MICROCHEMICAL REACTIONS OF CALCIUM 337 



c. Precipitate with dilute H 2 SO 4 , then heat, adding more acid if necessary, until 

 white fumes are given off, cool, breathe on the preparation and examine. Calcium 

 will separate either as the salt CaSO4, or as CaSO 4 -H 2 SO 4 . The crystal forms 

 most frequently met with are thin, rounded, prism-like plates or fusiform 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. A retardation of the reaction results. 



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 7, page 299. 



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 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 



