DESICCATION (from the

See also:

• LAT

Lat. desiccare, to dry up)  , the operation of drying or removing water from a substance . It is of particular importance in practical chemistry . If a substance admits of being heated to say loo°, the drying may be effected by means of an air-bath, which is simply an oven heated by gas or by steam . Otherwise a desiccator must be employed; this is essentially a closed vessel in which a hygroscopic substance is placed together with the substance to be dried . The 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 common use, phosphoric anhydride, concentrated sulphuric acid, and dry potassium hydrate are almost equal in power; sodium hydrate and 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 alcohols and several other compounds, since it forms compounds with these substances . Quicklime, barium oxide, and dehydrated copper sulphate are especially applicable to alcohol and 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 .

Baker on the influence of moisture on chemical 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 machines or filter-presses, are transported by means of a belt, screw, or other form of conveyer, on to trays staged in brick 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 part on a conveyer or in wagons . Rotating cylinders are also used; the material to be dried being placed inside, and the cylinder heated by a steam jacket or otherwise .