CHEMICAL MICROSCOPY 



but still gives similar precipitation and crj^s- 

 tals. 



Anionic Complexes of Central INIetals. 



These reagents are commonly called "gold 

 chloride", "platinum chloride", etc., and 

 their precipitates are called "double salts", 

 but actually the metals are in anionic com- 

 plexes and the precipitating agents are really 

 chlorauric acid, HAuCU , chloroplatinic 

 acid, H2PtCl6 , etc. Thus the reagents are 

 given by those metals a7id anions which will 

 form anionic complexes of this particular 

 type, and the group might be subdivided 

 either according to the metals or the simple 

 anions concerned. The metals are central to 

 the long periods of the periodic table; the 

 anions are halogens (Cl~, Br~, I~) and 

 "pseudohalogens", such as CN~, SCN~, 

 NO2-, N3-, etc. 



The iodide reagents of bismuth and plati- 

 num are colored and so sensitive and general 

 in aqueous solution that they are commonly 

 used for spraying to bring out the spots of 

 alkaloids or related compounds in paper 

 chromatography . 



The following table gives the reagents of 

 greatest value for microcrystals, designated 

 as R; other definite reagents within the scope 

 of this table are indicated by a small r. 



A "gold iodide" reagent is in use, made 

 from HI and chlorauric acid, but the HAUI4 

 immediately decomposes, and the chief 

 effects seem to be due to iodine-HI, rather 

 than any gold compound, with a few results 

 due to an aurous iodide complex, a gold 

 (-f-1) reagent. 



The following might also be mentioned: 

 chlorides (chloro-acids) of Fe, Zn, Sn (stan- 



nous and stannic), UO2 (uranyl), and VO 

 (vanadyl), which are most effective with a 

 high content of HCl to form the chloro-acid ; 

 iodides of Pb, Sn (+2), Zn, and Ag, with 

 alkali iodide; the complex cyanides of Fe, 

 both ferro- and ferri-, and also nitroprusside; 

 and complex thiocyanates of Ft, Hg, Sn 

 (-f2), Cd, Zn, Co (4-2), Ni (+2), Mn 

 (+2), and Cr (-F3). Most precipitates with 

 thiocyanate reagents do not crystallize very 

 readily. A remarkable exception is reinecke 

 salt, NH4Cr(NH3)2(SCN)4 , which is very 

 general and yet gives many crystals. 



Double complexes are also possible; e.g., 

 mercuric cyani-iodide or mercuric chloro- 

 iodide, made by dissolving the insoluble 

 Hgl2 in cyanid-? solution or in strong HCl, 

 respectively. 



On the whole this group is the most useful 

 of all fo^^ niicrocr^^stal tests, and by using 

 other strong acid; besides HCl the use of 

 these reagents may be extended over all 

 compounds of basic nitrogen. 



Simple Halides. These are useful with 

 relatively complex bases, particularly iodide 

 and thiocyanate, used as alkali salts. 



Organic Reagents. The best reagent of 

 this group is picric acid, and several others 

 are highly nitrated compounds, e.g., styph- 

 nic and trinitrobenzoic acids. A few other 

 kinds are known, e.g., sodium alizarin sul- 

 fonate. Sodium tetraphenyl-boron is a new 

 type of reagent (in some ways alhed to the 

 complex oxygen acids), recently introduced, 

 which is very useful for crystalline precipi- 

 tates with free amines in volatility tests 

 (without acidification), and quite sensitive 

 to the lower amines in this way. 



Reagents in Strong Acids. The study 

 of strong acids as media for the precipitation 

 of nitrogen-bases began with hydrochloric 

 acid, which, because many of the precipitat- 

 ing agents are halides, often has special 

 effects. Some reagents were already known 

 which require a high content of HCl either 

 to prevent precipitation of basic chloride 

 (e.g., SbCls) or to form the chloro-acid 



18 



