Desiccator (chemistry)
A desiccator (also: desiccator or desiccator , of lat. Exsiccare : dry) is a chemical laboratory device , primarily intended for drying solid chemical substances in the analytical and preparative chemistry is used.
A desiccator is a vessel usually made of thick-walled glass (or, more rarely, plastic), which is sealed airtight by a flat-ground lid. To ensure this closure, the joint is usually covered with a joint fat . The lower part of the desiccator is separated by a ceramic or plastic plate (sieve plate) and is filled with a desiccant underneath . On the screen plate is an open vessel ( watch glass , crystallizing dish , beaker o. Ä.) With the material to be dried.
The desiccant removes the evaporated solvent given off by the substance to be dried from the air inside the desiccator . This can cause the substance to lose more adsorbed solvent. This process continues until a certain residual content of solvent is reached, which, due to the intensity of the drying medium and the adsorptive properties of the substance, cannot be further reduced or until the absorption capacity of the drying agent (its capacity) is exhausted. If water is to be removed in the process, the primary property of the desiccant is called hygroscopicity . Calcium chloride , phosphorus pentoxide , sulfuric acid or silica gel are commonly used ; The latter can be accompanied by a moisture indicator ( blue gel, orange gel ). In addition to water, residues of other solvents can also be absorbed, depending on the choice of drying agent. A distinction is made between regenerable and non-regenerable drying agents. The former can be dried themselves by storage at elevated temperature and, if necessary, in a vacuum, while the latter absorb the moisture irreversibly. It should be noted that regenerable desiccants give off as much solvent as they absorb below a certain level of air humidity, which means that a completely dry atmosphere cannot be achieved with them.
In order to accelerate the drying process, suitable desiccators (“vacuum desiccators”) are evacuated if necessary . Such desiccators have a connection for evacuation that can be closed with a shut-off valve either in the middle of the lid or on the side wall . Desiccators that can be heated electrically are rare.
The desiccator is not only used to dry substances that contain solvents. It can also be connected to a vacuum pump , e.g. B. a water jet pump , are used to remove air from the system and the samples, e.g. B. wood to remove. This happens, for example, when impregnating wood test specimens in accordance with DIN EN 113 or when washing them out in accordance with DIN EN 84. The test specimens are placed in a separate soaking vessel in the desiccator and a vacuum is drawn using the pump, usually at 7 mbar absolute pressure. After 15 minutes, the impregnating agent is drawn onto the test specimens via a second access while maintaining the vacuum, so that they are completely soaked with the impregnating agent. In most cases, you have to come to a drinking volume of 700 L / m³.
Another application is the storage of samples at constant humidity. For this purpose, the lower part of the desiccator is filled with a saturated salt solution from which a body of the salt protrudes. Depending on the salt and temperature used, a certain water vapor partial pressure is then set above this. With this method it is possible to put samples that have a variable weight due to water absorption, depending on the humidity, into a defined and comparable state for weighing. The storage of samples or workpieces under inert gas is also possible and common.
Security notice
Brand new vacuum desiccators should be covered with a cloth or a wire cage when they are evacuated for the first time, since implosion can occur due to manufacturing defects or tension in the glass. Because of the risk of implosion, evacuated desiccators must not be transported. You should always stand on a soft surface under a protective cover made of wire mesh.
If concentrated sulfuric acid is used as a desiccant in a vacuum desiccator and the vacuum is generated with a water jet pump, it must be ensured that a Woulfe bottle is installed between the vacuum desiccator and the water jet pump in order to suck in the residual water from the pump in the event of a sudden drop in water pressure evacuated desiccator and thus a critical reaction with the concentrated sulfuric acid.
Individual evidence
- ↑ Kathy Barker: The Cold Spring Harbor Laboratory Manual for Beginners , Elsevier GmbH, Munich, 1st edition, 2006, p. 153. ISBN 978-3-8274-1656-8 .
- ^ Walter Wittenberger: Chemische Laboratoriumstechnik , Springer-Verlag, Vienna, New York, 7th edition, 1973, p. 133, ISBN 3-211-81116-8 .
- ↑ a b Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, pp. 386-387.
- ↑ Kathy Barker: The Cold Spring Harbor Laboratory Manual for Beginners , Elsevier GmbH, Munich, 1st edition, 2006, pp. 153-155, ISBN 978-3-8274-1656-8 .
- ↑ Erich Meister: Basic practical course in physical chemistry, theory and experiments. vdf Hochschulverlag, 2006; ISBN 978-3-825283292 .
- ^ Gerhard Meyendorf: laboratory equipment and chemicals , people and knowledge Volkseigener Verlag Berlin, 1965, pp. 21-22.
- ^ Organikum , Wiley-VCH Verlag GmbH, 23rd edition, 2009, p. 27, ISBN 978-3-527-32292-3 .
