1290 



Journal of Applied Microscopy 



Fig. 51. 



solutions containing strontium, yields bundles 

 of needles rapidly disintegrating to merely a 

 very fine granular precipitate. Unless the 

 preparation is examined immediately after the 

 addition of the reagent no acicular crystals will 

 be seen. 



If calcium is also present the grains of stron- 

 tium sulphate are generally larger and often ex- 

 hibit a tendency toward a spindle shape. 



In all cases recourse must be had to re- 

 crystallization. 



It is probable that the crystals of strontium 

 sulphate separating from hot concentrated sul- 

 phuric acid have a composition analagous to 

 calcium sulphate recrystallized under the same conditions. 



If after a short time no crystals appear in the drop of acid, breathe on the 

 preparation. 



It is imperative that the drop to be heated be placed at the very corner of 

 the slide, that the latter be inclined so as to keep the drop at the corner, and that 

 the " micro " flame be applied a little to one side, and nearer the center of the 

 slip. This procedure is necessary in order to avoid (1) the breaking of the 

 glass slide, and (2) the spreading of the sulphuric acid. This tendency of the 

 hot liquid to flow over the slip when it is placed in a horizontal position is so 

 great that it is generally advisable to transfer a part of the acid to a clean slip. 

 The transfer is accomplished by gradually raising the slip, which has been heated, 

 until it assumes an almost vertical position and the drop has flowed to the extreme 

 corner. The corner is then brought in contact with a clean glass slide and the 

 drop of solution caused to flow onto the latter by means of a glass rod. In this 

 way a clear, well rounded, deep drop is obtained in which good crystals of stron- 

 tium sulphate will form. 



When dealing with very minute quantities of material it is better to heat with 

 sulphuric acid on platinum foil, since the hot acid may extract sufficient mate- 

 rial from the glass to interfere with the reaction. 



The solubility of strontium sulphate in strong hydrochloric acid is quite low, 

 hence it is necessary to employ a considerable quantity of the solvent in order to 

 get satisfactory results. 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 insoluble in HCl. It is 

 of course essential in recrystallizing from HCl that only traces of free H2SO4 

 be present. Free nitric acid should also be absent. 



Before any attempt is made to recrystallize the precipitate of strontium sul- 

 phate, it is advisable, and usually Accessary, to remove any calcium which may 

 be present. This is accomplished by extracting the precipitate with hot water. 

 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 stron- 



