of the stock solution to 1L with
absorbing solution in a volumetric flask(1 mL equal to 10 µg NH3).
Prepare daily.
6.6 Glass Cleaning Solution. Dilute 10mL of concentrated HCL(12M) to 100 mL
with distilled water(molarity approximately 1.2M)
7. Procedure
Sample Collection: Place 10mL of absorbing solution in each bubbler for
samples and field blanks. Cap bubbler for transport. Assemble(in order)
prefilter and holder , flow meter bubbler and pump. Sample at the rate of 1
to 2 L/min for a sufficient time to obtain an adequate sample, usually 1h.
Record sampling time and flow rate. After sample collection, recap the
bubbler
Prefilters If prefilters are not used the method will determine both gaseous
ammonia and ammonium contained in particulates. At high humidity,
acid gas will promote reaction on the filter causing loss of ammonia gas
from the sample. In the absence of acid gases. Ammonia collected
momentarily on the filter during high humidity will be stripped of during
sampling with little loss.
The color may be developed in the flask 25mL, transfer contents to a
25.0 mL capacity glass graduated cylinder, being sure to blowout residual
sample from the frits if they are used. Maintain all solutions and sample at
25ºC. Add 2mL buffer. Add 5mL of working phenol solution, mix, fill to
about 22mL, then add 2.5mL of the working hypochlorite solution and
rapidly mix. Dilute to 25mL, mix, and store in the dark at 25ºC for 30 min
to develop color. Measure the absobance of the solution against a reagent
blank at 630nm, using 1cm cells.
Field Blanks. At least one bubbler of collecting solution is carried into the
field and treated in the same fashion as the actual samples except that no
air is drawn into the bubbler. It is treated in analysis as if it was a sample.
The value of the regent blank to determine either sampling glassware is
introducing appreciate contamination.
8. Calibration and standardization
Preparation of standards. Pipet 0.5,1.0 and 1.5mL of the working
standards solution into 25mL glass stoppered graduated cylinders. These
correspond to 5, 10 and 15µg of ammonia/25mL mark with absorbing
solution. Fill to the 10-mL mark with absorbing solution is also prepared.
Add reagents to each cylinder as in the procedure for analysis. Read the
absorbance of each standard against the reagent blank.

4. Standard Curve. Plot the absorbance as the ordinate versus the
concentration as abscissa on linear graph paper. Alternatively, determine
the slope by the method of least squares
9. Calculations
µg/m3
NH3=W/Vo
where W= µg NH3 in 25mL from standard curve.
Vo= Volume of air sample in m3
at
25º C and 101.3kPa
Vo=(F/1000) X (Ps/101.3) X (298/273+Ts)
Where F= Flow rate (L/min)
Ps=atmospheric pressure in kPa at sampling point
Ts=temperature, ºC at sampling point.
10. Effect of Storage
10.1 Sample of particulate matter may be stored indefinitely if protected from
contamination.
10.2 Changes in the precision of the method occur when storing the collectd
liquid samples during 2 days. Significantly lower values have been found on
replicates stored for several days