CHEMICAL MICROSCOPY 



for a volatile base is to catch the vapor in a substances, many such tests undoubledly 

 hang;ing drop of dilute HCl (say 1 % by will be found. However, the most familiar 

 volume of the concentrated acid), over an microcrystal tests at present are those for 

 alkaline drop on a cavity slide. Then the alkaloids and related substances, and related 

 slide with the hanging drop is reinverted and tests for other derivatives of basic nitrogen, 

 the drop allowed to dry up, leaving the hy- 

 drochloride of the volatile base. This solid Criteria for Selecting the Best Identi- 

 hydrochloride may then be examined micro- iicatioii lests 



scopically, if it crystallizes, or it may be (1) Highly characteristic crystals (which 

 tested with HAuBr4 in H3PO4 , or some other may be so by reason of two or more types 

 reagent of high acid content; or with this occurring in the same test -drop, or for other 

 purification it may be simply dissolved in reasons besides form); (2) Easily recogniz- 

 water for some test. able crystals; (3) Crystals forming very 

 Some substances decompose in alkaline readily; (4) Crystals of the same type or 

 solution and give off a volatile base. For types over a wide range of concentration or 

 example, the drugs levarterenol, epinephrine, with widely different amounts of material 

 and isoproterenol, all diphenols and of simi- (when direct addition is used); (5) Crystals 

 lar uses, give off ammonia, methylamine, reasonably stable; (6) Crystals not essen- 

 and isoproterenol give off ammonia, methyl- tially changed by the presence of nonreactive 

 amine, and isopropylamine, respectively, and material (e.g., tablet excipients); (7) Crys- 

 can be readily distinguished in this way, with tallization not prevented nor badly altered 

 a hanging drop of sodium tetraphenylboron when impurities of other nitrogenous bases 

 solution. The assm'ance of the identity of are present in quite minor amounts (ex- 

 the substance given off that is afforded by perience with actual uses of the test affords 

 microcrystals raises these tests much above the best information on this point); (8) High 

 the usual chemical decomposition tests. sensitivity; (9) Colored crystals are prefer- 

 Still, a decomposition test is usually not able (i.e., use of a colored precipitating com- 

 the best, if a direct test is available. In show- pound); (10) The reagent by itself (blank 

 ing contamination by rodent urine, instead test) should not give any possibly confusing 

 of the indirect test for urea by decomposition crystals; (11) A permanent reagent is prefer- 

 with urease and detection of the ammonia able; (12) A slow-evaporating or non-drying 

 evolved (a ubiquitous substance, and usu- reagent is preferable; (13) A reagent of gen- 

 ally detected merely by its alkalinity, al- eral usefulness for microcrystals is preferable, 

 though microcrystals can be used here, too) or alternatively (for some purposes) one that 

 a direct microcrystal test with xanthydrol does not precipitate at all with most other 

 can be used. In (2 -f 1) acetic acid this is alkaloids and amines; (14) Comparisons, 

 amazingly prompt and sensitive for an or- both in general, and with the most closely 

 ganic reaction, and results in crystals of a related compounds available, should estab- 

 urea compound, dixanthylurea, which are lish the meaning and valvie of the test for 

 observed directly, with obvious forensic and the particular substance. 

 other advantages for the test. This is an 

 example of the scattered microcrystal tests Future Lses 



which are possible and depend on varied It seems likel^M hat for the near future the 



organic reactions. usefulness of microcrystal tests will remain 



If analysts, generally, would try to find chiefly in the drug field, as regards the sub- 



microcrystal tests whenever this type would stances tested, and in general in regulatory 



be the most advantageous way of identifying and law-enforcement work. Here, the highly 



34 



