Practical organic and bio-chemistry / by R.H.A. Plimmer.

  • Robert Plimmer
Date:
1915
Catalogue details

Licence: Public Domain Mark

Credit: Practical organic and bio-chemistry / by R.H.A. Plimmer. Source: Wellcome Collection.

    29/660 (page 9)
    B. Fractional Distillation. A mixture of two or more miscible liquids may be separated from one another by fractional distillation if their boiling-points differ by 20-30. The mixture is distilled as described previously (p. 4), preferably froma flask with its side tube high in the neck and the thermometer is carefully watched. The first portion which distils will consist mainly or entirely of the more volatile constituent which has a higher vapour pressure ; the last portion will consist of the less volatile constituent having a lower vapour pressure or higher boiling-point. Between these portions there may be a small intermediate fraction consisting of a mixture of the liquids. The three portions or fractions are collected in separate receivers. B.g.— A mixture of equal volumes of chloroform and aniline will give on fractional distillation a fraction boiling at 61° (almost pure chloroform), an intermediate fraction, and a final fraction boiling at 183° (almost pure aniline), During the distillation of such a mixture when the boiling-point of a constituent exceeds 120° the water should be run out of the condenser. | | Re-distillation of the first and last fractions will : — sg give each of them in a state of purity. Ifa mixture of liquids contains constituents which have boiling-points fairly close to one another, the fractionation must be repeated several times until each fraction is found to have a constant boiling-point. The separation of such a mixture is greatly facili- tated by the use of a fractionating column or still-head. This is simply a device to lengthen the neck of the distilling flask so that the higher boiling fractions are exposed to the air and condensed before they reach the condenser and run back into the flask. Numerous forms have been invented; two efficient forms are those of Hempel and of Young (pear still-head, Fig. 4). The former consists of a’ glass tube filled with | glass heads and a side tube. The latter consists of a [ piece of glass tubing upon which are blown 2, 3, 4 or it more bulbs of a pear shape, and a side tube. The liquid is placed in a round-bottom flask, the fractional column inserted and this in turn connected by its side tube to the condenser. The mixture in the flask, to which several small pieces of Fic. 4.