THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 261 



APPLICATIONS OF THIS METHOD. 



Testing for acidity or alkalinity Litmus-silk 



Differentiating between strong mineral 



acids and organic acids Congo-red-silk 



Testing for boric acid, borates Turmeric- wool 



Group reagent for the heavy metals. . . . Guncotton-zinc-sulphide 

 Test for gold .... Adsorption upon silk, reduction with stannous 



chloride 

 For the methods for preparing the fibers, see page 271. 



EXPERIMENTS. 



a. Test a very dilute drop of an acidulated solution with blue litmus-silk. 



b. Test a dilute drop of alkaline solution with red litmus-silk. 



c. Place a drop of a dilute solution of borax upon an object slide, acidulate 

 with dilute HC1. Dip into the drop from a wax cone a fiber of turmeric-wool. 

 Allow to evaporate spontaneously to dryness. Examine the fiber under the 

 microscope. It should have a brownish color. Lay the fiber upon a slide and 

 moisten with a 10 to 15 per cent solution of NaOH. If borates were present the 

 fiber turns a bluish or lavender color. 



d. Into a tiny drop of a solution containing Au, lay a fiber of purified raw silk, 

 warm gently until evaporated to dryness; carefully avoid too high a temperature. 

 The fiber turns yellow or red. Treat with a dilute solution of SnCl 2 containing a 

 little tannic acid. A purple color results, due to the precipitation of metallic gold. 

 The beautiful red color of the silk fiber before the reducing agent is added is due to 

 colloidal gold; the agglutination of the colloidal particles by the SnCl 2 gives rise 

 to larger particles which appear purple. 



IX. The delicacy of the test is increased by taking advantage of 

 adsorption phenomena, or the test itself depends upon the adsorptive 

 properties of a compound. 



Although reactions of this type are those most frequently 

 employed in the differentiation of structures, tissues, cells and 

 cell contents in biology, histology and pathology, through the 

 use of differentiating stains or dyes, their applications in the 

 chemical laboratory to the common problems of qualitative 

 analysis are limited. 



The basis for selecting a reaction involving adsorption phe- 

 nomena or solid solution is that the resulting reaction shall con- 

 fer upon a practically colorless body a color of sufficient intensity 

 to render it more easily discerned. Whenever, therefore, stain- 



