and Laboratory Methods. 



1649 



MICRO-CHEMICAL ANALYSIS. 

 XIX. 



THE COMMON METALS.— SILVER GROUP. 



//. Ammonium Dichromate added to solutions of salts of Silver gives rise to 

 the separation of Silver Dichromate. 



2AgN03 + (NH^)2Cr20,=Ag2Cr.,0, + 2NH4N03. 



Method. — To the moderately concentrated test drop add a tiny drop of dilute 

 nitric acid, then a fragment of ammonium or potassium dichromate. Dark red 

 crystals of silver dichromate separate at once 

 in the form of prisms, plates and aggregates 

 belonging to the triclinic system (Fig. 77). 

 These crystals exhibit faint dichromism when 

 turned above the polarizing nicol. 



Remarks. — In neutral or very concentrated 

 solutions the crystals first formed consist of 

 tiny rods and needles so dark colored as to 

 appear black by transmitted light ; after a 

 time there are generally obtained, in addition 

 to these rods, the large plates and prisms 

 figured above. 



Silver dichromate can be recrystallized 

 from hot water, but better results follow the 

 use of nitric acid or ammonium hydroxide. 



From hot nitric acid very beautiful preparations can be obtained, 

 to some investigators the crystals which separate on cooling from a hot neutral 

 aqueous solution of the dichromate precipitate are not silver dichromate, but 

 normal silver chromate, Ag2Cr04. 



Ammonium hydroxide dissolves silver dichromate with ease. The crystals 

 separating from the ammoniacal solution are, according to some chemists, com- 

 plex salts containing one or more molecules of NHg. The recrystallized pro- 

 duct separates in the form of needles, skeleton crystals, and masses resembling 

 lichens. 



In the presence of much lead the reaction often fails. Instead of the dark 

 red salt, small yellow prisms of entirely different appearance separate. In such 

 an event either first remove the lead with a drop of dilute sulphuric acid and 

 then add the dichromate, or else add, immediately after the fragment of the 

 reagent, a drop or two of dilute sulphuric acid. Usually in a short time good 

 crystals can be obtained. The use of sulphuric acid in connection with the 

 dichromate complicates matters, since the crystals separating in the presence of 

 the silver sulphate formed in the reaction may be either those of the salt 

 Ag2Cr207 or the salt Ag.^CrO^ ; the latter compound is usually formed when 

 the amount of nitric acid is small and that of silver sulphate large. Normal 



According 



