1244 Journal of Applied Microscopy 



Sulphuric acid added to salts of strontium may, under exceptional conditions 

 (if the preparation be examined at once), yield a precipitate which closely resem- 

 bles that given by calcium. These crystals of strontium sulphate rapidly disin- 

 tegrate, however, and there results a fine granular deposit. This granular or 

 sandy precipitate is the form assumed by strontium sulphate under the condi- 

 tions which ordinarily obtain in this test. Barium is immediately precipitated in 

 an exceedingly finely divided condition, amorphous in appearance. Any lead 

 which may be present will also be precipitated as a dense white amorphous 

 powder. 



If calcium sulphate be heated with a drop or two of sulphuric acid until white 

 fumes (SO 3) are given off, and the preparation allowed to cool, the calcium will 

 separate either as the salt CaSO^ or as CaSO^* H2SO4. The crystal forms 

 most frequently met with are shown in Fig. 4.5. This modification of the test is 

 not satisfactory for calcium, but is characteristic for barium and strontium (q. v). 



It is not always wise to conclude that calcium is 

 present when crystals separate on the addition of sul- 

 phuric acid, which apparently resemble the star and 

 sheaf-like aggregates of calcium sulphate, even if the 

 crystals exhibit oblique extinction. For it sometimes 

 happens that other compounds, not calcium sulphate, 

 separate in forms not to be distinguished, at first sight, 

 from the crystals of the calcium salt. Such instances 

 are fortunately very rare. »pw-o.o.nv«v. 



It has been proposed to check this test as follows : ^'s- ^s- 



After allowing sufficient time for the separation of almost all the calcium as 

 CaS04» 2H2O, draw off the supernatant liquor; add to the residue a solution of 

 ammonium carbonate ; the crystals of calcium sulphate are dissolved and highly 

 refractive rhombs and grains of calcium carbonate appear, which are easily 

 found by examining the preparation between crossed nicols. A high power is 

 generally required. 



In the presence of borates, calcium cannot be satisfactorily detected by means 

 of sulphuric acid. In such an event Method II can be employed. 



Exercises for Practice. 



(See methods and exercises given under Strontium and Barium.) 



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

 solution. 



Try the effect of precipitating in the presence of free hydrochloric acid ; then 

 in the presence of free nitric acid. 



Precipitate with dilute sulphuric acid, then heat, adding more acid if neces- 

 sary, until white fumes are given off, cool, breathe on the preparation and 

 examine. 



Try testing for a trace of calcium in the presence of a large quantity of salts 

 of the elements of Group I. 



Try effect of a solution of ammonium carbonate on crystals of calcium sul- 

 phate. 



