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 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.
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)
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.
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.
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.
Note: 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.
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