358 ELEMENTARY CHEMICAL MICROSCOPY 



tions from their faces become noticeable. To be of value as a 

 test for chromium both the crystal form and color must be taken 

 into account. 



Free mineral acids should be absent, as also the salts of organic 

 acids. 



In general we must observe the same precautions as in test- 

 ing for aluminum with cesium sulphate. (See Aluminum, Method 

 A, page 342.) 



It is obvious that other "alum" forming elements, such as 

 aluminum, iron and manganese, must be absent or present only 

 in traces. 



Since all the alums are isomorphous it is often possible to start 

 crystallization by introducing into the test drop an infinitesimal 

 trace of potash alum when by chance the preparation shows a 

 tendency to supersaturate and no crystals form, or even better, 

 add a similar tiny fragment of cesium alum. In such an event 

 we must place our chief dependence upon the color of the crystals 

 separating. 



EXPERIMENTS. 



Test simple salts of Cr as described above, then employ more or less complex 

 mixtures. 



C. Detection of Chromium in Complex Mixtures such as 

 Alloys, etc. 



Method of Behrens. 1 Place the finely-divided material on 

 an object slide. Add a fair-sized drop of concentrated nitric 

 acid, heat to boiling, decant the acid to another slide and treat 

 the residue again in the same manner. Repeat until all is dis- 

 solved or until sufficient material has passed into solution. 

 Unite all the drops and evaporate to dryness. By means of a 

 tiny spatula carefully scrape off the dry mass into a platinum 

 cup or upon a piece of platinum foil. Add a very small quantity 

 of sodium carbonate-potassium nitrate fusing mixture (3:1) and 

 heat until a clear fusion results, adding more fusing mixture if 

 necessary, but being careful to use no more than absolutely neces- 

 sary. The yellow fused mass is dissolved in water, concentrated 



1 Behrens, Anleitung, i Auf. 186. 



