THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 307 



inclination of the slide at once so as to cause the reagent to flow 

 toward the material to be tested. Counteract any tendency of 

 the reagent to creep up by immediately increasing the inclina- 

 tion to an almost vertical position. 



Often the preparation cannot be laid flat upon the stage be- 

 cause of the instant spreading of the alcoholic solution. In such 

 an event, the corner of the object slide holding the liquid is in- 

 serted in the stage opening and may be held in place by another 

 slide placed upon the stage, carrying a piece of " plasticine" 

 against which the inclined slide is pressed. The preparation can 

 then be examined with a low power, focusing each different area 

 as it is brought into the field by means of the stage centering 

 screws. 



Because of the difficulties involved in the study of inclined 

 preparations it is always better to first evaporate to dryness the 

 drop of material to be tested so as to obtain a broad thin film 

 (see Method IV) and use a reagent solution made with as dilute 

 alcohol as will yield the proper conditions required in the test. 



EXPERIMENTS. 



a. Obtain a thin film of KCl at the corner of an object slide. Place near by 

 a drop of an alcoholic solution of freshly prepared sodium bismuth thiosulphate. 1 

 Tip the slide slightly and draw the reagent across the dry film. 



Yellow monoclinic 2 crystals of potassium bismuth thiosuiphate separate. The 

 salt is believed to have the formula 



3(K 2 S 2 3 ).Bi 2 (S 2 3 ) 3 -2H 2 0. 

 It is readily soluble in water, almost insoluble in alcohol. 



1 The reagent is prepared as follows: Place in a small watch glass (25 mm.) a 

 small drop of dilute hydrochloric acid; add repeatedly minute amounts of basic 

 bismuth nitrate, warming gently from time to time and stirring thoroughly, until 

 a trace of the basic nitrate remains undissolved; now add a bare trace of hydro- 

 chloric acid; just sufficient to dissolve the little residue of bismuth salt, but no more; 

 then add to the preparation a tiny drop of water. A permanent precipitate of 

 bismuthyl chloride should result. If the first drop of water does not produce 

 a permanent precipitate, another drop must be added. To this latter turbid solu- 

 tion a saturated solution of sodium thiosulphate is carefully added, with constant 

 stirring, a tiny drop at a time, as long as any of the precipitate remains undis- 

 solved. An excess of sodium thiosulphate is to be avoided. A perfectly clear, 

 faintly yellowish solution should result. To this clear liquid add alcohol (95 per 

 cent) drop-wise, until a permanent turbidity results, which is in turn cleared up 

 by the addition of a single drop of water. 



2 Hiiysse, Zeit. anal. Chem., 39 (1900), 9. 



