248 



reduces lead peroxide in iui acetic acid soliitinii in ilic presence of an ex- 

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

 thiosulphate. This method gives concordant results when proper precau- 

 tious are taken. The 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 objection to this niethotl, 

 since it usually recpiires considerable time and troulile to Itriui; ahdut tlie 

 complete solution of the lead iodide. 



A method which has given good i-csults is based upon the fact that 

 dilute nitric acid will dissolve the I'bO in red lead and leave Ix'hind the 

 rbO... 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 proceetled as fol- 

 lows : 



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

 warm dilute nitric acid (1 part acid, 10 parts water by A'olume) for 

 tliirty minutes. The sample is then liltered and the residue of lead perox- 

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

 parts of dilute nitric acid and hydrogen peroxide. 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 uitric acid and electx'oyzed in the usual way. This 

 method gives good results but requires considerable time and very careful 

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

 ing iieri-entagc of lead peroxide: .*^2.04, 31.84. .Sl.SO, 31.70. 31.S!). 



The most rapid method for the determination of PIiO. is distiihitiou 

 witli hydrochloric acid. The I'bO^ reacts witli tlie IKM to iibcrale free 

 CI according to the following reaction: 



rb( ),. 2rbO + 8ITcl -= SPbCl, + 4IL0 + CI,. 



Tlie chief objection to tliis niclhod, .ns descril>i-(l in the various Itooks, 

 is the cumbersome apparatus used. Cork or rubber stoppers on rubber 

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

 f»f the strong IIcl. Aftci- many trials we (innlly devised a very simple 

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

 (lask liavin.; a long I>eiit delivery tube and provided willi a i)erfectly lilting 

 C'ronnd glass sfopjier. The coniplele apparaliis is shown in the lignre. 



