and Laboratory Methods. 1611 



In addition to the crystalline salts given by the above reagents, the worker 

 should become familiar with the appearance of the precipitates formed by a 

 number of other reagents when brought in contact with silver salts. Chief 

 among these may be placed oxalic acid, primary sodium tartrate, primary sodium 

 phosphate, potassium antimonyl tartrate, sodium acetate, sodium sulphate, and 

 ammonium carbonate ; most of these silver compounds are of no real or actual 

 value so far as analytical work is concerned, but most of them are of consider- 

 able interest from a theoretical point of view, and because of the fact that in the 

 majority of cases excellent crystals, for crystallographic study with the micro- 

 scope, can be obtained ; for this reason it has been thought wise to devote a 

 little space to their consideration. 



/. Hydrochloric Acid added to solutions of salts of Silver, precipitates Silver 

 Chloride. 



AgNOg+HCl^AgCl+HNOg. 



Method. — Acidify the clear test drop with nitric acid, then, at the center, add 

 a drop of dilute hydrochloric acid. If silver is present an immediate precipitate 

 should result. Examine under the microscope. Silver chloride is so insoluble 

 in water that it is thrown down as an amorphous mass. If the precipitate is 

 wholly crystalline, either silver is absent 'or else present in very small amount. 

 In order to identify silver in an amorphous precipitate it is necessary to recrys- 

 tallize it. Before so doing it is always advisable, and often necessary, to first 

 remove the solution from the precipitate and wash the latter. If the hydrochloric 

 acid has been carefully added and the drop not stirred, it is easy to draw off the 

 clear solution from the curdy, heavy precipitate of silver chloride. When the 

 amount of precipitate is very small it is best to have recourse to the centrifuge 

 to accomplish the separation. After removing the supernatant liquor, wash the 

 precipitate once or twice with hot water acidified with nitric acid. The washed 

 precipitate is then recrystallized from {A) concentrated hydrochloric acid, or 

 (j9) ammonium hydroxide. 



A. To the precipitate of silver chloride, at the corner of a slide, add a drop 

 or two of concentrated hydrochloric acid, and heat the 

 preparation over the " micro " flame. If the precipi- <J 

 tate is not completely dissolved, draw off rapidly the gl ^O ^^ r\ 

 hot acid, without exercising any great care, to obtain -^^ ^i /V ^^^4!!% 



a perfectly clear drop. On cooling, tiny crystals of ^Q ^ ^ ^^^ 



silver chloride separate (Fig. 75). Octahedral crystals <:^ ^^p^ ^^ 



predominate. ^ "^ ^ [^ 



B. To the washed precipitate add one or two drops ' '"TrtvTT^o.oi >%*«,. 



of strong ammonium hydroxide. After a second or p- y- 



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 



