Can solids be pressurized?

Department of biology, chemistry, pharmacy

Many inorganic compounds are sensitive to air and humidity. In order to be able to represent and handle them, one must therefore use suitable methods:

  • Working under protective gas (argon) according to the countercurrent principle
  • Use of absolute solvents
  • Storage and handling of substances in a dry box (glovebox)

1. Working under protective gas

In order to protect a substance or reaction from air, equipment is used which is under an inert atmosphere. To do this, the apparatus is first completely assembled and evacuated (at least 5 minutes, check for leaks!) In order to largely remove the atmospheric oxygen (it is sufficient to exclude oxygen Notto purge the apparatus with argon only). The apparatus is then carefully heated from all sides with further evacuation with a hot air blower so that all the water evaporates from the glass wall ("bakeout"). Then it is flooded with argon. Every apparatus is to be equipped with a pressure relief valve; it allows a slight overpressure in the apparatus, which prevents the ingress of air, but avoids excessively high pressures that could lead to the apparatus exploding. It is advisable to present insensitive solids before evacuation, and the magnetic stir bar should also not be forgotten. In any case, it is to be avoided to permanently pass argon through a running apparatus, instead argon is led past it by means of a T-piece (as shown in Figure 3.1, top right).

2. Countercurrent principle

If an apparatus has to be opened, e.g. to add a reagent or to rebuild the apparatus, one works according to the so-called countercurrent principle in order to avoid air penetration. In doing so, argon flows out of the apparatus against the penetrating air. If you want to assemble two parts of the apparatus without air contact, argon is allowed to flow from both parts in the direction of the connection point.

Fig. 1: Filling in a substance and assembling an apparatus according to the countercurrent principle

3. Transfer of air-sensitive substances

There are several possibilities to transfer a substance from one reaction vessel to another without bringing it into contact with air:

The safest way to get liquids is to use a direct connection between the two vessels such. B. transfer an elbow. However, depending on the solvent, ground joint grease can dissolve and contaminate the substance. In addition, it is not always possible to tilt complicated equipment.

Fig. 2: Transfer of air-sensitive liquids by means of a direct connection

A can be used much more flexibly Teflon hosethrough which the liquid is forced into another vessel by means of argon pressure. Since you only purge it with argon before pushing it over, there is little contact with air; Extremely sensitive connections cannot be transferred in this way, but in most cases there are hardly any problems.

Fig. 3: Transferring air-sensitive liquids using a Teflon hose

The same applies to transferring with a syringewhich, in addition to flexibility, also allows certain quantities to be measured. Here, too, you only have the option of flushing the syringe several times with argon in order to exclude air contact as far as possible.

Highly volatile substances (gases and low to medium-high boiling liquids, in individual cases also highly volatile solids) can pass through most comfortably Recondensation be transferred. For this purpose, the substance to be transferred is first frozen to nitrogen temperature and evacuated; the target vessel is also evacuated. Then the main tap 2 closed, the target vessel is cooled and the substance to be transferred is slowly warmed to room temperature after opening all the taps between the two vessels. The volatile components then condense in the target flask, provided the static vacuum is sufficient to evaporate the substance.

Fig. 4: Recondensation of volatile compounds in a static vacuum

4. Use of a dry box (glove box)

A dry box is used to handle air-sensitive solids. It consists of an airtight box with a window, a pair of gloves and a lock. In order to bring substances into the box, they are put into the lock from the outside and this at least 30 minutes evacuated before flooding them with argon. The user can then slip into the gloves and open the lock from the inside in order to transfer the substances, weigh them, etc. Since the lock must always be evacuated, only open or evacuated vessels can be introduced, otherwise they will burst! Liquids are generally not fed into the dry box. Infiltration of cork rings, paper and other porous materials requires longer evacuation in order not to let moisture get into the protective gas atmosphere. They are therefore not allowed in the dry box during the internship. The best way to weigh substances is to use aluminum foil.

As of 10/08