INDUSTRIAL RESEARCH 



in the aqueous solution. If the sohitions are 

 placed together in a container, the chloro- 

 form solution sinks to the bottom of the 

 dish, and acts as a reservoir to replenish the 

 aqueous KCN with reagent 'as it is depleted 

 from solution in the latter. When the surface 

 of the polished and etched specimen was ex- 

 posed to the aqueous solution no migration 

 of the reaction product occurred. The pres- 

 ence of lead chromate was later shown to be 

 unessential in providing sufficient quantity 

 of lead-salt reactant; in fact, it was observed 

 to have migrated disturbingly. Therefore, 

 the potassium chi'omate reaction was omit- 

 ted. Instead it was found that the picric acid 

 and alcohol etchant had created a surface 

 film of nonmigrating reaction product which 

 was present in quantities quite adequate to 

 yield microscopically visible lead dithi- 

 zonate (13). 



Figure 3 show^s a photomicrograph of a 

 leaded steel specimen treated in this manner. 

 The structures at the ends of the man- 

 ganese-sulfide inclusions were stained red 

 indicating that they are indeed lead-bearing. 

 No other structures reacted to yield this 

 stain. Due to the random nature of the 

 plane of metallographic sectioning, it is 

 possible for some of these structures to ap- 

 pear dissociated from the manganese sul- 

 fide. 



Examples of other spot-test reactions used 

 to show chemical distributions by micros- 

 copy are rubeanic acid for copper, 5(7>-di- 

 methylaminobenzylidene) rhodanine for sil- 

 ver, and manganous sulfate and silver nitrate 

 for hydroxyl ion. The last example is one in 

 which the hydrox}^ ion triggers a redox reac- 

 tion between manganous and silver ion, pro- 

 ducing precipitates of black manganese 

 dioxide, and black finely divided metallic 

 silver. No insoluble reaction product forms 

 between the reagent and the ion for which 

 the test is performed. This ion merely influ- 

 ences the components of the reagent to 

 react with one another. This illustrates the 

 point that indirect as well as direct reactions 

 which result in insoluble products should be 



V ^ ' 



A 



Fig. 3. A polished and etched leaded steel 

 specimen stained with dithizone to show the dis- 

 tribution of lead. The arrows indicate the location 

 of the lead bearing structures which stained red. 



investigated when attempting to develop a 

 microscopical spot-test technicpe. 



New spot-test reagents are rapidly being 

 discovered; however, most of them are used 

 in the conventional manner. It is hoped that 

 increased selectivity by means of more ap- 

 propriate reagents for microscopy will make 

 it feasible to devise relativelj^ simple system- 

 atic methods of analysis. Ultimately, it 

 may be possible to place these tests on a 

 quantitative basis through microspectro- 

 photometry. 



Physical Methods. Microspectrophotome- 

 try is an example of the combination of 

 physical methods (14). Some absorption 

 spectra may be obtained by this means from 

 microscopical specimens for purposes of 

 identification and anal3^sis. Biologists have 

 been able to gain insight into the chemistry 

 of components of single cells with this tool. 

 Spectra in the ultraviolet, ^'isible, and the 

 near infrared have been studied. This science 

 is in the beginning stages of development, 

 and, therefore, suffers from rapidlj^ changing 

 instrumentation, but holds great future 

 promise. (See the "Encyclopedia of Spectros- 

 copy".) 



37a 



