348 
ELEMENTARY CHEMICAL MICROSCOPY 
precipitated by bichromate from neither acid, neutral nor am* 
moniacal solutions. 
The strontium salt of the formula SrCr04 appears from 
ammoniacal solution as exceedingly tiny yellow globulites or 
dumb-bell-like aggregates; it is dimorphic, being either ortho¬ 
rhombic or monoclinic. If the former, it is isomorphous with 
the barium salt. 
When this test is used, acidify the dilute drop with acetic 
acid, then add the fragment of bichromate. Do not stir, and 
avoid rubbing the glass with rod or wire. Barium chromate 
separates at once if present. After several minutes decant if 
a precipitate has formed. To the decanted solution or clear 
drop add a small drop of ammonium hydroxide and examine 
the preparation for dumb-bells of strontium chromate. 
If both barium and strontium are believed to be present it 
is best to warm the preparation to cause as complete a precipi¬ 
tation of barium chromate as possible before adding the am¬ 
monium hydroxide, but care must be taken to avoid unduly 
concentrating the drop. It is also usually better to allow the 
ammonium hydroxide to flow into the drop from one side rather 
than add it directly to the middle of the drop. 
Normal potassium chromate produces, with barium salts, a 
precipitate similar to that obtained with dichromate, but is not 
to be recommended as a reagent because of its property of also 
precipitating strontium compounds in acid solution. 
Ordinarily the precipitate of barium chromate is mostly 
amorphous in appearance. Here and there, however, will be 
found areas where there are recognizable crystals. A high 
power is always required for the recognition of the form of the 
crystals, hence the drop to be studied must be spread out quite 
thin. 
Free mineral acids interfere with the test. 
In addition to barium and strontium, it must be remembered 
that dichromate will also yield crystalline precipitates with 
silver, lead, mercury and thallium, but in these cases nitric acid 
may be present. 
