Hydrogen sulfide (H2S)

 

 radiello components to be used

 

white diffusive body code 120
      supporting plate code 121
              vertical adapter code 122 (optional)
                      chemiadsorbing cartridge code 170


 
 

Principle

   
 

The cartridge code 170 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.

 
methylene blue
 

Methylene blue is quantified by visible spectrometry.

   

Sampling rate

   
 

Sampling rate Q298 at 298 K (25°C) and 1013 hPa is 0.096±0.005 ng·ppb-1·min-1 or 69 ml·min-1.

   

Effect of temperature, humidity and wind speed

   
 

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

 
 

where QK is the sampling rate at the temperature K and Q298 is the reference value at 298 K. This produces a variation of ±5% for 10 °C variation (upwards or downwards) from 25 °C.
Sampling rate is invariant with humidity in the range 15-90% and with wind speed between 0.1 and 10 m·s-1.

Calculations

   
 

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

 
calcolo della concentrazione
 

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

   

Exposure

   
 

Exposure duration is allowed to vary from 1 hour to 15 days.
Sampling is linear from 2,000-50,000,000 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 hours exposures.
The uncertainty at 2s is 8.7% over the whole allowed 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 belonging to the same lot (lot number and expiry date are printed onto the plastic bag): 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. Be careful to apply the same dilution ratio to the samples and the blanks.

 

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.

NEVER USE WATER TO DILUTE.

     

Calibration

   
 

Calibration curves may be prepared by sodium sulfide standard solutions, that have to be titrated just before use. As diluted sodium sulfide are very unstable (the sulfide content can diminish as much as the 10% in an hour time) it is strongly recommended to make use of the calibration solution code 171, following the instructions included.

 

user tip

Code 171 calibration solution
relieves you from the task of
preparation and titration of the
sodium sulfide solutions.