1294 



Journal of Applied Microscopy 



\P\V.= O.Olr 



Fig. 54. 



/. Sulphuric Acid added to solutions containing Bariu7n p)-ecipitaTcs Barium 

 Sulphate. 



BaCU + H2SO4 == BaS04 + -HCl. 



Method.— A.dd to the test drop dilute sulphuric acid as long as any precipi- 

 tate is formed ; draw off, treat the residue with a large drop of the reagent, and 

 heat until copious white fumes are given off. Cool, breathe on the preparation, and 



examine. Barium sulphate separates, first as 

 tiny rectangular plates and X-shaped skeletons ; 

 then, in a short time, much larger crystallites 

 appear with more or less feathery arms which 

 still retain the X-form. See Fig. 54. These 

 crystals apparently belong to the orthorhombic 

 /J7 "^ 1^^ system. 



\J "i^T^ ^3- ^ Remarks. — Owing to the low solubility of 



barium sulphate, a considerable amount of sul- 

 phuric acid is necessary and the preparation must 

 be strongly heated in order to obtain a solution of 

 the precipitate. In this operation, the precau- 

 tions mentioned under Strontium must be ob- 

 served. 



In the event of a heavy precipitate being obtained with the reagent, it is wise 

 to remove a small portion to another slide for recrystallization, rather than 

 attempt to dissolve the whole mass. 



Recrystallization in the presence of much calcium is to be avoided. First 

 extract the calcium sulphate with hot water. 



In the presence of moderate amounts of strontium the crystallites of barium 

 sulphate are generally not well formed. If strontium is in excess, the crystals 

 separating from the hot sulphuric acid have the general type of strontium sul- 

 phate, but are not well developed and exhibit an inclination to approach X-forms 

 of barium sulphate. For this reason it is advisable to remove any strontium 

 which may be present by repeatedly heating with hydrochloric acid, in which 

 strontium sulphate is soluble while the barium compound remains undissolved 

 and can then be recrystallized by heating with sulphuric acid. 



Any lead sulphate which may be present will appear, first, in crystals very 

 suggestive of strontium sulphate, then, in a short time, in larger crystallites which 

 may at times be mistaken for barium sulphate. Treatment with hydrochloric 

 acid or, better, with sodium hydroxide will remove the lead, leaving the barium 

 salt unacted upon. 



It is sometimes desirable to apply other tests to the precipitated sulphate in 

 order to confirm the presence of barium. In such an event, transfer the washed 

 precipitate to platinum foil or to a platinum cup and fuse with potassium carbon- 

 ate. The fused mass is then extracted with water and the residue of barium car- 

 bonate dissolved in hydrochloric acid. This solution can then be tested for 

 barium by any of the tests given below. 



Since chlorides of the trivalent .metals sometimes interfere with the formation 



