1246 Journal of Applied Microscopy 



Exercises for Practice. 



Try reaction after the manner given above, on a salt of calcium in neutral 

 solution. Try again in the presence of free HCl ; then in the presence of free 

 HNO3. 



Precipitate calcium oxalate, draw off the supernatant liquor, and treat the resi- 

 due with dilute H2SO4. After examining the preparation, add more acid, and 

 heat until white fumes appear, cool and examine again. 



(See also suggestions under Barium.) 



///. Sodium lartrate added to neutral or acetic acid solutions of salts of Calcium 

 catises the precipitation of Calcium lartrafe. 



CaCl2 + HNaC^H^Oe = CaC^H^O,. 4H2O + NaCl + HCl. 



Method. — To a drop of the solution to be tested add a little sodium acetate 

 and a little acetic acid, then add a fragment of sodium tartrate. In a few 

 moments crystals of calcium tartrate separate near the spot where the reagent 

 was added. These crystals are large, colorless, transparent, and well developed 

 prisms belonging to the orthorhombic system (Fig. 47). 



Remarks. — The reaction is apt to fail in the pres- 

 ence of free mineral acids owing to the solubility of 

 the calcium tartrate ; hence the reason for the addi- 

 ^^_ \\ <» tion of the sodium acetate. The calcium salt is also 



\ \ IS t^^^ "^ soluble in sodium and potassium hydroxides. 



A little free acetic acid favors the formation of 

 well developed crystals. 



If the solution is too dilute no crystals will appear 

 Fig. 47. for some little time. On the other hand, too con- 



centrated solutions give rise to the immediate precipitation of crystallites and 

 imperfectly developed prisms. 



Strontium gives a tartrate isomorphous with that of calcium and hence not to 

 be distinguished from the latter, although there is a decided tendency on the 

 part of the strontium salt to form shorter and therefore proportionally stouter 

 prisms. 



Barium is precipitated in the form of a fine powder. 



Lead is at first thrown down as a fine sandy precipitate soon crystallizing in 

 the form of irregular crystallites not to be confused with either calcium or 

 strontium. 



In the presence of magnesium the formation of the crystals of calcium tar- 

 trate is greatly retarded, and according to Behrens the crystals then formed are 

 more slender and rod-like ; in the experience of the writer, however, the forma- 

 tion of slender mixed crystals is seldom observed. 



The tartrates of potassium and ammonium may sometimes be precipitated in 



forms which at first sight are difficult to distinguish from those of the calcium salt. 



The testing for calcium with sodium tartrate is of little value when dealing 



with unknown mixtures, for in addition to the fact that the crystals of calcium 



tartrate cannot be distinguished from those of strontium tartrate, salts of barium. 



