Theoretical organic chemistry / by Julius B. Cohen.

  • Cohen, Julius B. (Julius Berend), 1859-1935.
Date:
1902
Catalogue details

Licence: Public Domain Mark

Credit: Theoretical organic chemistry / by Julius B. Cohen. Source: Wellcome Collection.

    30/604 (page 10)
    separation, a portion of the less volatile liquid being carried over in the first distillate, together with the more volatile body, the temperature gradually rising throughout the distillation. In order to effect separation of the several substances, recourse is had to the method of fractional distillation. The liquid is dis¬ tilled in a round flask, which is surmounted with a fractionating column, holding the thermometer. Various forms of fractionating columns are used (Fig. 4.) The effect of the column may be explained as follows. The vapour given off from a mixture of liquids contains, as a rule, a larger proportion of the more volatile constituent than the liquid.1 If this vapour is partly condensed in its ascent, the vapour above this condensed liquid will be still richer in the more volatile constituent. If, by a series of constrictions or diaphragms, the condensed liquid is obstructed in its return flow, a momentary equilibrium between liquid and vapour is estab¬ lished at each diaphragm, and the longer the column the greater will be the amount of the more volatile constituent in the last portion of vapour to undergo condensation. This passes off by the condenser, and is collected in the receiver. By this means a partial separation is effected, and the portions distilling within a range of a small number of degrees are collected in separate flasks. Each of these portions or fractions is redistilled, and col¬ lected within still narrower limits of temperature, until at length the mixture is separated into certain portions, the boiling-points of which are nearly constant, and these may be regarded as pure. The following tables, I. and II., illustrate two series of frac¬ tional distillations of coal-tar naphtha containing a small quantity of paraffin boiling below 8o°; benzene, b.p. 8o~ ; toluene, b.p. 1 ioc ; and xylene, b.p. 140°. In the first fractionation (Table I.) the distillate is collected between every 5 degrees. TABLE I. A 71 5-85 B 85 -90 C 90-95' D 0 0 95 -100 E ’ IOO —105 F 105-1 IO° G 0 0 no -115 Residue. ' 19 c.c. 1 ' 1 53 c.c. 26 c.c. 15 c.c. 13 c.c. 17 c.c. 21 C.C. 33 c.c. l This does not include the case of a constant boiling mixture, whose vapour pressure is greater or less than that of its constituents. Vide J. Walker, Intro¬ duction to Physical Chemistry> chap, ix., p. 73 (Macmillan).