CHEMICAL MICROSCOPY 



eutectic composition will crystallize rapidly without appreciable change in velocity as 

 throughout the mixing zone. The appearance the preparation is cooled. The resultant solid 

 of a eutectic zone is quite characteristic and crystals will appear homogeneous through 

 easily recognized once the observer is famil- the preparation and the polarization colors 

 iar with the phenomenon. will be uniform. On heating, the entire prep- 

 Molecular addition compound formation aration will melt at the same temperature as 

 is readily observed in that a third solid the two starting components. Small amounts 

 phase may be seen to crystallize in the mix- of impurities modify the behavior only 

 ing zone of the two-component system. One slightly. If the two components are not 

 or two eutectic zones are also observed, de- identical, there will normally be a marked 

 pending on whether the addition compound depression of the melting point in the mixing 

 melts incongruently or congruently. The zone. 



various types of solid solution are also read- Frequently, if a contact preparation be- 

 ily detected on solidification of the melt, tween an unknown and an easily supercooled 

 With solid solution, both component A material such as thymol is allowed to digest 

 and component B crystals are seen to grow on the hotbar, the unknown will develop well 

 through the mixing zone with a change in defined crystal faces in the mixing zone. It 

 growth velocity but without the appearance is then possible to measure accurately such 

 of an area of eutectic composition or a mo- quantities as profile angles, extinction an- 

 lecular addition compound. If there is partial gles, dichroisom, and refractive index rela- 

 miscibility of liquid A and liquid B, the two tive to the melt for identification purposes, 

 liquid phases may be seen in the mixing It is also sometimes possible, with the proper 

 zone. choice of second component, to nucleate un- 

 Observations made during heating of a stable polymorphic forms of a given com- 

 contact preparation serve to confij-m the pound. This enables the investigation of 

 more rapidly obtained conclusions drawn polymorphic forms difficultly available or 

 from observations made on cooling. In addi- unattainable by other means, 

 tion, it is possible to measure the various An identification scheme based upon eu- 

 melting points such as the eutectic tempera- tectic melting points has been published by 

 tures, molecular compound melting points, the Koflers. Unknown compounds are sub- 

 maxima or minima in solid solution systems, divided into various restricted temperature 

 and the melting points of the two starting ranges and two reagents are used for each 

 components. The accuracy of such measure- temperature range. The eutectic temperature 

 ments is dependent on the accuracy of between the unknown and the two reagents, 

 calibration of the hotstage and the care with together with the refractive index of the melt 

 which the proper heating rate is maintained, determined by the Koflers glass powder 

 Aside from the general determination of method serves to identify the unknown com- 

 the type of phase diagram between two com- pound. Over 1200 compounds have been so 

 ponents and the determination of the signifi- catalogued and either the contact prepara- 

 cant temperatures, there are other applica- tion method or the method of mixtures may 

 tions of the contact preparation method, be used. In principle, this method may be 

 Identity or non-identity of an unknown com- applied to any fusible compound provided 

 pound and a suspect compound may be rap- suitable reagents are chosen, 

 idly established both by observation on A different method of identification based 

 cooling and by observation on heating. If upon molecular addition compound forma- 

 the two are identical, the growing crystal tion and applicable to aromatic compounds 

 front will pass through the mixing zone has been published by Laskowski, Grabar, 



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