CHEMICAL MICROSCOPY 



slide ; as the slide is removed from the source 

 of heat, a coverglass from which a drop of 

 quinoline is han<>;ing is placed on the solids 

 and when the slide has cooled the prepara- 

 tion is examined under a microscope. If 

 larger, more euhedral crystals are desired, 

 the slide may be gently warmed and then 

 allowed to cool, but the cupric chloride- 

 quinoline compound, if present, should be 

 identified prior to this because its color is 

 destroyed by heating. If the sample is a dry 

 solid, it may be mounted in quinoline to 

 establish the presence of the above-named 

 cations as chlorides. 



The characteristic crystals formed by 

 quinoline with the metal chlorides are de- 

 scribed below, grouped by color. 



Yellow crystals indicate cuprous copper or 

 ferrous iron. The copper compound appears 

 as needles or rhomb-shaped plates and 

 tablets; the iron compound appears as pleo- 

 chroic (pale yellow to yellow) rectangular 

 plates. 



Blue crystals indicate cobalt, nickel, or 

 cupric copper. The cobalt compound forms 

 pleochroic (light blue to blue) rhomb-shaped 

 or rectangular plates and tablets; the nickel 

 compound forms pleochroic (violet to blue) 

 blue-violet plates and tablets; the copper 

 compound forms pleochroic (green to blue 

 to blue-violet) crystals shaped like elongated 

 hexagons or footballs. 



Colorless, needle-like crystals indicate cal- 

 cium or cadmium; these two are not readily 

 differentiated from each other. 



Colorless, well-defined crystals indicate 

 manganese, mercury, or zinc. The manganese 

 compound forms small elongated rectangu- 

 lar plates; the mercury compound forms 

 pseudo-hexagonal plates; the zinc compound 

 forms rhomb-shaped plates and tablets. 



The chlorides of the less commonly en- 

 countered elements indium and thallium 

 (thallous) also react with quinoline to yield 

 colorless crystals. The indium compound 

 forms diamond -shaped and rectangular 



plates; the thallium compound forms hexag- 

 onal, rhomb-shaped, and rectangular plates. 

 Primarily, (juinoline serves as a group re- 

 agent; a positive test indicates that one (or 

 more) of the above-named metal-chlorides 

 is present and a negative test indicates ab- 

 sence of an apprecial)le ([uantity of any of 

 them; under favorable conditions qviinoline 

 offers a specific test for these cations. When 

 the sample is a solid, ciuinoline may distin- 

 guish these metal chlorides from their oxides, 

 or sulfates, or free metals, and it simultane- 

 ously distinguishes the valence state in the 

 cases of copper chlorides or iron chlorides. 



J. M. MUTCHLER 



REAGENTS FOR MICROCRYSTAL 

 IDENTIFICATIONS 



The reagents given here are primarily 

 precipitant s of organic compounds contain- 

 ing basic nitrogen. They include, but go far 

 beyond, traditional alkaloidal reagents. A 

 few give good inorganic tests for certain ions; 

 a few extend to nitrogenous compounds that 

 are almost completely acidic, such as the 

 barbiturates. 



Each of the outstanding precipitating 

 compounds, HAuBr4 for example, makes a 

 number of quite different reagents, by the 

 use of different solvent media, particularly: 



(1) Syrupy H3PO4 ; diluted H3PO4 , 



(2) Diluted H2SO4 (up to (1 + 1) for 

 chlorides, (2 + 3) for bromides, (1 -f 3) for 

 iodides), 



(3) Water; and aqueous solutions only 

 slightly acid, or made acid only by the pre- 

 cipitating compound itself; sometimes even 

 neutral, slightly basic, or alkaline, 



(4) Concentrated HBr (40%); diluted 

 HBr, 



(5) Concentrated HCl (38%); diluted 

 HCl, 



(6) Acetic acid (usually diluted at least 

 (2 -|- 1) simply to prevent its spreading all 

 over the slide). 



These are given above in the order of de- 



58 



