Theoretical organic chemistry / by Julius B. Cohen.

  • Julius B. Cohen
Date:
1928
Catalogue details

Licence: In copyright

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

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    crop of crystals will be deposited; which will contain a larger pro¬ portion of the more soluble constituent. The mother-liquors from these will contain a still greater proportion of the more soluble constituent and so on. By a repetition of this process, which is termed fractional crystallisation, the mixture may be separated more or less completely into its constituents. Microscopic examin¬ ation will often show if the crystals are homogeneous or not by the difference in crystalline form. The process of crystallising requires practice and skill, and is one of the most important operations in organic chemistry. Sublimation.—Another method of purification which is occasion¬ ally employed, is sublimation. The process may be carried out in various ways. One method is to place the substance in a large watch-glass on a sand-tray which is heated by a small flame. The substance is covered with a sheet of asbestos-paper, held in position by a second inverted watch-glass or funnel. The volatile sub¬ stance sublimes on the asbestos-paper whilst the non-volatile com¬ pound remains on the watch-glass. Distillation in steam may be used occasionally for effecting the separation of solids and liquids. An experimental illustration of the process will be given later (p. 417). Melting-Point Determination.—It is well known that the pres¬ ence of a “ foreign ingredient ” lowers the melting-point of a substance. Fusible metals are made on this principle. If we made various mixtures of two substances A and B and plotted their melting-points as ordinates and the quantities as abscissae we should obtain a curve something like that in the accompanying Fig. 1. The melting-point of each would fall with successive additions of the second substance until it reached a minimum and would then rise until the second pure substance alone was present. Successive crystallisations would show by a change or otherwise in melting-point if the substance were pure. Slow liquefaction is an indication that the substance is impure for the following reason : on cooling a mixture of two substances, the one that predominates would separate until the mixture of minimum melting-point (eutectic point) is attained, when the whole would solidify. On heating a mixed solid the reverse occurs, and if the process takes place slowly some of the more fusible mixture will melt, leaving
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