249 



The deteriuiiiation is carried out as follows : One gram of the sample 

 is introduced into the distilling flask, together with a few lumps of pure 

 magnesite. The neck of the flask is washed down with 5 to 10 c. c. of 

 distilled water, then 40 to 45 c. c. of concentrated HCl is added and the 

 flask quickly stoppered. The delivery tuhe of the flask, which is drawn 

 out to point, dips into a 100 c. c. Nessler tube containing a 3 to 4 per cent, 

 solution of potassium iodide. The chlorine given off liberates free iodine 

 which is soluble in the excess of potassium iodide present. The amount 



of iodine liberated Is determined by titration with N/10 normal sodium 

 thiosulphate solution. One c. c. of N/10 sodium thiosulphate .01195 gm. 

 of PbOj. 



The flask should be gently heated at the beginning of the reaction and 

 strongly again at the end. During the intervening time, heating is un- 

 necessary and undesirable, since it causes a too rapid evolution of gas. 

 The action of hydrochloric acid on the magnesite causes the evolution 

 of enough carbon dioxide to carry over all the chlorine except the last 

 traces. Twenty to twenty-five minutes should be allowed for the copi- 



