MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 335 



The crystals separating from the ammoniacal solution are, accord- 

 ing to some chemists, complex salts, containing one or more mole- 

 cules of NH 3 . The recrystallized product separates in the form 

 of needles, skeleton crystals and masses resembling lichens. 



Unless the original precipitation was made in nitric acid solu- 

 tion both strontium and barium may, under unusual conditions, 

 be precipitated. It is well to bear this in mind when recrystal- 

 lizing from ammonia. 



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 bichro- 

 mate, 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 bichromate complicates matters, since the 

 crystals separating in the presence of the silver sulphate formed 

 in the reaction may be either those of the salt Ag 2 Cr 2 O7 or the 

 salt Ag 2 Cr04; the latter compound is usually formed when the 

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

 Normal silver chromate is isomorphous with normal silver sul- 

 phate, normal silver selenate, and anhydrous sodium sulphate; 

 all are to be referred to the orthorhombic system. Because of 

 this isomorphism of the sulphate and chromate very interesting 

 and instructive preparations may be obtained. Silver sulphate 

 separates from solution generally in the form of highly refrac- 

 tive, transparent, colorless, rhombic octahedra, but in the pres- 

 ence of silver chromate these colorless octahedra increase in size, 

 turn first yellow, and finally a more or less intense brownish 

 red. 



Normal potassium chromate added to neutral solutions of 

 silver causes the precipitation of normal silver chromate; but 

 when the test drop is first acidified with nitric acid the crystals 

 separating probably consist of both the chromate and bichro- 

 mate. When recrystallized from hot nitric acid the precipitate 

 will usually consist of the bichromate alone. When ammonium 

 hydroxide is the solvent employed to recrystallize the silver chro- 



