MICROCHEMICAL REACTIONS OF SILVER 
381 
Ammonium hydroxide dissolves silver bichromate with ease. 
The crystals separating from the ammoniacal solution are, accord¬ 
ing to some chemists, complex salts, containing one or more mole¬ 
cules of NH3. 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 Ag2Cr207 or the 
salt Ag2Cr04; 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- 
