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The activity of autochim in the field of the water treatment has developed around the detection of hydrogen sulphide (H2S), a highly toxic and mortal gas.

Its detection, generally problematic because of the constraints of environment (high moisture and continuous presence of H2S in weak or strong concentration), is ensured perfectly thanks to the technology semiconductor Thin film of General monitors, which also has the advantage of being very specific.

In this very important application, this gas can be generated by two phenomena:

• The first is related to the stability of sulphides in solution,

• stagnant water is brought by the second one.

Stability of sulphides in solution:

According to the pH, there is prevalence of one of the three sulphured components: H2S, HS, and S2. As shown in the curve of the figure 1, H2S appears for pH lower than 10. Beyond that, there is prevalence of the sulfhydrate ions, then sulphide. If the concentration of sulphide, in a sufficiently acid solution, exceeds the limit of solubility of the H2S (0.099 m at 25°C), the latter is found in a gas state.

Stagnant water:

The generation of H2S intervenes where water stagnates in a place not very well ventilated. The aerobic bacteria develop and consume the dissolved oxygen of water, without it being able to be renewed. The produced substances create a pH variation. Because of the lack of oxygen, the development of the aerobic bacteria is stopped, in aid of the anaerobes. The latter ones use sulphates dissolved in water for their growth and produce H2S, as well as other substances.

These bacteria are known under the name of reducing sulphate bacteria. So far,the signs are:

• The absence of oxygen,

• the presence of H2S,

• and often a reduction of the pH.

Experimental measurements showed that as soon as water becomes stagnant, the production of H2S is very fast. The concentration can double every 15 hours and reach 50 to 60 ppm ; if the oxygen renewal is very weak, the concentration in the air in ppm volume can be a few hundreds times larger than the one in the water expressed in ppm weight by volume. So it will be necessary not to extrapolate water/air.

That is how one meets H2S concentrations, sometimes very important in the safety rooms, during pre-treatment, or in the screen cleaners' zones.

At the same time as the detection of H2S, the activity of autochim developed around other combustible and toxic gases at fixed or portable station, such as methane (CH4), ammoniac (NH3), ozone (O3) CoV' S etc....

The detectors / analyzers proposed use photo-ionisation technologies (PID) coupled or not with chromatography, infra-red, photo acoustic, flame ionization (FID),electrochemical cells etc...