382 
ELEMENTARY CHEMICAL MICROSCOPY 
mate, the compound separating is thought to have the formula 
Ag2Cr04 • 4 NHgd 
Normal potassium chromate produces in neutral or slightly acid 
solutions of manganous salts sheaves and bundles of a cinnamon 
brown manganous chromate soluble in excess of acid. Bichro¬ 
mates cause no precipitates in solutions of manganous salts. 
Precautions. 
The test drop must be moderately concentrated with respect 
to silver. 
When working with test drops acidified with nitric acid there 
is Httle danger of any interference by members of the calcium 
group. 
Large amounts of the salts of the alkalies seem to have an 
injurious effect when but little silver is present. 
In all analytical work it is safe to assume that the presence of 
any elements which are precipitated as chromate or bichromate 
in acid solution will interfere with the reaction for silver, particu¬ 
larly when such elements are in excess of the latter. 
White alloys believed to contain silver can be tested for this 
element by drawing across them a streak of a solution of ammo¬ 
nium bichromate in nitric acid. The color of the streak is gener¬ 
ally sufficient to indicate the presence or absence of silver, but 
if the streak of the reagent be examined under the microscope 
(best with an illuminating objective or some form of vertical 
illuminator) in the presence of silver the characteristic dark red 
crystals of silver bichromate will be easily distinguished. 
EXPERIMENTS. 
a. To a moderately concentrated neutral test drop add a fragment of 
(NH 4 ) 2 Cr 207 . Then try K 2 Cr 04 . 
b. Acidify test drops with HNO3, then add the above reagents in turn. 
c. Decant the mother liquor from a precipitated test drop and recrystaUize the 
Ag salt by heating with H2O. Try another preparation by heating with dilute 
HNO3. Recrystallize a third portion of the Ag compound, using NH4OH. 
d. Make a mixture of AgNOa and PbNOs, acidify with HNO3, then add a drop 
or two of dilute H2SO4 and finally a fragment of (NH 4 ) 2 Cr 207 . 
^ Ladenburg, Handworterbuch, 10, 713 . 
