CHEMICAL MICROCRYSTAL IDENTIFICATIONS 



tion in a strong acid, most often syrupy 

 H3PO4 , (2 + 3) H2SO4 , concentrated HCl, 

 or (2 + 1) acetic acid — acids which give re- 

 agents of very different effects even with the 

 same precipitating compound. 



With a medium of strong phosphoric or 

 sulfuric acid the drop cannot dry up, but its 

 moisture content may gradually change on 

 standing (generally increasing, sometimes 

 considerably), depending on the humidity or 

 dryness of the air. These tests are usually 

 observed during only about two hours, so 

 that ordinarily no special precautions are 

 needed. 



Small cover-glasses of 12-mm diameter are 

 obtainable. A test made under this size can 

 be inverted over a cavity slide, a procedm'e 

 which will very nearly prevent any evapora- 

 tion of a volatile acid or water, or changes 

 due to humidity or dryness of the air. This 

 is generally sufficient control for at least 24 

 hours without sealing-in. A further refine- 

 ment is to put a drop of diluted sulfuric acid 

 of a certain strength in the cavity of the 

 lower slide, to regulate the humidity. H2SO4 

 (45 -j- 55) will correspond pretty closely to 

 a reagent of syrupy H3PO4 ; H2SO4 (1 -f 5) 

 will provide controlled humidity; H2SO4 

 (55 + 45) provides gentle drying for H3PO4 

 reagents. 



Reagents in strong acids may also be 

 added to aqueous solutions, or applied to 

 solids in a form somewhat diluted with wa- 

 ter, without a cover-glass. Then formation 

 of a precipitate and crystallization may occur 

 promptly, or only as the drop evaporates to 

 a higher acid strength, particularly in us- 

 ing phosphoric acid. Sometimes drying in a 

 desiccator may be useful, e.g., with HAuCU 

 in H3PO4 , or a controlled humidity to allow 

 drying just to a certain degree, or to prevent 

 complete drying when the air is very dry, 

 e.g., with the reagent HsBile in (1 -f 7) 

 H2SO4 . 



A hanging drop of reagent is also used in 

 tests for volatile substances. This is more or 

 less familiar in chemical and microcrystal 



tests for ammonia from ammonium salts, and 

 as a test for urea when it is decomposed to 

 ammonia by urease. Microcrystals tests in 

 the hanging drop for the readily volatile d- 

 and c?/-amphet amine were perhaps first used 

 by Griebel, with aqueous reagents. The ma- 

 terial (e.g., a small amount of scrapings from 

 a tablet containing an amphetamine salt), 

 or its solution, is treated with dilute alkali 

 on a cavity slide, and a hanging drop is in- 

 verted over it. There are now at least a 

 dozen such sympathomimetic drugs regu- 

 larly on the market, that are readily volatile, 

 to be tested in this way, and others that are 

 less readily volatile. 



It is certainly not yet realized how far 

 such tests can be extended, how many sub- 

 stances usually considered nonvolatile are in 

 fact slightly volatile with water vapor even 

 at room temperature and can be detected in 

 a hanging drop with an exposure of, say, two 

 hours, for example, I- and c?/-ephedrine, and 

 phenmetrazine. Gentle heat from above, e.g., 

 from a desk lamp, may be used to shorten 

 the time required, and may improve results, 

 e.g., in the vaporization of pentylenetetrazol, 

 but a longer exposure at room temperature 

 is as good or better for most compounds. 



On the acid side benzoic and salicylic 

 acids may be mentioned as sufficiently vola- 

 tile with water vapor for vaporization tests. 



A hanging drop of an aqueous solution of 

 reagent, above an aqueous solution on a 

 cavity slide, does not dry up or expand un- 

 less considerable hygroscopic material is 

 present in one solution, and a long exposiu'e 

 may be used. For basic vapors, a reagent in 

 diluted H3PO4 , (1 + 4) to (1 + 2), may be 

 exposed for some two hours or more, with 

 some increase in the size of the hanging drop 

 due to absorption of water. The slide with 

 the hanging drop is then reinverted to allow 

 evaporation, even put in the desiccator if 

 necessary, and tests can be obtained for a 

 great many basic substances even if only 

 slightly volatile. 



A convenient purification as well as a test 



33 



