QUESTIONS ON CHAPTER IV. VAPORIZATION. phere? What is vaporization? What is sublimation? What is meant by ebullition or boiling? What is meant by the boiling-point of a liquid? Is the boiling-point of a liquid a definite temperature? Is the evaporating-point of a liquid a definite temperature? What is the boiling-point of water under the ordinary pressure of the atmos When confined under pressure, how high can its temperature be raised without boiling? What will be the effect upon boiling if the pressure is made much less than that of the atmosphere? How may the boiling-point of a liquid be determined accurately? What effect have cold and pressure upon vapors? What effect have heat and removal of pressure upon vapors? Upon what does the quantity of vapor that will form in a confined space depend? How is the evaporation of a liquid influenced by the pressure or absence of aqueous vapor in the air? How may the rapidity of evaporation be increased? Why does increase of temperature hasten evaporation? In evaporation by boiling, temperature, pressure, etc., being equal, upon what does the rapidity of the process depend? When a pure volatile liquid is evaporated by boiling in the open air, does the temperature change with the amount evaporated? If there is solid matter dissolved in the liquid, what takes place on evaporation? What inference should be drawn from this,-for example, in the preparation of extracts? Which are most easily evaporated,-thin, mobile liquids or dense and thick ones, and why? Does the depth of a liquid influence its boiling-point? Why are rough metallic surfaces better for evaporation than smooth ones? In evaporating liquids below the boiling-point, temperature, pressure, etc., being equal, upon what does the rapidity of the process depend? Therefore, what shaped vessels should be used for evaporating liquids below the boiling-point? What effect is produced by stirring an evaporating liquid? Will water boil at a higher or lower temperature in more elevated positions, and why? What application is made of this principle in pharmacy? What is an evaporating chamber, and how should it be constructed? What is the objection to evaporating liquids by direct heat? In evaporating a liquid to a definite measure, how may it be ascertained when the measure has been reached? What is a hood, and what is its use? What is a grommet, and what are its uses? 10 CHAPTER V. DISTILLATION. THE first part of the process of distillation is identical with that of evaporation, for it is simply the vaporization of the volatile liquid through the application of heat. The next step is distinctive and opposite, and consists in the conversion of this vapor into a liquid by the application of cold: this part of the process is called condensation. The elements of distillation are: 1, vaporization, and, 2, condensation. The subject of vaporization was treated of in the preceding chapter: we have now to consider condensation. Condensation. It has been already shown (page 128) that when a liquid is vaporized a certain amount of heat disappears or is rendered latent, and when a vapor is liquefied a corresponding degree of heat is evolved or reappears. Whilst the practical application of this law is of great service in the use of steam for heating purposes, the opposite process of condensation shows its disadvantages, because of the relatively large quantity of cold water necessary to liquefy vapors. The greater the difference in temperature between the condensing surface and the vapor, the more rapid is the condensation; and it has been computed that steam at 100° C. (212° F.) requires about twentyfive times its weight of water at 20° C. (68° F.) to condense it. The proper relation between the heating and condensing surfaces of apparatus used in distillation can only be known by careful study of the laws governing vaporization and condensation, or by practical experience. Apparatus used in Distillation.-In considering the many kinds of apparatus used in distillation, two typical forms are presented : 1, the alembic form, in which the vapor is condensed in an enclosed space immediately above the heated liquid, and, 2, the retort form, in which the vapor is condensed in a vessel placed at one side of that containing the heated liquid, and connected with it by a suitable tube or pipe. The Alembic,-This is probably the most ancient kind of distillatory apparatus, and in its original form it is now rarely employed. The body, or cucurbit, is usually globular or oval in shape, and at its junction with the hemispherical head or dome there is a gutter or groove. This serves to collect the condensed vapor or distillate, which is carried off by a tube, as shown in the illustration (see Fig. 134). Fig. 135 shows an alembic which, according to Mr. Brady, of Newcastle, England, is still frequently employed in Japan. Into the boiler, a, is fitted a short cylinder, b: this has a perforated bottom, and the gutter communicates with the spout; the condenser, c, has a cover, d, and is also furnished with a spout for carrying off the water used in cooling the doine after it has been heated by the vapors rising from the boiling liquid in the body. This still, or, as it is called by the Japanese, lambik, is shown because it represents the alembic probably in one of its best practical forms, and the advantages possessed by this kind of distilling apparatus are seen to be compactness and simplicity. It is, however, not fitted for distilling very volatile liquids, is very inconvenient and troublesome, because of the necessity for constantly replacing the water used in the condenser, and distillation is slow and tedious. The Retort.-This form of distillatory apparatus has many advantages over the alembic, which it has almost entirely replaced. A retort, in its FIG. 136. simplest form, may be described as a long-necked glass flask in which the neck, after being heated thoroughly, is bent over until its axis makes an acute angle with that of the bowl of the flask. Retorts are of two kinds,-plain and tubulated. A plain retort has just been described (see Fig. 136). If it has a tubulure or orifice at the top of the bowl for the purpose of introducing the liquid to be distilled, it is said to be tubulated (see Fig. 137). Retorts are made of glass, porcelain, earthenware, platinum, iron, lead, etc., according to the purposes for which they are designed. Plain retort. Porcelain and earthen-ware retorts are used in the distillation of phosphorus, mercury, etc.; platinum and iron retorts, in destructive distillation; leaden retorts, in making hydrofluoric acid, ether, etc. FIG. 137. The glass retort is the only kind, however, which will be considered here in detail. The shape of a retort is an important point to regard in its selection: retorts having deep bowls are best suited for very volatile liquids. The lower surface of the neck of the retort should form a decidedly acute angle with the surface of the bowl if tubulated; a line drawn from the centre of the stopper should touch the centre of the bowl, A, so that when a funnel is introduced into the tubulure, T, to charge the retort, the contents may all be delivered into the bowl without soiling or splashing the neck. The neck of the retort should gradually taper to the end, and the beak, B, should never be larger in diameter than any other A Tubulated retort. B portion of the neck, otherwise difficulty may be experienced in making joints with adapters, receivers, or condensers (see Fig. 138). Such a fault may prevent the use of a cork ring in joining, for this ring should be made tight by forcing it up the gradually-increasing diameter of the neck. The glass forming the retort should be carefully annealed and have a uniform thickness: extremes should be carefully guarded against. If the glass be too thick, the sudden changes in tem FIG. 138. Badly-formed retort. perature, to which retorts are constantly subjected in distillation, may cause fracture; on the other hand, if too thin, they are easily broken in cleaning. Scratches and imperfections in the bowl should cause retorts to be rejected. The objections to the use of retorts are mainly due to 1 Although there is no disposition on the part of the author to weaken the force of this conventional advice, the possession of a retort having a large bubble in the bottom of the bowl, which has been in successful use for ten years and has outlasted many of its more perfect fellows, supplies the needed exception. their peculiar shape, the principal one being the difficulty of cleaning FIG. 189. Distilling flask. them thoroughly. The necessity for having a variety of different shapes and sizes to suit special operations is greater than when distillation is performed with flasks and bent tubes. Flask distillation will be usually found most practicable for pharmaceutical work. The best shape for a flask is shown in Fig. 139. The bowl should have a flat bottom, so that it will stand unsupported; the neck should be wide, to admit a large cork, so that there will be plenty of room for a wide bent tube, a thermometer, and a safety or charging tube (see Fig. 140). The advantages of using a flask instead of a retort for all distillations that will allow of it are several: it is FIG. 140. easily cleaned, it is useful for other purposes, for instance as a measure, as a container, or as a receiver for the distillate, and the parts are readily replaced in case of breakage. One of the most important parts of this apparatus is the bent tube for conducting the vapor to the receiver or condenser: the diameter of the tube should be as large as the receiver or condenser will conveniently accommodate. Cutting Glass Tubes.-The glass should be moderately thick and of the proper length. A glass tube may be broken neatly by scratching it across with a sharp threecornered file, and then grasping it with both hands, one on each side of the scratch, and making a slight outward pressure, which will generally produce a clean fracture: by holding the sharp edges for a few moments in the flame the corners Bent tube, etc., for distilling flask. |