MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 333 



such conditions it is necessary to exercise the greatest care in 

 order to avoid missing the little silver which is present. 



Elements forming oxychlorides may under exceptional con- 

 ditions be precipitated with the silver. 



It is also well to bear in mind that the addition of hydrochloric 

 acid may force back the dissociation of certain salts to a degree 

 causing the separation of a solid phase. 



Precautions. 



When working with concentrated hydrochloric acid or strong 

 ammonia, great care must be used to avoid spoiling the micro- 

 scope and objectives. It is essential to work rapidly. 



The drop is acidified with nitric acid because the presence of 

 this reagent favors the agglutination of the particles of silver 

 chloride, and hinders at the same time the precipitation of oxy- 

 chlorides, etc. 



Decanting after precipitation is advisable, since the crystal 

 form of silver chloride is changed by many compounds when the 

 former is crystallized in the presence of the latter. Still other 

 compounds completely ruin the test. Although there is, of 

 course, danger of the occlusion of some of these objectionable 

 salts by the silver chloride, this difficulty is reduced to a mini- 

 mum by avoiding too concentrated test drops and washing the 

 precipitate. 



Washing the precipitated silver chloride with hot water re- 

 moves the greater part of the lead chloride which may have been 

 carried down with the silver. 



EXPERIMENTS. 



a. Precipitate with dilute HC1, a test drop containing AgNO 3 . Separate and 

 wash the precipitate; then recrystallize it by the above described method, using 

 concentrated HCl. Then repeat the experiment, using NHUOH as the solvent. 



b. Make a mixture of Ag and Pb, test by both recrystallization methods. 



c. In like manner test a mixture of AgNO 3 and HgNOs. 



d. Precipitate with HCl a test drop containing Pb and Ag; recrystallize the 

 precipitate without drawing off the solution. In like manner test mixtures of Ag 

 and Zn, Ag and Cd, Ag and Sb, Ag and Pt, Ag and Sn, Ag and Cu. 



e. Try recrystallization of AgCl in the presence of phosphates, in the presence 

 of sulphates and in the presence of molybdates. 



