1 6 MICRO-ANALYTICAL METHODS 



acid, ammonia and ammonium sulphide by bringing a drop of the reagent in contact 

 with the thread upon the slide. (See also Biological Test for Arsenic in Part II.) 



b. Zinc Test. — Dip cotton fibers into the suspected solution, allow the moisture 

 to evaporate, and then dip the threads into a solution of gold chloride. A violet 

 coloration develops which remains in the presence of acids but vanishes in the presence 

 of chlorine water, indicating the presence of zinc chlorite. The reaction is appreciable 

 in the presence of 0.003 Mg of zinc chlorite, whereas in the form of the sulphite, 

 0.1 Mg of zinc is required to show the reaction. 



c. Antimony Test. — Dip a sulphide thread into the solution, allow solution to 

 evaporate and then expose the thread to the vapor of ammonium sulphide. If 

 the solution to be tested contains considerable hydrochloric acid, sulphide of anti- 

 mony is formed upon evaporation. 



d. Gold Test. — Gives a brown coloration with the sulphide thread, which color 

 disappears upon prolonged exposure to ammonium sulphide, more quickly on ex- 

 posure to chlorine, bromine and sodium hypochlorite. The threads which have 

 been decolorized with chlorine are colored blue to black with iron chlorite and violet 

 to red with zinc chlorite. 



e. Silver Test. — A neutral or faintly acid silver nitrate solution gives a brown to 

 black coloration with the sulphide thread, the depth of the reaction depending upon 

 the concentration of the solution. The fibers can be decolorized by placing in sodium 

 hypochlorite, and the color can be restored by means of zinc chlorite or an alkaline 

 solution of grape sugar. Sulphuric acid will again decolorize. 



/. Mercuric Chloride. — Cotton threads dipped into a solution containing mer- 

 curic chloride and exposed to the vapors of ammonium sulphide or ammonia, are 

 colored black. The color is quite permanent in the presence of acids. A sulphide 

 thread is colored yellow in neutral solution of mercuric chloride, changing to black 

 in the ammonium sulphide vapor. 



g. Lead Test. — Neutral lead solutions (lead nitrate) turn the sulphide threads 

 yeUow and black on prolonged exposure to ammonium sulphide. In acid solutions 

 the color reaction with the sulphide thread is black. The yellow coloration is 

 promptly changed to black upon exposure to ammonium sulphide, or when placed 

 in weak sulphuric acid (i : 15). The latter reaction distinguishes between lead and 

 mercury, as the yellow coloration of the mercury is changed very slowly with dilute 

 sulphuric acid. 



h. Bismuth Test. — Solutions color the sulphide thread reddish-brown. Bromine 

 causes the color to disappear. Potassium dichromate causes a yellow coloration, 

 while alkaline solutions of zinc chlorite produce a black coloration. Lead solutions 

 are not reduced by alkaline solutions of zinc chlorite. 



i. Iron Test.— Ammonium sulphide vapor gives a black precipitate which is 

 soluble in weak solutions of hydrochloric acid. Potassium ferrocyanide gives a 

 blue coloration. 



j. Copper Test— Solutions of copper sulphate give a brown coloration to the 

 sulphide thread, which color persists in 10 per cent, hydrochloric acid, but disappears 

 on exposure to bromine vapor. The threads which have been bleached with bro- 



