378 ELEMENTARY CHEMICAL MICROSCOPY 



To the washed precipitate add one or two drops of strong 

 ammonium hydroxide. After a second or two of contact, draw 

 off the ammoniacal solution from any undissolved precipitate. 

 Do not heat the preparation. Allow the preparation to stand. 

 Almost immediately the drop becomes turbid around the edges, 

 because of the separation of minute crystals of silver chloride; 

 these crystals increase slowly in size, but are always very small, 

 requiring a moderately high power for distinguishing their form. 

 From ammoniacal solutions silver chloride seems to separate 

 almost invariably in the form of cubes and hexagonal and rec- 

 tangular plates. Only rarely are octahedral crystals obtained. 



Of the two recrystallization methods, that with ammonium 

 hydroxide will be found to be the better, as well as also the more 

 convenient, because of the greater solubility of the precipitate 

 in this reagent, and because the employment of ammonium 

 hydroxide eliminates many interfering substances. 



Lead chloride is precipitated in the form of white acicular 

 crystals, irregular crystallites and X-like dendrites, soluble in 

 hot water and therefore easily removed. 



Mercurous salts yield a granular precipitate, but sometimes 

 minute needles. Recrystallized from concentrated hydrochloric 

 acid tetragonal crystals may be obtained but no cubes and part 

 of the salt is converted into soluble mercuric chloride. Mercu- 

 rous salts therefore interfere with the satisfactory detection of 

 traces of silver by masking the tiny cubes of silver chloride. 



Thallous salts yield cubes and stars. 



Treated with ammonium hydroxide, silver chloride dissolves 

 with the formation of the compound AgCl-2NH 3 (Isambert). 

 If mercurous chloride is present the precipitate turns black under 

 the action of the reagent, an insoluble compound being formed 

 which Barfoed has shown to be a mixture of metallic mercury 

 and the compound Hg-NH 2 'Cl. If, therefore, silver chloride 

 is present only in traces in a precipitate consisting chiefly of 

 mercurous chloride, ammonium hydroxide may dissolve practi- 

 cally no silver chloride, since the finely divided metallic mercury 

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

 (Silver follows mercury in the electrochemical series.) Under 



