radiello^® Diffusive Air Sampling Application - Ammonia

radiello Table of Contents

radiello Components used:

Blue diffusive body Product No. RAD1201
Supporting plate Product No. RAD121
Vertical adapter Product No. RAD122 (optional)
Chemiadsorbing cartridge Product No. RAD168

Principle

Cartridge RAD168 is made of microporous polyethylene and impregnated with phosphoric acid. Ammonia is adsorbed as ammonium ion. Airborne ammonium salts dispersed as particulate matter do not cross the diffusive membrane of radiello.

Ammonium ion is quantified by visible spectrometry as indophenol: at basic buffered pH ammonium ion reacts with phenol and sodium hypochlorite, with pentacyanonitrosylferrate catalysis (in the following cyanoferrate), to form indophenol. The reaction product is intensely coloured in blue, and its absorbance measured at 635 nm.

Sampling Rate

Sampling rate Q at 298 K (25 °C) and 1013 hPa is 235 ml·min^-1.

Effect of Temperature, Humidity, and Wind Speed

The effect of temperature on sampling rate is negligible (<0.1%/ °C) in the range from 275 - 312 K (2 - 39 °C). Sampling rate is invariant with humidity in the range 10 - 90% and with wind speed between 0.1 and 10 m·s^-1.

Calculations

The concentration C in μg·m^-3 is obtained according to the equation:

where m is the mass of ammonium ion in μg found onto the cartridge and t is exposure time in minutes.
0.944 is the numerical factor necessary to convert ammonium ion into ammonia (Analysis)

Exposure

Introduce the cartridge in the diffusive body and make sure that the PTFE cap is positioned at the same end of the screw.
Ammonia is sampled linearly in the range from 2,000 - 20,000,000 μg·m^-3·min.
Exposure time is allowed to range from 1 hour to 14 days.

IMPORTANT
Do not touch the microporous portion of the cartridge with your fingers: sweat contains ammonium ions.

Limit of Detection and Uncertainty

The limit of detection is 1 μg·m^-3 for 24 hour exposure. The uncertainty at 2σ is 6.5% over the whole allowed exposure range.

Storage

The cartridges are stable at least for 12 months before and after exposure if kept at room temperature in an ammonia free environment. Do not expose all of the cartridges of the same lot: keep at least two of them as blanks.

Analysis

Materials

• plastic or glass tube, volume 12 mL, with cap
• micropipet with variable volume from 0.1 to 1.0 mL
• 5 mL glass pipet

Reactives

• buffer solution (pH 10.6): dissolve 1.1 g of NaOH and 3.04 g of NaHCO₃ in one liter of water
• phenol: dissolve 10 g of phenol in 100 mL of ethanol
• cyanoferrate: dissolve 0.5 g of sodium pentacyanonitrosylferrate dihydrate (Na₂Fe(CN)₅NO·2H₂O) in 100 mL of water and add a few drops of 10% NaOH. Keep this solution in a dark bottle and prepare it freshly.
• oxidising solution: sodium hypochlorite with 1% of active chlorine in 0.2 M NaOH. Keep cool in a dark bottle.

Ammonium Ion Quantification

Open radiello tube and cautiously discard the cartridge PTFE cap (it may have been contaminated with handling). Help yourself with a pair of pliers.

Add 10 mL of deionised water to the cartridge in its tube (make sure that no trace of ammonium ion is found in the water you use). Recap the tube and stir vigorously by a VORTEX stirrer for at least 15 seconds.

Transfer 1 mL of the solution into another tube along with 0.4 mL of phenol, 0.4 mL of cyanoferrate, 5 mL of buffer solution and 1 mL of oxidising solution.

Wait for 1 hour and then measure the absorbance of the solution at 635 nm using water to zero the spectrophotometer.

Do the same with two unexposed cartridges and subtract their absorbance value to the samples. Generally, the blank value does not exceed 0.040 absorbance units.

For exposure value higher than 500,000 μg·m^-3·min the absorbance value is no longer linear: dilute a known fraction of the coloured solution with the buffer.

Calibration curves are conveniently prepared with ammonium chloride solutions in the range from 0.5 to 10 mg·l^-1 as ammonium ion.

IMPORTANT
If sample is too concentrated (absorbance no longer linear) DO NOT DILUTE WITH WATER: the pH value is critical in the determination of the colour intensity.