Part 19 (1/2)

[Ill.u.s.tration: FIG. 71.]

[Ill.u.s.tration: FIG. 72.]

Active dimethyl tartrate melts at 43.3; racemic dimethyl tartrate at 89.4. Active mandelic acid melts at 132.8; the racemic acid at 118.0. In the one case, therefore, the racemic compound has a higher, in the other a lower melting point than the active forms. {219}

In the case of partially racemic compounds (_i.e._ the compound of a racemate with an optically active substance) the type of curve will be the same, but the figure will no longer be symmetrical. Such a curve has been found in the case of the l-menthyl esters of d- and l-mandelic acid (Fig.

72).[300] The freezing point of l-menthyl d-mandelate is 97.2, of l-menthyl l-mandelate 77.6, and of l-menthyl r-mandelate 83.7. It will be observed that the summit of the curve for the partially racemic mandelate is very flat, indicating that the compound is largely dissociated into its components at the temperature of fusion.

III. _The inactive substance is a pseudo-racemic mixed crystal._

In cases where the active components can form mixed crystals, the freezing-point curve will exhibit one of the forms given in Fig. 65. The inactive mixed crystal containing 50 per cent. of the dextro and laevo compound, is known as a pseudo-racemic mixed crystal.[301] So far, only curves of the types I. and II. have been obtained.

Examples.--The two active camphor oximes are of interest from the fact that they form a continuous series of mixed crystals, _all of which have the same melting point_. The curve which is obtained in this case is, therefore, a straight line joining the melting points of the pure active components; the melting point of the active isomerides and of the whole series of mixed crystals being 118.8.

[Ill.u.s.tration: FIG. 73.]

In the case of the carvoximes mixed crystals are also formed, but the equilibrium curve in this case exhibits a maximum (Fig. 73). At this maximum point the composition of the solid and of the liquid solution is the same. Since the curve must be symmetrical, this maximum point must occur in the case of the solution containing 50 per cent. {220} of each component, which will therefore be inactive. Further, this inactive mixed crystal will melt and solidify at the same temperature, and behave, therefore, like a chemical compound (p. 187). The melting point of the active compounds is 72; that of the inactive pseudo-racemic mixed crystal is 91.4

Transformations.--As has already been remarked, the conclusions which can be drawn from the fusion curves regarding the nature of the inactive substances formed hold only for temperatures in the neighbourhood of the melting points. At temperatures below the melting point transformation may occur; _e.g._ a racemate may break up into a _dl_-mixture, or a pseudo-racemic mixed crystal may form a racemic compound. We shall at a later point meet with examples of a racemic compound changing into a _dl_-mixture at a definite transition point; and the pseudo-racemic mixed crystal of camphoroxime is an example of the second transformation.

Although at temperatures in the neighbourhood of the melting point the two active camphoroximes form only mixed crystals but no compound, a racemic compound is formed at temperatures below 103. At this temperature the inactive pseudo-racemic mixed crystal changes into a racemic compound; and in the case of the other mixed crystals transformation to racemate and (excess of) active component also occurs, although at a lower temperature than in the case of the inactive mixed crystal. Although this behaviour is one of considerable importance, this brief reference to it must suffice here.[302]

3. _Alloys._

One of the most important cla.s.ses of substances in the study of which the Phase Rule has been of very considerable importance, is that formed by the mixtures or compounds of metals with one another known as alloys. Although in the investigation of the nature of these bodies various methods are employed, one of the most important is the determination of the character of the freezing-point curve; for from the form of this, valuable information can, as we have already learned, be {221} obtained regarding the nature of the solid substances which separate out from the molten mixture.

Although it is impossible here to discuss fully the experimental results and the oftentimes very complicated relations.h.i.+ps which the study of the alloys has brought to light, a brief reference to these bodies will be advisable on account both of the scientific interest and of the industrial importance attaching to them.[303]

We have already seen that there are three chief types of freezing-point curves in systems of two components, viz. those obtained when (1) the pure components crystallize out from the molten ma.s.s; (2) the components form one or more compounds; (3) the components form mixed crystals. In the case of the metals, representatives of these three cla.s.ses are also found.

1. _The components separate out in the pure state._

In this case the freezing-point curve is of the simple type, Fig. 63, I.

Such curves have been obtained in the case of a number of pairs of metals, _e.g._ zinc--cadmium, zinc--aluminium, copper--silver (Heyc.o.c.k and Neville), tin--zinc, bis.m.u.th--lead (Gautier), and in other cases. From molten mixtures represented by one branch of the freezing-point curve one of the metals will be deposited; while from mixtures represented by the other branch, the other metal will separate out. At the eutectic point the molten ma.s.s will solidify to a _heterogeneous mixture_ of the two metals, forming what is known as the _eutectic alloy_. Such an alloy, therefore, will melt at a definite temperature lower than the melting point of either of the pure metals.

{222}

In the following table are given the temperature and the composition of the liquid at the eutectic point, for three pairs of metals:--

-------------------------------------------------------------------

Temperature.

Composition of liquid.

------------------------------------------------------------------- Zinc--cadmium

264.5

73.5 atoms per cent. of cadmium.

Zinc--aluminium

380.5

11 ” ” aluminium.

Copper--silver

778

40 ” ” copper.