298 ELEMENTARY CHEMICAL MICROSCOPY 



B. By Means of Oxalic Acid. 



Read carefully the discussion of this test as given under 

 Calcium and Strontium, pages 292 and 293. 



Barium oxalate BaC2O4 wH 2 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 



