28 A MODIFICATION OF THE PROCEDURE 



The usual source of error at this point is of course, that 

 when barium is present alone and in considerable quantity, 

 there may not be sufficient quantity of acetic acid present to 

 hold all of it in solution as the chromate, and as a result, some 

 of the barium may be precipitated in place of strontium as 

 the sulphate. Tests were made, therefore, to determine the 

 maximum amounts of barium which could be present under 

 the conditions of the procedure. It was found that a solution 

 containing 400 mgs. of barium and no strontium, failed to 

 give the slighest precipitate as the sulphate after having been 

 removed as the chromate in presence of 3 cc's of acid. 



Separation of Strontium and Calcium 



The problem to be studied in this separation was the ex- 

 tent to which calcium sulphate is dissolved by ammonium ace- 

 tate under the conditions of experiment, and also to find the 

 minimum quantity of strontium that could be detected by 

 precipitation as the sulphate. In an investigation of this sep- 

 aration using ammonium sulphate and ammonium chloride, 

 Vickery* has found that 80 cc's of a 10% solution of ammonium 

 chloride were necessary to prevent all precipitation of calcium 

 sulphate in a solution containing 200 mgs. of calcium, and that 

 under similar conditions, 3 mgs. of strontium gave a faint tur- 

 bidity. 



Solutions were prepared containing varying quantities of 

 strontium and calcium, and separations were carried out accord- 

 ing to the procedure given. It was found that 3 mgs. of stron- 

 tium could readily be detected in the presence of 400 mgs. of 

 calcium. • Also it was found that a solution containing only 

 400 mgs. of strontium gave not the slightest test for calcium. 



It is apparent therefore, that the substitution of ammonium 

 acetate for the chloride or sulphate offers a wider limit for the 

 delicacy of the strontium-calcium separation. The final de- 

 tection of calcium in the filtrate from strontium sulphate de- 

 pends upon the well known oxalate precipitation, and of course, 



*These Transactions, Vol. XIV, page 30. 



