CHAPTER IV.

VAPORIZATION.

UNDER this head will be included those pharmaceutical operations in which volatile substances are separated from fixed bodies, or from others which are less volatile, by the action of heat at varying temperatures. Vaporization is frequently employed in pharmacy, and it will be most convenient to consider its applications in the order of their importance: 1. To Liquids. 2. To Solids.

1. When vaporization is used to separate a volatile liquid from a less volatile liquid, it is called evaporation.

2. When the object sought is the volatile liquid, it is called distilla

tion.

3. When it is used to separate a volatile liquid from a solid, it is called desiccation, exsiccation, or granulation.

4. When it is used to separate a volatile solid from another body, it is called sublimation.

The following diagram may serve to impress the definitions on the

The subjects of Evaporation, Distillation, Sublimation, and Desiccation will be considered in the chapters which immediately follow. Vaporization, as applied to Granulation and Exsiccation, will be more appropriately considered after the chapters on Solution and Crystallization.

EVAPORATION.

Although this term has in its more popular sense the signification of the separation of moisture from any body, whether solid or liquid, in pharmacy the word has a more restricted meaning, and signifies the driving off of the more volatile or less valuable portions of a liquid by the application of heat, with the object of purifying it or obtaining the less volatile portion. Illustrations are found in the concentration of syrups and liquids intended for crystallization, and in the treatment of weak tinctures in making fluid-extracts and extracts.

As ebullition, or boiling, is an important form of evaporation, it will be necessary first to consider the essential points concerned therein. Ebullition in a heated liquid is caused by the formation of bubbles of vapor upon the surface of the vessel, which, rising to the surface of the liquid and bursting, permit the vapor to become diffused in the space above the boiling liquid. The boiling-point of a liquid may be defined as the temperature at which the tension of its vapor is equal to the pressure of the atmosphere, this point being definite, whilst evaporation takes place in the same liquid at nearly all degrees of FIG. 113. heat, and hence the evaporating point is an indefinite temperature. The point at which a liquid boils varies with the liquid, and in the U. S. Pharmacopoeia and other authoritative works the boiling-point is frequently considered an important test in establishing the identity or purity of a liquid. The table at the end of this chapter shows the boiling-point of the official liquids arranged in order, beginning with the

lowest.

The boiling-point of a liquid is affected by the cohesion of the liquid and the degree of pressure upon its surface. Water under the ordinary pressure of the atmosphere boils at 100° C. (212° F.). When confined in a steam boiler it has been shown that water can have a temperature considerably over 200° C. (392° F.) without boiling, the bubbles being prevented from rising to the top on account of the pressure of the steam in the confined space above the liquid. On the other hand, the removal of pressure causes a liquid to boil below its normal boiling-point, as will be explained in the chapter on vacuum apparatus. The character of the vessel in which a liquid is boiled has also a slight effect in modifying the boiling-point. (See Evaporation by Boiling.)

Determination of Boiling-Points.-One of the simplest methods of ascertaining the boiling-point of a liquid is illustrated in the cut (see Fig. 113). The liquid is introduced into a test-tube, and a glass tube is selected of such diameter as will permit a tube-thermometer to pass easily through it and leave a small space between; the tube should be about one inch shorter than the thermometer. A short piece of wire should be passed through the glass ring of the thermometer, and slightly bent to hold it in position; a perforated cork should now be fitted tightly to the test-tube, and the tube carrying the thermometer-tube pushed through the perforation in the cork until the bulb of the thermometer is just above the liquid; heat should be applied cautiously by a sand-bath or Boiling-point water-bath. The vapor from the boiling liquid passes upward through the whole length of the thermometer, escaping at the top, and thus the error common to some methods, due to the difference in temperature between the portion of the thermometer in the test-tube and that outside of the test-tube, is measurably avoided.

test.

Tension of Vapors.-If a glass tube, thirty-six inches long, closed at one end, is filled with mercury, and the open end, after closing it

with the finger, carefully inverted in a beaker containing mercury, it will be found that the mercury will run out from the tube into the beaker until a column of mercury about thirty inches in height is left: this column is sustained by the pressure of the atmosphere, and is, in fact, the well-known mercurial barometer-tube: the six inches of space in the tube above the level of the mercury is of course empty, or vacuous. Now, if a few drops of water are passed into the tube by a dropper, they immediately rise to the level of the mercury in the tube, and, although the temperature has not been increased, a portion of the water is vaporized, and the column of mercury is proportionately depressed: this depression is due to the elasticity or tension of the aqueous vapor. If the tube be forcibly pushed down into the mercury, the increased pressure will be found to have liquefied the vapor, and the original quantity of water is recovered; but the depression in the column of mercury may be increased by heat, and when a sufficient amount of heat has been applied to the tube to expel the mercury until none is left in the tube, it will be found that the temperature marks 100° C. (212° F.), which is exactly the boiling-point of the liquid (water), showing that this point must be reached in order to overcome the pressure of the atmosphere. If alcohol or ether be substituted for water, it will be found that the mercury will be depressed in a far greater ratio,-this being due to the greater volatility and lower boiling-point of these liquids. The maximum density of the vapor of a volatile liquid in a confined space in contact with the corresponding liquid is reached when its elastic force attains the limit beyond which pressure produces the liquefaction of the vapor. When this limit is reached, the vapor is said to be saturated: maximum density varies with the temperature. If a saturated vapor in an enclosed tube is not in contact with an excess of liquid, increase of temperature lowers its density or expands it. On the other hand, when a saturated vapor is cooled, liquefaction gradually takes place, the vapor above the liquid remaining in the condition of maximum density until converted into the liquid: so that cold and pressure have the effect of converting vapors into liquids, whilst heat and the removal of pressure have the reverse effect,-i.e., the conversion of liquids into vapors. The phenomena above described characterize evaporation into a space filled with air as well as evaporation into a vacuum, the only difference being that more time is required to produce the same effects when evaporating in contact with air, for volatile liquids are instantly converted into vapor in a vacuum, while the presence of air retards, but does not prevent, vaporization. A consideration of the foregoing facts leads to the following deductions:

1. The quantity of vapor that will form in a confined space depends upon the amount of pressure and heat to which the liquid is subjected; and when the point of maximum density of the vapor is reached, evaporation ceases if the pressure and temperature remain the

same.

2. The rapidity of evaporation of an aqueous liquid in the open air is influenced by the condition of the aqueous vapor always present in the air. If it has the greatest density possible for the degree of heat, evaporation is retarded; but if the aqueous vapor in the atmosphere

is much below the state of maximum density, as is usually the case, evaporation is promoted.

3. Rapidity of evaporation is increased by removing the pressure of the atmosphere.

4. Increase of temperature obviously accelerates evaporation, by increasing the formation of vapor.

FIG. 114.

Evaporation of Liquids by Boiling.-In evaporating by boiling, temperature, pressure, etc., being equal, the rapidity of the process depends upon the extent of surface exposed to the heat. Fig. 114 represents a profile view of two evaporators, A and B. The corrugated bottom of A gives twice as much surface as the smooth bottom of B, and hence if the same quantity of a liquid is made to boil in each, at the same temperature, the bubbles of vapor given off from the corrugated bottom will be Evaporation by boiling. twice as numerous as those formed on the plain bottom.

The superiority of tubular boilers over the ordinary plain or Cornish boiler also affords a good illustration of this fact (see Fig. 105).

When a pure, volatile liquid is heated to the boiling-point in the open air, its temperature remains the same until the whole of the liquid has evaporated. If, on the other hand, solid matter is dissolved in the liquid, the temperature of the solution is gradually increased until saturation is reached this fact is well illustrated by considering boiling-points of saturated solutions of various salts (see page 127), and it shows the importance of diminishing the heat in the evaporation of solutions of organic substances as evaporation progresses, as, for example, in the making of extracts, etc.

The cohesion of a liquid affects its boiling-point, dense, thick, and sticky liquids offering more resistance to the escape of the bubbles of vapor than rare, mobile, or thin liquids.

The relative depth of liquid also influences the boiling-point. Shallow vessels favor ebullition, because they afford proportionally less weight of liquid above the bottom of the dish for the bubbles to escape through than deep ones. Rough metallic surfaces favor evaporation by boiling, and are better than smooth surfaces, because they expose a greater amount of surface to the source of heat.

FIG. 115. FIG. 116.

Evaporation below the Boiling-Point.-In evaporating liquids below their boiling-point, temperature, pressure, etc., being equal, rapidity of evaporation depends upon the extent of surface exposed to the air. Figs. 115 and 116 show two vessels of exactly the same diameter, but of different capacity, containing water: both expose the same amount of surface to the air, but that of Fig. 116 contains eight times more liquid than that of Fig. 115.

If both be subjected to the same temperature, provided it be below 100° C. (212° F.),

Evaporation below the boiling

point.

the water will evaporate as rapidly from one as from the other. Proper Shape of Vessels for Evaporating Liquids.-Broad,

shallow vessels should be used for evaporating below the boiling-point, because the extent of surface is proportionally greater in vessels of this shape. Fig. 117 is an illustration of a porcelain evaporating dish having

the proper shape: the chief objection to dishes of this kind is their liability to breakage. Care should be taken to dry the bottom of the dish

FIG. 119.

thoroughly before placing it over a gas-flame. A glass evaporating dish is shown in Fig. 118. This should always be used in a sandbath, or should be otherwise protected from direct heat. Enamelled cast-iron dishes are very useful, notwithstanding the lack of durability of the enamel. Enamelled sheet-iron dishes, called "agate-ware," are very light, and are much more lasting than the ordinary enamelled cast-iron dishes (see Fig. 119).

Use of Stirrers.-By stirring an evaporating liquid the surface is largely increased, whilst the currents of air produced at the same time greatly assist in dissipating the vapors which rise. Upon the small

scale, porcelain, horn, or wooden stirrers are used (see Figs. 120, 121, and 122), whilst mechanical stirrers are usually employed in the labo