The Still

If a glass flask is used it should be globular, because a flat-bottomed flask is liable to crack when heated with a naked flame. It should not be larger than is necessary for the amount of liquid to be distilled.

The Still-Head

The still-head should not be very narrow, or the thermometer may be cooled slightly below the temperature of the vapour. It is a good plan to seal a short length of wider tubing to the still-head near the bottom, leaving a sufficient length of the narrower tubing below to pass through the cork in the still, as shown in Fig. 3.

The still-head, as supplied by dealers, is often too short. It should, if possible, be long enough for the thermometer to be placed in such a position that not only the mercury in the bulb but also that in the stem is heated by the vapour of the boiling liquid ; otherwise a troublesome and somewhat uncertain correction must be applied (p. 11), and, if the distillation is not proceeding quite steadily, a little air may be carried back from time to time as far as the thermometer bulb and the temperature registered by the thermometer will then fluctuate and will, on the whole, be too low (p. 25).

Fig. 3. - Ordinary still, with Liebig's condenser.

Fig. 4. - Modified form of still with condenser.

The longer and wider the still-head and the higher the boiling point of the liquid distilled, the greater will be the amount of condensed liquid flowing back to the still. The lower end of the still-head should be wide enough to ensure that no priming takes place. With the bottom ground obliquely as in Fig. 6 a much narrower tube may be used than when the end is cut off horizontally (Fig. 3).

The Condenser

If the boiling point of the liquid to be distilled is higher than about 170°, the condensing tube should not be cooled by running water for fear of fracture. A long tube should be used and the cooling effect of the surrounding air will then be sufficient.

When a Liebig's condenser is used there is no advantage in having either the inner or the outer tube very wide ; an internal diameter of 7 or 8 mm. is sufficient for the inner, and of 15 mm. for the outer tube. If the outer tube is much wider it is unwieldy, and, when filled with water, it is inconveniently heavy. A mistake that is rather frequently made may be referred to here. It is usual to seal a short wide tube to the long, narrow condensing tube for the insertion of the delivery tube from the still-head. The tubes are often sealed together in such a way that when the distillation is proceeding a little pool of liquid collects at the junction (Fig. 5, a), and, in the fractional distillation of a small quantity of liquid, the error thus introduced may be serious. The fault is easily remedied by heating the wide tube close to the junction with the narrow one until the glass is soft, and then drawing it out very gently until it has the form shown in Fig. 5, b.

When a long still-head is used, it is advisable to bend the narrow tube just below its junction with the wider one, so that the condenser may be vertical in position instead of sloping gently downwards. Much less space is thus taken up on the laboratory bench, and the receivers are somewhat more conveniently manipulated.

The Source Of Heat

For laboratory purposes an ordinary Bunsen burner is usually employed. Wire gauze, asbestos cardboard, sand baths, or water or oil baths are not, as a rule, to be recommended because the supply of heat can be much more easily regulated without them, and a round-bottomed flask, if properly blown, is so thin walled that there is no danger of fracture when the naked flame is applied. The flask should be so placed that the flame actually comes in contact with the bottom of it; this is especially necessary when the liquid to be distilled is liable to "bump." Many substances, such as carbon disulphide, which boil quite regularly under the ordinary atmospheric pressure, bump more or less violently when the pressure is greatly reduced, unless special precautions are taken. Under a pressure of 361 mm. carbon disulphide boils at 25°, and if a quantity of it be distilled under this pressure with the flame placed some distance below the bottom of the flask, it may happen that the whole of the carbon disulphide will pass over without any ebullition whatever taking place. The liquid, however, in these circumstances, becomes considerably superheated, and if a bubble does form there will be a sudden and perhaps violent rush of the extremely inflammable vapour. If, however, the top of the burner be placed only about 2 mm. below the bottom of the flask, so that the minute flame touches the glass, ebullition will take place quietly and regularly.

Fig. 5. - Condensing tube of (a) faulty, (b) correct construction.

There are liquids which cannot be prevented from bumping in this way, and the best plan is then to add a few small fragments of porous porcelain [a clay pipe broken in small pieces answers the purpose very well] or pumice-stone, or both, or a number of small tetrahedra of silver or platinum. A method frequently employed is to pass a very slow current of air through the liquid, but a small error in the boiling point is thereby introduced. The explanation of this is given on p. 24. A suitable flask, described by Wade and Merri-man,1 is shown in Fig. 6. A water or oil bath need only be used when a solid substance is present in the flask, as, for instance, when a liquid is distilled over lime or phosphorus pent-oxide, or when the liquid is liable to decompose when heated with the naked flame.

It is customary to employ a water-bath for the distillation of ether, but it is doubtful whether this is necessary or even advisable except in the case of the ethereal solution of a solid substance or one that will not bear heating much above 100°. When an accident occurs it is almost invariably because, owing to "bumping," or to the distillation being carried on too rapidly, some of the vapour escapes condensation and comes in contact with a flame in the neighbourhood, generally that below the water-bath. If a naked flame were used the distillation could be much more easily regulated, and there would probably be really less danger than if a water bath were employed.