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DESICCATION (from the Lat. desiccare,...

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Originally appearing in Volume V08, Page 94 of the 1911 Encyclopedia Britannica.
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DESICCATION (from the See also:Lat. desiccare, to dry up) , the operation of drying or removing See also:water from a substance. It is of particular importance in See also:practical See also:chemistry. If a substance admits of being heated to say loo°, the drying may be effected by means of an See also:air-See also:bath, which is simply an See also:oven heated by See also:gas or by See also:steam. Otherwise a desiccator must be employed; this is essentially a closed See also:vessel in which a hygroscopic substance is placed together with the substance to be dried. The See also:process may be accelerated by exhausting the desiccator; this so-called vacuum desiccation is especially suitable for the concentration of aqueous solutions of readily decomposable substances. Of the hygroscopic substances in See also:common use, phosphoric anhydride, concentrated sulphuric See also:acid, and dry See also:potassium See also:hydrate are almost equal in See also:power; See also:sodium hydrate and See also:calcium chloride are not much behind. Two common types of desiccato? are in use. In one the absorbent is placed at the bottom, and the substance to be dried above. Hempel pointed out that the efficiency would be increased by inverting this arrangement, since water vapour is lighter than air and consequently rises. Liquids are dried either by means of the desiccator, or, as is more usual, by shaking with a substance which removes the water. Fused calcium chloride is the commonest absorbent; but it must not be used with See also:alcohols and several other compounds, since it forms compounds with these substances. Quicklime, See also:barium See also:oxide, and dehydrated See also:copper sulphate are especially applicable to See also:alcohol and See also:ether; the last traces of water may be removed by adding metallic sodium and distilling.

Gases are dried by leading them. through towers or tubes containing an appropriate drying material. The experiments of H. B. See also:

Baker on the See also:influence of moisture on chemical See also:combination have shown the difficulty of removing the last traces of water. In chemical technology, apparatus on the principle of the laboratory air-bath are mainly used. Crystals and precipitates, deprived of as much water as possible by centrifugal See also:machines or See also:filter-presses, are transported by means of a See also:belt, See also:screw, or other See also:form of conveyer, on to trays staged in See also:brick See also:chambers heated directly by flue gases or steam pipes; the latter are easily controlled, and if the steam be superheated a temperature of 300° and over may be maintained. In some cases the material traverses the chamber from the coolest to the hottest See also:part on a conveyer or in wagons. Rotating cylinders are also used; the material to be dried being placed inside, and the See also:cylinder heated by a steam jacket or otherwise.

End of Article: DESICCATION (from the Lat. desiccare, to dry up)

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