Categorization of an unknown chemical substance

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Categorization of an unknown chemical substance

In many situations, personnel may be in the presence of an unknown chemical substance.  It may be a first responder on a mission during an intervention in a university chemistry laboratory (I have personally experienced such a situation!), a chemical waste manager who has just been informed of old full cans that have been dumped in the wild, a judicial operator looking for evidence or CBRN military personnel.

What can be done in front of such a product to eliminate it in the best conditions?





What is the problem ? 

Complete identification of the chemical substance would require sampling and characterization in a laboratory. This takes a long time and its transport can be risky because the product can be explosive, flammable or even toxic…

We will, therefore “categorize” the product, i.e. determine in a few simple tests and, in the field, its main physicochemical characteristics allowing the most appropriate safety measures to be taken quickly.

Field measurements should be simple to perform by non-specialized chemical personnel, with easy-to-use equipment and a “step-by-step” procedure to perform the manipulations in the right order.

The unknown product may be in gaseous form (air, gas, vapor) or in solid or liquid form.



A very simple test makes it possible to identify several existing “TICs” in gaseous form by simply exposing a Multi-Tic detector card to gas. Coloured indicators show the possible presence of Hydrogen Cyanide (HCN), Hydrogen Chloride (HCl), Hydrogen Fluoride (HF), Chlorine, Bromine, Phosgene and Ammonia.


Solid and liquid

  • Flammability test

The flash point is the minimum temperature at which the concentration of the vapors emitted is sufficient to produce a deflagration in contact with a flame or hot spot under standard conditions.

A minimum temperature is shown at which the emission of vapors is sufficient to form the flammable gas mixture with air under the external action of a flame. This minimum temperature level is called flash point.
The lower the flash point, the more flammable and therefore dangerous the substance.

There are several field tests based on the behavior of a sample at a distance or in contact with a flame. These are semi-quantitative tests very useful in the field and require the use of a bunsen-type nozzle on a gas cartridge. They are made with a very small sample deposited on copper wire with a loop at its end. A small amount of product is placed and brought near the flame.

Flash Pt < 40°C: extremely flammable. The product ignites at 1 cm from the flame

Flash point between 40°C – 60°C: flammable. The product ignites out of the flame after touching it.

Flash point 60°C – 90°C : flammability limit or combustible. The product burns out of the flame after maintaining 2 seconds of contact with it.

Flash Pt > 90°C . Requires to be in the flame to burn: combustible

If the product explodes in contact with the flame it is potentially explosive!

To highlight the explosive aspect of the product, the so-called “hairpin test” can also be performed. The solid sample the size of a grain or a drop of liquid is placed on a watch glass. We take a hairpin and heat one end of it in the flame of the beak until it is red. The heated end touches the unknown solid or liquid. When the product is explosive, a volatile explosive release is created.

This test can be completed by the Beilstein Test.  It consists of bringing the copper wire to the incandescence and bringing it into contact with the sample. In the presence of halogen, the flame will turn green after reintroducing the wire into the flame, following the formation of a copper halide. Clean the copper wire in the flame. Take a sample again when the wire is cooled and reintroduce it into the flame and observe the color of the flame. The appearance of a bright green color indicates the presence of an organic product containing halogens (Cl, Br, I). This is the case, for example, for polychlorinated biphenyl (PCBs).

  • Water search and behavior in water

If the sample contains water, it will be indicated by a colored indicator on a test paper.

Water solubility will be tested by introducing a drop of liquid product or the equivalent for a solid product in 1 mL of water. Observe any signs of reaction: the release of heat, presence of bubbles or vapors (formation of toxic product or flammable gas). If there is no sign of a reaction, gradually supplement to about 1 gram of solid sample/1 ml of a liquid sample.

If the product is completely dissolved or forms a density gradient it is soluble in water (ionic/polar). A water-insoluble precipitate may form (non-polar, specific gravity > 1.0). This is the case for halogenated hydrocarbons. The undissolved product can float on the surface of the water: it is insoluble in water (non-polar, specific gravity less than 1.0, possibly flammable). This is the case for hydrocarbons. It can emulsify like cream on coffee.


  • pH measurement

It is performed with pH paper on a liquid sample or on a solid diluted in water (provided that the product does not react with water!).

If the pH <2 or pH>12.5 the product is corrosive (strong acid, strong base).


  • Oxidizing properties

In the same way, test papers containing colored indicators reveal oxidants, peroxide, nitrates, perchlorates.

Cyanide or sulfide-based products can also be searched for.

  • Additional tests

Char Test

Its purpose is to determine whether it is an organic or inorganic substance.

Introduce a small amount of solid or liquid product (tested not to be explosive!) into a test tube. Heat the tube until there is no further reaction or until the test piece melts. When it is an organic product, the vapors that are released are ignited with a lighter (exception: halogenated hydrocarbons do not ignite).

In the case of an inorganic product, the vapors in the headspace do not ignite. Observation of the color of the vapors, the presence or not of water on the walls of the tube, odors, etc. also provides valuable information on the composition of the unknown product.

It is also interesting to test these vapors with the Multi-Tic detector card that we already saw at the beginning of the chapter.


Detection of cyanide, sulfide, oil, fluoride. Colored indicators


Temperature measurement (using an infrared camera to determine, for example, whether an exothermic reaction inside a container can lead to an explosion).


Raman Spectroscopy 

It allows to compare extremely flammable, flammable or potentially flammable profiles. It can, therefore, assess the risk associated with heat.


Identification of war chemicals using Detection Paper such as PDF-1 and CALID 3 (GD, HD, VX).



We can, therefore, categorize explosive products, acids/basics/oxidants (corrosive), flammable, combustible, dangerous in the presence of water, oxidants, toxic to humans and the environment (halogenated, cyanides, sulfides…).

Hazardous gaseous products are also classified as HCN, HCl, HF, Chlorine, Bromine, Phosgene, Ammonia.

Rapidly, in the field, appropriate measures can be taken to treat unknown products under the best possible safety conditions.



These tests only work well with pure products and are therefore not suitable for chemical waste.

Like all semi-quantitative screening methods, there are many false positives and false negatives, but they are known and well identified in the instructions for use.

The company Ouvry® distributes a kit to characterize hazardous chemicals in the field: HAZCAT



° Techniques for identifying unknown materials. Hazard Categorization. Washington State Deparment of Ecology Spill Response Section. Lacey, WA.

° A guidebook intended for use by first responders during the initial phase of a tranposrtation indicent involving dangerous goods/hazardus materials. 2016 Emergency response Guidebook. 20016 Emergency response guidebook.

° HazMat Field Screening Equipment Manual  03/13/17 California CUPA Forum.

° Mode d’emploi HAZMAT (Ouvry SAS)




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