Protection Of Flame From Draughts

In order that satisfactory results may be obtained it is necessary that the distillation should proceed with great regularity, and the heat supply must therefore not be subject to fluctuations. The most important point is to guard against draughts, and, to do this, the ordinary conical flame protector may be used, or a simple and efficient guard may be made from a large beaker by cutting off the bottom and taking a piece out of the side (Fig. 7).

1 "Apparatus for Fractional Distillation at Pressures other than the Atmospheric Pressure." Trans. Chem. Soc, 1911, 99, 994.

Fig. 6.

Electrical Heating

For many purposes, notably for distillation under reduced pressure, it is convenient to employ an electrically heated coil of platinum wire as the source of heat. T. W. Richards and J. H. Matthews 1 strongly recommend this method of heating, and consider that electrical heating gives slightly better separation and far less superheating than ordinary flame heating.

In order that the fine platinum wire may still be completely immersed when the quantity of liquid in the flask has become very small, Richards and Matthews recommend a vessel of the form shown in Fig. 8. The coil actually employed consisted of about 40 cm. of platinum wire and had a resistance of 0-7 ohm. A current of ten to fifteen amperes was passed through the coil, the ends of which were sealed into two glass tubes in which were stout copper wires. Connection between the platinum and copper wires was effected by means of a little mercury.

A similar method was recommended by Beckmann.2

H. S. Bailey 3 makes use of a flask which is so narrow at the bottom that 90 per cent of the liquid can be distilled with the coil still completely immersed. Bailey recommends a coil of German silver or nichrome wire instead of platinum.

Rosanoff and Easley4 wind the heating wire on a glass rod bent in the form of a conical spiral, the apex of which extends almost to the bottom of the vessel.

W. R. G. Atkins finds it convenient for many purposes to employ an electrically heated metal plate, placed a little distance below the flask. The amount of heat reaching the flask is regulated by moving a piece of asbestos pasteboard over the heated plate. Superheating of the vapour, when the amount of liquid has become small, can be prevented by allowing the flask to rest on a sheet of asbestos pasteboard with a circular hole cut in the centre, so that the heat only reaches a small area at the bottom of the flask.

1 Richards and Matthews, "Electrical Heating in Fractional Distillation," J. Amer. Chem. Soc., 1908, 30, 1282 ; 1909, 31, 1200 ; Zeilschr. physik. Chem., 1908, 64, 120.

2 Beckmann, "Erfahrungen uber elektrisches Heizen bei ebullioskopischen Bestimmungen und bei der fraktionierten Destination," Zeitschr. physik. Chem., 1908, 64, 506.

3 Bailey, "An Electrically Heated Vacuum Fractionation Apparatus," J. Amer. Chem. Soc, 1911, 33, 447.

4 Rosanoff and Easley, "Partial Pressures of Binary Mixtures (Apparatus)," ibid., 1909, 31, 964.

Fig. 7. Simple flame protector.

Fig. 8.

Allen and Jacobs 1 encase the distillation flask in the two halves of a pear-shaped mould on the inner side of which is wound a resistance wire which is suitably heated electrically.

Steam As Source Of Heat

On the large scale the still is frequently heated by steam under ordinary or increased pressure (Fig. 9). The steam may be introduced through the pipe a, and the condensed water run off at b (see also pp. 331, 398).

The Thermometer

In carrying out a fractional distillation one must be able, not only to read a constant or nearly constant temperature with great accuracy, but also to take readings of rapidly rising temperatures. These requirements are best fulfilled by the ordinary mercurial thermometer, which is therefore, notwithstanding its many drawbacks, used in preference to the air or the platinum resistance thermometer. If accurate results are to be obtained the following points must be attended to.

1. Calibration. - The thermometer must be carefullv calibrated, and it would be a great advantage if all thermometers were compared with an air thermometer, for two mercurial thermometers, constructed of different varieties of glass, even if correct at 0° and 100°, will give different and incorrect readings at other temperatures, more especially at high ones, for various reasons :

(a) In the first place, it is impossible to obtain an absolutely cylindrical capillary tube, and therefore the volume corresponding to a scale division cannot be quite the same in all parts of the tube. Various methods have been devised for calibrating the stem,23 but even when this is done there remain other sources of error.

(b) The position of the mercury in the stem at any temperature depends on the expansion both of the mercury and the glass, and, for both substances, the rate of expansion increases with rise of temperature.

(c) Different kinds of glass have different rates of expansion, so that two thermometers made of different materials - even if the capillary tubes were perfectly cylindrical - would give different readings at the same temperature. It is therefore necessary to compare the readings of a mercurial thermometer with those of an air thermometer, or of another mercurial thermometer which has previously been standardised by means of an air thermometer. Or, instead of this, a number of fixed points may be determined by heating the thermometer with the vapours of a series of pure liquids boiling under known pressures.

1 Allen and Jacobs, " Electrically Heated Still for Fractional Distillation," Dept. of Inter., Bur. of Mines, U.S.A., Bull. 19, 1 ; J. Soc. Chem. Ind., 1912, 31, 18.

2 " Methods employed in Calibration of Mercurial Thermometers," British Association Report for 1882, 145.

3 Guillaume, " Traite pratique de la thermometrie de precision," p. 112.

Fig. 9. - Still with steam jacket.

Table 1 contains a list of suitable substances with their boiling points, and the variation of temperature for a difference of 10 mm. from the normal atmospheric pressure.