GENERAL MICKOSCOPY 



Combination of Disciplines. Chemical 

 Microscopy. Chemistry and microscopy have 

 been used in combination for many years 

 (10). Chamot and Mason in their "Hand- 

 book of Chemical Microscopy" have pre- 

 sented one of the most authoritative works 

 in this field. It is felt, therefore, that no 

 lengthy discussion of their techniques be- 

 longs here. These authors have emphasized 

 the identification of inorganic ions in small 

 specimens by means of microscopical recog- 

 nition of characteristic crystalline forms in 

 reaction products. They recommend using a 

 polarizing chemical microscope to facilitate 

 identification. These methods are covered in 

 some detail in pages 13 to 72 of this Ency- 

 clopedia. 



Wet Chemistry and Microscopy. Another 

 combination with microscopy is straight wet 

 chemistry on a reduced scale to provide di- 

 rect microscopic observation of the course of 

 the reaction. In this area, the specimen con- 

 tained in small vessels, pipettes, etc., is 

 handled with a micromanipulator while un- 

 der observation. Strikingly quantitative 

 methods have been developed; even ti- 

 trimetry is possible. 



Spot-Test Reactions and Microscopy. Al- 

 though the use of spot-test reactions (12, 13) 

 in conjunction with microscopy is not gen- 

 erally recognized, it has been found to be 

 quite informative in the authors' laboratory. 

 Spot tests yield colored reaction products 

 which indicate the presence of ions or groups, 

 and are most applicable m inorganic micro- 

 scopical analysis, but also have been found 

 useful for organics. If these reactions are 

 chosen judiciously, distributions as well as 

 species of ions or organic groups frequently 

 may be shown. Some biological stains fall 

 into this category, but very few of these have 

 proved applicable to industrial specimens. 

 More often greater success has resulted from 

 adapting spot tests which had not been used 

 previously in connection with microscopy. 

 The criteria for selecting a spot test have 

 been that it must have the desired selectivity 

 within the bounds of the system, it must 



produce a very intense color which is, in low 

 concentration, visible microscopically, and it 

 should proferal)ly form a product which does 

 not migrate seriously from the site of reac- 

 tion. Adsorption complexes and precipitates, 

 for example, are the most generally suitable 

 reaction products in this sense. Although 

 many of these reactions have been success- 

 fully employed here, only one typical ex- 

 ample will be cited. The following detailed 

 description of adaptation of a spot test to a 

 metallographic specimen serves to illustrate 

 the nature of the specific difficulties as well 

 as advantages. 



A steel to which lead had been added to 

 improve machinability was being examined 

 to locate the lead. The samples were polished 

 and etched with picral. The most predom- 

 inant structures were elongated manganese- 

 sulfide inclusions in both the leaded specimen 

 and the control specimen containing no lead, 

 but the leaded specimen exhibited small 

 structures, not evident in the control speci- 

 men, at the ends of the manganese-sulfide 

 inclusions. Consequently, it was immediately 

 suspected that the observed structures were 

 lead-bearing. A search made for a suitable 

 spot-test reaction resulted m the choice of 

 the reagent diphenylthiocarbazone (dithi- 

 zone). Reacted with lead, it yields a water- 

 insoluble red inner complex (11). 



First the dithizone was applied in chloro- 

 form solution to the sample after the lead 

 had been reacted superficially with potas- 

 sium chromate solution to produce a film of 

 lead chromate ; however, the lead dithizonate 

 was soluble in the chloroform. Further litera- 

 ture search (12) provided a satisfactory 

 means of preserving the reaction product at 

 the site of the reaction. Dithizone is itself 

 water insoluble ; however, it was learned that 

 dithizone would transfer from a chloroform 

 solution to a water solution of potassium 

 cyanide. The chloroform and water are im- 

 miscible, and the dithizone is only sparingly 

 soluble in the aqueous potassium cyanide 

 solution. The reaction product is insoluble 



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