26 A MODIFICATION OF THE PROCEDURE 



Boil the solution until free from H2S. Filter if necessary 

 and add NH4CI, (if it has not already been added). Make 

 ammoniacal and add 10-20 cc of ammonium carbonate solu- 

 tion. (The temperature of the solution at the time of the addi- 

 tion of ammonium carbonate should not exceed 70° C.)* Filter 

 and test for completeness of precipitation. The precipitate 

 may now contain the carbonates of barium, strontium and 

 calcium; the filtrate will contain the ions of the alkali group 

 with magnesium ion. 



Treatment of the Precipitate. 



Dissolve the precipitate by pouring over it 10 cc of 3 N. 

 acetic acid. Continue this treatment, using the same 10 cc of 

 acid, until all action is over. Carefully neutralize the acetic 

 acid solution with ammonium hydroxide and then add 3 cc 

 of acetic acid in excess. Next add about 10-15 cc of potassium 

 chromate solution: a yellow precipitate is barium chromate 

 and confirms the presence of barium. Heat to boiling, let 

 stand 10 minutes, and filter. To the clear filtrate, which should 

 be orange yellow in color with the excess of di-chromate ions, 

 add 10 cc of acetic acid, a strong excess of ammonium hydr- 

 oxide, and 10 cc of ammonium sulphate solution. (The solu- 

 tion should have a volume of approximately 100 cc at this 

 point.) Boil for about two minutes; a white precipitate is 

 strontium sulphate and confirms the presence of strontium. 

 Let stand for about ten minutes and filter. 



To the clear filtrate add 10 cc of ammonium oxalate solu- 

 tion. If no precipitation occurs at once, boil for a few mo- 

 ments; a white precipitate is calcium oxalate and confirms the 

 presence of calcium. 



The usual and most satisfactory method of studying pro- 

 blems of this kind is to determine the analytical limits involved. 

 That is, to find the minimum quantity of one ion which can 

 be detected in presence of a maximum quantity of the other. 

 With this idea in view solutions were prepared of the following 



*H. B. Vickery: these Transactions, Vol. XIV, page 31. 



