340 ELEMENTARY CHEMICAL MICROSCOPY 



after heating, and before crystallization (due to cooling) sets in. 

 The resulting crystals are quite small and of varied form. The 

 results are less satisfactory than with sulphuric acid, but there 

 is, on the other hand, the advantage that barium sulphate is 

 practically insoluble in hydrochloric acid. It is of course essen- 

 tial in recrystallizing from hydrochloric acid that not more than 

 mere traces of free sulphuric acid be present. Free nitric acid 

 should be absent. 



Before any attempt is made to recrystallize the precipitate 

 of strontium sulphate, it is advisable, and usually necessary, 

 to remove any calcium which may be present. This is accom- 

 plished by extracting the precipitate with hot water in which 

 the calcium salt is soluble. Unless this is done, peculiar crystal 

 forms are obtained which are difficult to interpret. 



If only a small amount of barium is present, characteristic 

 crystals of strontium sulphate are still obtained from hot sul- 

 phuric acid, but much barium is apt to alter the usual crystal 

 form, although the appearance of the crystals separating still 

 suggests the strontium sulphate type. An excess of barium 

 seems to cause the majority of the crystals to assume forms 

 somewhat resembling barium sulphate. But, in general, crystals 

 of both strontium and barium sulphate can be distinguished in 

 mixtures of these two elements. 



Any lead which may be present will be precipitated in an 

 amorphous condition by the dilute acid, although under rare 

 conditions it may appear crystalline. Recrystallized from hot 

 sulphuric acid, the lead sulphate, as stated above, will separate 

 in forms which at first closely resemble those of strontium sul- 

 phate and which, later, grow to forms which may be mistaken 

 for barium sulphate. Recrystallized from hydrochloric acid 

 there is less danger of error. If in doubt, extract the precipi- 

 tated sulphates with a solution of potassium or sodium hydrox- 

 ide in which lead sulphate is soluble. 



Silver sulphate will appear as already described under calcium. 

 Hence silver as well as most of the lead should first be removed 

 with hydrochloric acid. 



As in the case of calcium, chlorides of the trivalent metals 



