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Acidic Reaction Gas

Do not blow it into the atmosphere!

In environment regulations the protection of territorial waters seemes to be much more important than the protection of the atmosphere. Nevertheless you may not blow acid gases like hydrogen chloride, hydrogen bromide or sulfur dioxide into the atmosphere.

As a rule all these gases are easily absorbed in sodium hydroxide solution. But there are two problems:

  1. A lot of heat may be developed during the reaction.
  2. The absorption is very vigorous and if there is not enough gas running to the absorption solution it is the absorption solution which will run back to the source of the gas. This may happen with the speed of a flash!
It is very important to use PVC-tubings for all connections described in the following equipments. With rubber tubings you will produce crumbly "rubber bromide" or "rubber chloride" respectively!
If you use a gas washing bottle to absorb the gas usually a safety bottle (see left) is needed as a trap, if the absorption solution is sucked back. The problem is that if the absorption solution is sucked back it will move back and forth between the two bottles.

You may avoid this problem if you do not immerse the gas inlet pipe into the absorption solution but let it end very close to the surface. Use your lab glass ware to fabricate your own gas washing bottle, where the height of the gas inlet pipe can be adjusted manually. See the pictures right hand to get an idea how to do this. The left apparatus has a free outlet and therefore has to be placed close to the fume hood flue. In both equipments the round bottom flask is the main part and contains the absorption liquid. The flask should be filled to the half of its volume, because in this case the surface of the solution is on its maximum. Adjust the size of the flask and the concentration of the absorption solution in a way that all of the gas can be absorbed. Since hydrogen chloride or hydrogen bromide dissolve very effectively in water approximately 80 % of the are removed. (Consider that the gas is heavier than the air and will therefore spread onto the water surface where it is absorbed.) Stirring is not necessary because the acid solution is also heavier than the pure water and will sink to the bottom of the flask. (You will see the streaks moving down to the bottom.)

If you are performing a reaction where cooling water is needed you may use this cooling water and the equipment shown in the picture to build up a simple gas washer. Consider that no water should flow through the cooling coils of the dimroth condenser. The cooling coils are only needed to distribute the water which is feed through the opening (A) of the adapter over a large surface. You may also use a Vigreux-column instead of the condensor. The opening (B) of the vacuum adapter is connected to the apparatus where the gas is formed. (A straight adapter does only look better and has no other advantages.)


The cooling water must not plug the outlet of the condensor, but has to rinse down without any pressure instead. Otherwise the water will move into your reaction apparatus! Plugging happens,

  • if glassware with NS14 standard joints is used instead of NS29-glassware,
  • if the cooling water is running too powerful or
  • if you swap (A) and (B).

Place the end of the cooler over a sink. It is not needed to neutralize the diluted acid running out of the condensor because the lab waste water is neutralized automatically by the technical equipment of the institute.