249 



The (letenniiintion is carried mit as folluws: One grain of Ihe sample 

 is iutrodueed into the distilling tlaslv. together with a few lumps of pure 

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

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

 flask quickly stoppered. The delivery tul>e of the flask, which is drawn 

 out to point, dips into a 100 c. c. Nessler tnhe 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 iiberated is determined by titration with N/10 normal sodium 

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

 of PbO,. 



The tlask 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 niagnesite causes the evolution 

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

 traces. Twenty to twenty-five minules should lie allowed for the com- 



