MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 2QI 



crystals with tufted ends. This modification of the test is not satisfactory for 

 Ca, but is characteristic for Ba and for Sr (q.v.). 



d. Try testing for a trace of Ca in the presence of a large quantity of salts of 

 the elements of Group I. 



e. Try effect of a solution of (NH 4 ) 2 CO 3 on crystals of CaSO 4 2 H 2 O. 



B. By Means of Oxalic Acid. 



Apply the reagent according to Method /, page 251. 



The oxalate which separates at room temperature from neutral 

 or slightly alkaline solution has the formula CaC 2 O 4 3 H 2 O, 

 and belongs to the tetragonal system. The crystals are tiny, 

 highly refractive octahedra, or rectangular or square plates. If 

 rapidly formed, crosses and bundles or sheaves of crystallites 

 will be seen. From hot or acid solutions a monoclinic oxalate 

 CaC2O4 H 2 O separates which is practically valueless as an 

 identity test for calcium. This same salt appears to sometimes, 

 separate if a large excess of oxalic acid has been added. In addi- 

 tion to changing the crystal form free mineral acids so increase 

 the solubility of calcium oxalate as to sometimes prevent its. 

 precipitation. 



Strontium gives with oxalic acid an identical reaction, save that 

 the crystals of strontium oxalate are generally somewhat larger. 



Barium oxalate takes the form of fibrous bundles of needles, 

 and is not likely to be mistaken for either calcium or strontium. 



Zinc under certain conditions may yield a zinc oxalate difficult 

 to distinguish from the oxalates of calcium and strontium. 



Magnesium oxalate will separate in forms not to be distin- 

 guished from calcium oxalate if the test drop contains much 

 acetic acid, but in the absence of this acid magnesium oxalate 

 will not appear. 



Manganese forms groups of radiating needles (see Manganese). 



Lead oxalate may also assume forms somewhat resembling 

 those of calcium oxalate, but after a short time these crystals, 

 grow into large, well-developed prisms. 



Silver oxalate separates first as a granular deposit, soon 

 changing to crystals of a great variety of forms, hexagonal 

 plates, six-sided plate-like prisms and stout prisms with obliquely 

 truncated ends. 



