Read Time: 2 mins
Hits: 106122

 The colorimetric methods are able to identify, in many cases in a quantitative way, the presence of a gas. It is possible to use patches or colorimetric tubes or badges.

Badges.

  The badges are a one-use piece of cardboard with a small surface coated with a chemical that changes its colour when exposed to a certain gas. Both the time exposure and the intensity of the colour change are important. The badges show an integrated averaged value not very precise but useful when dealing with personal protection exposure.

 

These devices might be hanged anywhere, or be carried on the worker's cloths or they might be combined with small fans for different applications. Hydrogen sulphide badges are well used in farming, for example to limit H2S exposure in workers.

 

Colorimetric tubes.

  There are different types of colorimetric tubes to measure a wide range of gases. The Colorimetric tubes are glass tubes with both ends closed. To perform a measurement with a colorimetric gas tube, both ends of the tube must be broken. Then the tube is connected to a pump (manual or electronic). The pump makes the gas to circulate through the tube with a known volume of odorous gas. The media inside the tube reacts with the gas and change its colour according with the type of gas sampled. A scale is used to measure the amount of gas that reacts with the media. This scale indicates the concentration of the gas detected.

The colorimetric tubes usually have limited scales and the precision is around 10-20% of the full reading scale of the tube. The colorimetric tubes offer almost instant readings with detection limits down to 0,2 ppm in the case of Hydrogen sulphide (H2S). The cost of a manual pump stay between 300-500 €. The diffusion tubes that measure a mean concentration are also available for certain gases.

It is possible to find colorimetric tubes that work by means of a mechanism of diffusion, without the need of a pump. To use these passive samplers, one of the ends of the tube is opened and then the tube is placed in the site to be monitored. After a known period of time (normally 6 to 8 hours), a reading is taken by the colour change observed in the tube. This parameter and the exposition time are used to calculate average concentration for the time period sampled. This tubes have a price of around 6-8 € each.

You can check the different colorimetric tube makers in the directory of this site, or just clicking here.

Read Time: 4 mins
Hits: 11735

   Both olfactometry and physical-chemical analysis are two methods used to evaluate the somehow relative annoyance caused by odorous substances. These techniques have several advantages and also some drawbacks although they can provide complementary information.

  With the olfactometry technique odour threshold can be quantified but in most cases these parameters cannot be chemically linked to a substance or group of substances. On the other hand, a physical-chemical analysis can identify single substances that could be related with odorous episodes, but in most of the cases these relation is not possible. Physical-chemical analysis proportionates also information about the toxicity of certain odorous gases, however, there will hardly be a link between concentration of a substance and odour annoyance using this technique. As a short example, more than 500 molecules have been identified in a sample taken from a landfill (terpenes, sulphides, aldehydes, acids, acetones, alcohols, aromatic compounds, chlorides, esters, etc.).

  An odorous gas can be sampled with different sampling devices such as packed tubes, Tedlar®/Nalophan® bags, canisters, etc. The election of the sampling method has to be evaluated as previous step in the planning process. The criteria to use one or the other method is out of the scope of this article but as a general rule, when odour sensation on site is well distinct, Tedlar®/Nalophan® bags can perfectly be used both for physical-chemical analysis and dynamic olfactometry. In any case, independently of the sampling technique, the substance sampled has to remain in stable conditions when transported to the lab for analysis.

  In certain cases, it is possible to measure directly on site. This is quiet convenient when the influence of changes in processes on a direct emission of odours is the object of the study or when there is a incident with immediate risks to the population. In this case there are several methods such as colorimetric passive tubes, electronic detectors or portable Gas Chromatography analysers coupled with several detectors (MS, NPD, FID, PID, FPD, etc). Several pros and cons can be outlined but in any case and when money is not problem, a portable GC/MS is a very good option. As an example, the regional environmental ministry of Andalusia is regularly using a GC/MS to measure high hydrocarbon emissions in oil refineries in the Gibraltar area to identify toxic substances on site.

 It is also possible to evaluate odour concentration on site by means of mobile labs in vans or similar. In this case the calibrated odour panel is on site and samples are evaluated inside the van. These vans or similar have to be odourless and perfectly isolated from the ambient air. Some companies nowadays use these mobile olfactometric laboratories, eg. EOG (France), ECOMA (Germany) or the Odour Unit Pty Ltd (Australia) which are able to relocate to meet clients' specific needs.

 Due to the fact that in most cases an odour is made of several substances at extremely low concentration, it is very difficult and in some cases costly to find out the exact composition of an odour. In any case, physical-chemical analysis is always a good add on for any evaluation of odour annoyance.

There is a link between odour threshold and chemical composition for some substances. Here, there is one small compilation for some compounds.

chemical compound

Odour threshold, ppm

 

Aldehidos

  

Acetaldehido

0.21

Propionadehido

0.0095

 

Acidos grasos volátiles

  

Ácido acético

1.0

Ácido Propionico

20.0

Ácido Butyrico

0.001

 

Compuestos nitrogenados

  

Metilamina

0.021

Dimetilamina

0.047

Trimetilamina

0.00021

Skatol

0.019

Amoniaco

46.8

 

Compuestos azufrados

  

Metanotiol

0.0021

Etanotiol

0.001

Propanotiol

0.00074

t-Butiltiol

0.00009

Dimetil sulfito

0.001

Hydrogen sulfide

0.0072
Read Time: 3 mins
Hits: 11338

  Both olfactometry and physical-chemical analysis are two methods used to evaluate the somehow relative annoyance caused by odorous substances. These techniques have several advantages and also some drawbacks although they can provide complementary information.

  With the olfactometry technique odour threshold can be quantified but in most cases these parameters cannot be chemically linked to a substance or group of substances. On the other hand, a physical-chemical analysis can identify single substances that could be related with odorous episodes, but in most of the cases these relation is not possible. Physical-chemical analysis proportionates also information about the toxicity of certain odorous gases, however, there will hardly be a link between concentration of a substance and odour annoyance using this technique. As a short example, more than 500 molecules have been identified in a sample taken from a landfill (terpenes, sulphides, aldehydes, acids, acetones, alcohols, aromatic compounds, chlorides, esters, etc.).

Page 5 of 5

All the content here under Creative Commons license