radiello^® Diffusive Air Sampling Application - Hydrogen Sulfide

radiello^® Components Used

White diffusive body (RAD120)
Supporting plate (RAD121)
Vertical adapter (RAD122) (optional)
Chemiadsorbing cartridge (RAD170)

Or try: Hydrogen Sulfide Starter Kit (RAD170S)

• 1 Support plate
  
• 1 White diffusive body
  
• 1 Vertical adapter
  
• 2 Microporous PE coated w/ zinc acetate

Principle

Cartridge (RAD170) is made of microporous polyethylene and impregnated with zinc acetate. Hydrogen sulphide is chemiadsorbed by zinc acetate and transformed into stable zinc sulfide. The sulfide is recovered by extraction with water. In contact with an oxidizing agent as ferric chloride in a strongly acid solution it reacts with the N,N-dimethyl-p-phenylendiammonium ion to yield methylene blue.

Sampling Rate

Sampling rate Q at 298 K (25°C) and 1013 hPa is 0.096 ± 0.005 ng·ppb^-1·min^-1.

Effect of Temperature, Humidity, and Wind Speed

Sampling rate varies from the value at 298 K on the effect of temperature (in Kelvin) as expressed by the following equation:

where Q_K is the sampling rate at the temperature K ranging from 268 to 313 K (from -5 to 40 °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

Once Q_K at the sampling temperature has been calculated, the concentration C is obtained according to the equation:

where m is the mass of sulphide ion in μg found onto the cartridge and t is exposure time in minutes.

Exposure

Exposure duration may vary from 1 hour to 15 days. Sampling is linear from 2,000 to 50,000,000 ppb·min of H₂S.

Limit of Detection and Uncertainty

The limit of detection is 30 ppb for 1 hour exposure or 1 ppb for 24 hour exposure. The uncertainty at 2σ is 8.7% over the whole exposure range.

Storage

The cartridges are stable at least for 12 months before and 6 months after exposure. Do not expose all of the cartridges of the same lot: keep at least two of them as blanks.

Analysis

Reactives

• sulphuric acid: slowly add 25 mL of concentrated sulphuric acid to 10 mL water and let the solution cool;
• amine: dissolve 6.75 g of N,N-dimethyl-p-phenylendiammonium oxalate in the sulphuric acid solution. Dilute this solution to 1 liter with sulphuric acid - water 1:1 v/v. Kept in a dark bottle and well capped, this solution is stable for at least four weeks. CAUTION: this solution is very poisonous.
• ferric chloride: dissolve 100 g of ferric chloride hexahydrate (FeCl₃·6H₂O) in 40 mL of water.
• ferric chloride-amine: mix 10 mL of ferric chloride solution with 50 mL of amine solution. This solution has to be freshly prepared;
• sulphuric acid for dilution: slowly dissolve 40 mL of concentrated sulphuric acid in 900 mL of water, let the solution cool and make up to 1,000 mL.

Procedure

Add 10 mL of water to the plastic tube containing the cartridge, recap and stir vigorously, preferably by a VORTEX stirrer.
Add 0.5 mL of ferric chloride - amine solution, recap immediately and stir. The tube must be capped immediately in order to avoid that the developed hydrogen sulfide can escape from the tube before reacting.
Wait for 30 minutes and measure absorbance at 665 nm using water to zero the spectrophotometer. The colour is stable for several weeks.
Do the same with two or three unexposed cartridges of the same lot and obtain the average blank value, then subtract it to the samples.

IMPORTANT
Absorbance is linear up to 1,200 absorbance units, corresponding to an exposure value of about 80,000 ppb·min. If higher absorbance values are obtained, dilute the samples with the sulphuric acid for dilution.
Be careful to apply the same dilution ratio to the samples and the blanks.
NEVER USE WATER TO DILUTE.

Calibration

Calibration curves may be prepared by sodium sulfide standard solutions, which have to be titrated just before use. As diluted sodium sulfide solutions are very unstable (the sulfide content can diminish as much as the 10% in an hour) it is strongly recommended to make use of the calibration solution (RAD171), following the instructions included.

USER TIP
Calibration solution (RAD171) relieves you from the task of preparation and titration of the sodium sulfide solutions.