250 



pJete reaction to take place. Care must be taken during the last stages 

 of tlie reaction, since the maguesite is used up and the HCl gas given off 

 being extremely soluble allows the potassium iodide solution to suck back 

 into the flask. This is prevented by heating. Heating at this i>oint not 

 only prevents the sucking back of the KI solution, but is necessary in 

 (iider to exiiel the last traces of chlorine from the flask. A second Nessler 

 tube should be inserted and the heating continued a few minutes, in order 

 to make sure that the reaction is complete. During the distillation the 

 Xessler tul)o is surrounded by a beaker of cold water, in order to keep 

 Ihc temperature of the potassium iodide i'juluticn as low as possible so as to 

 lirevent the volatilization of the iodine. 



This method is iKitli (piick and accurate. A series of analyses on one 

 sample gave the following results: 31.84, .'il.!>2, 31.!i2 per cent, lead per- 

 oxide. In all about fifty different samples were run by this method and 

 in every case it was easy t(t check the results to within 0.10 per cent. 



If a distilling flask with a ground gla.ss stoppt'i" i« not at band, onr* 

 can be made in a few minutes. Select a glass stopper of the proi»er size 

 to fit the neck of the flask and fasten it in an horizontal position to the 

 end of a slowly rotating shaft or axle fit the flask over the rotating stop- 

 jicr and grind with fine emery tliist moistened will a nii.xture of equal parts 

 of ether, turpentine, and alcohol. 

 (lie mica I l.ithonitorii. 

 Iiididiiu Viiirvrsitji. 



