All the carbonates (those of the alkalies and alkaline earths excepted)
are completely decomposed on ignition into the oxide of the metal and
carbon dioxide; but the temperature required for this decomposition
varies with the nature of the base. All carbonates are soluble with
effervescence in dilute acids; some, such as chalybite and magnesite,
require the aid of heat. The alkaline carbonates are soluble in water;
the rest, with the exception of the bicarbonates, are insoluble therein.
Carbonates are recognised by their effervescence with acids--a stream of
bubbles of gas are given off which collect in the tube, and possess the
property of extinguishing a lighted match. The most characteristic test
for the gas is a white precipitate, which is produced by passing it into
lime or baryta-water, or into a solution of subacetate of lead.
The expulsion of carbon dioxide by the stronger acids serves for the
separation of this body from the other acids and bases.
~Dry Assay.~--There is no dry assay in use. Any method which may be
adopted will necessarily be applicable only to special compounds.
WET METHODS.
There are several methods in use which leave little to be desired either
in speed or accuracy. We will give (1) a gravimetric method in which the
estimation may be made directly by weighing the carbonic acid, or,
indirectly, by estimating the carbon dioxide from the loss; (2) a
volumetric one, by which an indirect determination is made of the gas;
and (3) a gasometric method, in which the volume of carbon dioxide given
off is measured, and its weight deducted.
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