CHAPTER IV.

APPLICATIONS OF HEAT IN VARIOUS OPERATIONS.

Under this head, those operations which require a high degree of temperature will be considered first.

USE OF THE BLOWPIPE.-A knowledge of the uses of the blowpipe is important, because of its various applications in analytical chemistry. For a thorough treatise upon this subject, the student is referred to Plattner's " Blowpipe Analysis." In using the blowpipe (Fig. 93), a constant and uniform blast of air must be kept

up through the tube by the operator, who must use the muscles of his cheeks as bellows, keeping them constantly distended, supplying air as needed. This blast of air passing through the dark cone of any illuminated flame, produces the same effect as that of the Bunsen flame, only that the flame becomes long and pointed and of a very high temperature, being concentrated on a very small amount of space. The flame (Fig. 92) is divided into

two parts, the inner or deoxidizing zone at a a (or a, Fig. 94), for here, where some of the gas is yet unoxidized or unconsumed, oxygen is removed from metallic oxides. The outer zone at e (or b, Fig. 94)

FIG. 94.

Blowpipe Flame.

is called the oxidizing flame, for by the high temperature and free access of oxygen, metals are easily oxidized. When a flame of very high temperature is desired, the Blastlamp (Figs. 77-78) is best employed.

OPERATIONS WITH THE CRUCIBLE.-Crucibles are cup-shaped vessels of round or triangular shape, made of such material that they will stand extremely high temperatures. They are employed in carrying on the process of ignition or smelting. For large operations the Hessian or graphite crucible is employed. The former is somewhat porous, hence only adapted for crude work; the latter withstands a very high degree of heat and rapid changes of temperature without fracture. For general operations of the chemist or apothecary, the porcelain, platinum, silver, or

Crucibles. a, b. Hessian. c. Graphite. d. Platinum. e, f. Porcelain. g. Clay. h. Copper.

nickel crucible is employed. Porcelain crucibles are best adapted for general use, because of their cheapness, and also because less care is necessary to keep them in order. Moist substances should never be heated in such a crucible over the naked flame; the

substance should be first dried by placing the crucible and moist contents on a water bath or in a drying oven, and when thoroughly dry, placing the substance in the crucible and the latter on a pipe-stem triangle and carefully heating, increasing gradually to redness. These crucibles should not be employed for fusing caustic alkalies. Platinum crucibles are best adapted for all kinds of work; wet precipitates may be thrown in and ignited at once. They will stand an intense degree of heat, and may be also rapidly heated and cooled. Mixtures that give off free chlorine or contain lead compounds should not be heated in them.* They should be cleansed from time to time by scouring with moist rotten stone; sand should not be used. Silver or nickel crucibles are best adapted for fusing caustic alkalies. Care should be observed not to heat too strongly, for there is danger of melting the silver.

The following operations require a high degree of heat.

FIG. 96.

IGNITION. Ignition is employed in analysis to remove the last portions of moisture, or of organic or volatile constituents, from inorganic substances, in order to convert the latter into stable weighable compounds. The filter containing the precipitate is folded, laid in the crucible, which is then placed on the triangle and heated slowly, until the filter is reduced to ash; then the heat is increased to dull redness, in which case the blast lamp is necessary; when sufficiently cool it is placed in the desiccator, being handled with a pair of crucible tongs. In the early stage of the operation the crucible lid is removed, until all carbonaceous matter is burned off; then the cover is replaced and strong heat applied.

Examples:-A solution of ferric salt is precipitated by an alkali, and the precipitate washed and dried; ignition then converts the ferric hydrate into ferric oxide, in which condition it is weighed. Fe(OH), Fe2O3 + 3H2O. Phosphoric acid is precipitated as magnesium ammonium phosphate (2MgNH,PO, + 6H2O), this on ignition vields 2NH,, 7H2O and magnesium pyrophosphate, Mg2P2O7. The latter, a stable compound, is weighed.

=

FUSION. This is liquefaction of solids by heat. We fuse those solids which liquefy and do not suffer chemical change by heat. In analytical operations, we fuse silver chloride, to preclude the presence of any moisture. Caustic potash and soda are fused to

Also caustic alkalies, cyanides, or metallic salts mixed with organic matters.

remove all traces of moisture, then moulded into sticks for the sake of convenience.

CALCINATION.-Some inorganic substances are strongly heated (until of constant weight), to remove some volatile constituent, as water of hydration or carbonic acid gas.

=

Example.-Limestone or marble, on being strongly heated, yields calcium oxide (burnt lime). CaCO, CaO + CO2. Magnesium carbonate (MgCO3), Mg(OH)2, when calcined, yields magnesia (MgO), water, and carbonic acid gas.

DEFLAGRATION.-The subjecting of inorganic salts to strong heat, whereby decrepitation takes place, with the giving off of

oxygen.

Example:NaNO, 2KCI +302 Sodium Nitrate. Sodium Nitrite. Oxygen. Potassium Chlorate. Potassium Chloride. Oxygen.

NaNO + 0.

2KCIO =

TORREFACTION.—The subjection of drugs to roasting, whereby an alteration in their properties and constituents takes place.

Example.-Roasting of coffee. Rhubarb possesses cathartic properties, but on roasting, the cathartic principles are destroyed and an astringency is developed.

CARBONIZATION.-The subjection of organic substances to strong heat, out of contact with air.

Example.-Wood charcoal and animal charcoal. When these are burnt in open air, ashes result.

CHAPTER V.

VAPORIZATION.

OPERATIONS REQUIRING A LOWER DEGREE OF HEAT. The slow conversion of a liquid into the gaseous condition is designated by the general term-Vaporization. As applied in pharmacy, when it is intended to separate a volatile liquid or solvent from a solid, or a more volatile from a less volatile liquid, or for the purpose of concentrating a liquid, the process is calledEvaporation. When the volatile portion or portions are sought for, it is called-Distillation.

When a volatile is to be separated from a non-volatile solid, it is called-Sublimation.

When solids are deprived of moisture at a low temperature, it is called-Desiccation.

When crystalline salts are deprived of their water of crystallization by means of heat, it is called-Exsiccation.

EVAPORATION.

In pharmaceutical operations, we resort to evaporation for the concentration of liquids, the collection of a dissolved body (extracts), or for the purposes of crystallization. For pharmaceutical or technico-chemical purposes, the principal object to be considered, is the saving of time and fuel. In quantitative analysis this is not considered, care and attention being directed to guard against loss or contamination of the substance operated

upon.

According to the nature of the solvent and dissolved body, the method of evaporation may vary, as to the vessel in which it is carried on, or the nature of the source of heat. The rapidity of evaporation depends upon :

1st. The amount of surface of the liquid exposed. 2d. The temperature of the liquid.

3d. The nature of the liquid and dissolved body.

4th. The atmospheric pressure, also the atmospheric humidity. 5th. The nature of the vessel.

1st. The larger the amount of surface exposed to the atmosphere, the more rapid is the rate of evaporation, for it is evident, that a liquid, contained in a shallow evaporating dish, will evaporate more rapidly if exposed to the same temperature, than the same quantity contained in a deep narrow vessel. Shallow vessels favor ebullition, since there is a lesser weight of liquid above the source of heat, thereby affording less resistance to the