344 ELEMENTARY CHEMICAL MICROSCOPY 
B. By Means of Oxalic Acid. 
Read carefully the discussion of this test as given under 
Calcium and Strontium, pages 338 and 339. 
Barium oxalate BaC204 • WH2O forms large branching aggre¬ 
gates, radiating bundles of branching crystallites and sheaves of 
bristling fibrous needles. Rarely, well-developed monoclinic 
prisms may be obtained. The branching crystallites are char¬ 
acteristic of barium and are never given by calcium or by 
strontium. 
Precautions. 
The solution to be tested should be neutral; even a very little 
trace of acid is apt to prevent the separation of the character¬ 
istic crystals. 
If no crystals appear after a short time, add a fragment of 
sodium or ammonium acetate. 
When calcium or strontium are present the characteristic 
crystal forms of barium oxalate will rarely be obtained. Re¬ 
course may then be had to testing in dilute nitric acid. From 
nitric acid solutions the barium salt will not separate, while the 
oxalates of calcium and strontium will slowly crystallize in their 
usual form. After allowing sufficient time for the complete 
separation of calcium and strontium, decant, concentrate the 
solution and add sodium acetate. Barium oxalate now appears, 
usually in the form of rosettes of thin prisms. 
Barium oxalate, like the oxalates of calcium and strontium, 
assumes different crystal forms, according as the test drop is hot 
or cold. Hot solutions give rise to the production of strongly 
polarizing orthorhombic plates. 
Since, in order to facilitate the separation of barium oxalate, 
sodium acetate has been added, it is well to bear in mind that 
there is danger of interference from members of the magnesium 
group. 
Borates present in the test drop, if in large amount, may 
prevent the formation of characteristic crystals of barium 
oxalate. 
Although chlorides of iron and aluminum have, as has been 
