1612 Journal of Applied Microscopy 



<^ chloride seems to separate almost invariably in the 



A\ y\^ y^\^ form of cubes and hexagonal and rectangular plates 



rally J ^ (Fig. 76). More rarely octahedral crystals are ob- 



O ^^ ^ tained. 



^^ O ^— — ^ ^ <^ rs ^^ ^^^ ^^^ recrystallization methods, that with am- 



^^f^ A (^ ^ monium hydroxide will be found to be the better, as 



^.^ y\ ^^ -^^ well as also the more convenient, because of the 



^11 <^::;^^^ / \ greater solubility of the precipitate in this reagent. 



iff, and because the employment of ammonium hydroxide 



xpw.»o.o\vY\wry. eliminates many interfering substances. 



Fig. 76. , . . 



Remarks.- — When workmg with concentrated hy- 

 drochloric acid or strong ammonia, great care must be used to avoid spoiling 

 the microscope 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 oxychlorides, etc. 



Decanting after precipitation is advisable, since it has been shown by a num- 

 ber of investigators that 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 minimum by avoiding too concentrated test drops and 

 washing the precipitate. 



Washing the precipitated silver chloride with hot water removes the greater 

 part of the lead chloride which may have been carried down with the silver. 



Treated with ammonium hydroxide, silver chloride dissolves with the forma- 

 tion of the compound AgCl • ^NH, (Isambert). If mercurous chloride is present 

 the precipitate turns black under the action of the reagent ; an insoluble com- 

 pound being formed which Barfoed has shown to be a mixture of metallic mercury 

 and the compound Hg • NHo • CI. If, therefore, silver chloride is present only 

 in traces in a precipitate consisting chiefly of mercurous chloride, ammonium 

 hydroxide may dissolve practically no silver chloride, since the finely divided 

 metallic mercury may reduce the greater part of the silver salt to metallic silver. 

 Under such conditions it is necessary to exercise the greatest care in order to 

 avoid missing the little silver which is present. 



Thallous chloride is precipitated by hydrochloric acid in the form of cubes 

 and stars. 



Exercises for Practice. 



Precipitate with dilute HCl a test drop containing AgNO^. Separate and 

 wash the precipitate ; then recrystalHze it by the above described method {A), 

 using concentrated HCl. Then repeat the experiment, using NH^OH as the 

 solvent {B). 



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



In like manner test a mixture of AgNOg and HgNOg. 



Precipitate with HCl a test drop containing Pb and Ag ; recrystalHze the 

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

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



Try recrystallization of the silver chloride in the presence of phosphates, in 

 the presence of sulphates, in the presence of molybdates. 

 Cornell University. E. M. Chamot. 



