248 



reduces lead peroxide in an acetic acid solution in the presence of an ex- 

 cess of alkali acetate. The iodine liberhted is titrated with N/10 sodium 

 thiosulphate. This method gives concordant results when proper precau- 

 tions are taken. ITie best results were obtained when the potassium iodide 

 and sodum acetate were ground in a mortar, dissolved in 50% acetic acid, 

 the sample then added and the solution diluted. It is essential that all of 

 the lead iodide be dissolved. This is the chief ol)jection to this method, 

 since it usually requires considerable time and trouble to bring about the 

 complete solution of the lead iodide. 



A method which has given good results is l>ased niton the fact that 

 dilute nitric acid will dissolve the I*b() in red lead and leave behind the 

 PbOo. Careful tests have shown that the concentration of the acid should 

 be at least 1 to 20 and not stronger than 1 to 10. We proceeded as fol- 

 lows : 



Digest a weighed quantity of about one gram in about 100 c. c. of 

 warm dibit.- nilric acid (1 part acid. 10 parts water by volume) for 

 lliirty niimites. The saiiiiilc is then liltcrcd and the residue of h-ad i>erox- 

 ide washed with dilute (1 to 10) nitric acid, and tlien dissolved in equal 

 parts of dilute nitric acid and hydrogen ixn-oxide. This solution is evap- 

 orated to dryness to remove oxides of nitrogen. The evaporation carried 

 out in a Kjeldahl flask to prevent spattering. This residue of lead nitrate 

 is dissolved dilute nitric acid and electroyzed in \\\v usual way. This 

 iiietiiod gives good resull.> Imt re(|nires considerable time .-ind very careful 

 manipulation. A .'series of determinations on one sample gave the follow- 

 ing percentage of lead peroxide: .•',2.04, .31.S4. .-'.I. SO. .",1.70. 31. SO. 



Tlu> most rapid method for the deterniinat ion of I*i)(», is distillation 

 witli hydrochloric add. Tlic I'bO. reacts witli tlie IK'l to lilterate free 

 ('] according to the following reaction: 



Vh(h. 2rbO -I- 8Hcl = .SPbCL + 4H,0 + CI... 



Tlie chief (ili.jection to tliis method, as described in the various books, 

 is the cumbersome apitaratus used. Cork or ruidter stoppers on rubber 

 connections of any kind can not be used because of the corrosive action 

 of the strong lid. After many trials we finally d<'vised a very simple 

 and workable apjiaralns. It consists essentially of a KH) c. c. distilling 

 flask having a long bent delivery tube and provided with a perfectly fitting 

 ground glass stopper. The c-omplete apparatus is shown in the figure. 



