CHEMICAL MICKOCRYSTAL IDENTIFICATIOxNS 



ties. In the microscopic chemical tests, on 

 the other hand, one can test directly for a 

 basic drug (e.g., morphine, or amphetamine) 

 without necessarily being concerned with 

 whether it is originally present as the sulfate 

 or hydrochloride or in some other combina- 

 tion, and similarly for other kinds of sub- 

 stances. 



Indeed, when a substance must be sepa- 

 rated from a complex mixture, whether by 

 extraction, chromatography, or some other 

 means, its original state of combination is 

 lost, and to use optical crystallography, or 

 for that matter, to get a melting-point or 

 x-ray diffraction pattern, it must be obtained 

 in some kind of crystalline form. The great- 

 est sensitivity, simplicity, and directness for 

 any test requiring crystals exist if the crys- 

 tals are obtained as some very insoluble com- 

 pound which still crj^stallizes quite readily 

 and in forms that are recognizable by direct 

 inspection and observation. This is precisely 

 the idea of the chemical microcrystal tests. 



This subject is, therefore, a branch of 

 chemistry. It involves the use of the micro- 

 scope in making identifications by means of 

 chemical reactions, especially precipitation 

 reactions, yielding crystals. The optical 

 crystallography used is limited and usually 

 need not be of a specialized kind. In fact the 

 chemistry used is not very unusual either, 

 but naturally the reagents and reactions are 

 selected as those especially useful in con- 

 junction with the microscope. The science is 

 to some extent a blend of chemistry and 

 microscopy, but analysts should particularly 

 keep in mind the chemical meaning of the 

 tests, and development of the field will pro- 

 ceed along chemical lines. It is strange indeed 

 that it has been so generally neglected by 

 chemists, over a period of a hundred years. 



Advantages and Disadvantages. The 

 outstanding advantages of this kind of iden- 

 tification tests are: 



(1) The high assurance with which micro- 

 crystal identifications can be made. 



(2) The direct character of most of these 

 tests. 



(3) Their simplicity, convenience, and 

 speed. 



(4) Selectivity, in the sense of noninter- 

 ference, or no great interference, by most 

 impurities. 



(5) High sensitivity, not necessarily in 

 degree of dilution, but especially, with the 

 aid of the microscope, in respect to amount 

 of the substance for which the test is made. 



In addition to various objections that are 

 often made but have no very substantial 

 ba^is, the main disadvantages are: 



(1) The results are not readily classifiable. 



(2) Applicability of coverage limited in 

 organic chemistry. 



(3) The general neglect. 



When an identification is wanted for a 

 chemical substance the identification should 

 be specific. However, it is seldom reached by 

 a single specific test, but usually as the only 

 conclusion possible from two or more — com- 

 monly several — group tests or characteristic 

 tests and properties. Often most of the 

 chemical tests used are rather general, and 

 are then supplemented with some physical 

 measurement, often made on a derivative, 

 to show differences between closely related 

 compounds within the ascertained group, or 

 provide final confirmation. The combined 

 characteristics then become specific. 



Much misunderstanding of the microcrys- 

 tal tests seems to result from a failure to 

 appreciate their chemical character. In a 

 chemical microcrystal test we know, fij'st of 

 all, what reagent we used; and we know, or 

 ought to know, what its chemical effects may 

 be. We take a close look through the micro- 

 scope at the crystals formed. Often we can 

 recognize them immediately by their visible 

 characteristics, assuming we have seen them 

 before, and always remembering that we 

 know what reagent was used. 



The results are far more definite for indi- 

 vidual compounds than in most chemical 

 tests, l)ut the procedure of identification is 



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