36 AN INVESTIGATION OF THE " CHROMATE METHOD " 



of ammonium chloride is perhaps somewhat excessive, but 

 it was found that when much smaller quantities were used, 

 the calcium was liable to fall when the solution was boiled 

 or digested on the water-bath. It is quite easy to tell from 

 the character of the precipitate at this point if the calcium 

 is precipitating. The strontium precipitate is very fine 

 grained and falls immediately. The calcium sulphate pre- 

 cipitate, especially if ammonium chloride is present, is 

 feathery, crj^stalline and forms slowly. It usually forms 

 first at the surface, and is particularly liable to come down 

 on boiling. 



CALCIUM OXALATE 



0.1 mg. of calcium gave a perceptible precipitate on 

 standing for a few minutes. The effect of ammonium 

 chloride in considerable quantities seemed to be to make the 

 precipitation rather slower, but 0.1 mg. could still be easily 

 detected. 



SENSITIVENESS OF METHOD 



The next step in the investigation was to carrj^ out a 

 series of analyses under normal conditions, using large excess 

 of two of the metals and varying small quantities of the 

 third until it could no longer be detected by the methods 

 used. In this way the limit of detectability for each metal 

 in the presence of excess of the others was found when the 

 complete analysis is carried through. 



A solution containing 4 mg. of barium and 200 mg. each 

 of calcium and strontium was made up. The carbonates 

 were precipitated and allowed to digest at 70° for 15 minutes, 

 and then filtered and dissolved as usual. A slight precipitate 

 formed in 5 minutes after adding 5cc of the chromate and 

 on boiling and filtering a distinct yellow deposit was left 

 on the filter after washing the potassium chromate out. 

 A. similar solution, but containing only o mg. of barium 



