88 



DIVERS AND HAÜA. 



the lead replaced by hydrogen. The weight of the precipitate 

 obtained from a given quantity of trisodium salt, is greater with lead 

 acetate than with tlie nitrate, and greater as the excess of acetate used 

 is greater. Tlie acetate mother-liquors, though alkaline to methyl 

 orange, are acid to phenol-] )hthaleïn, and after replaciijg the lead in 

 them by sodium, Iw ])reci[)i fating with normal sodium oxalate, can be 

 titrated h\ sodium liydroxide and phenol-] )hthaleïn as indicator. 'I'hus 

 tested these liquors j)roved to contain free acetic acid, and in increasing- 

 quantity with that of the acetate used, and, therefore, of the jDrecipitate 

 ]3roduced. In these experiments |3articularly excellent normal lead 

 acetate was used. It ap]3eared from tlie ex]3eriments made that about 

 one-eighth of the excess of lead acetate used must react with disodium 

 imidosul])honate, a ])roduct of the main reaction, as already ]winted 

 out, the change being as follows : — 



3Pb(OÄ^)2+ 3OH2 + HNa.N(S03\, = (HOPb)3N(S03)2-|- 2NaOÄ^ -1-4H(3.4^ 



The acetic acid tlnis generated, acting so as to reverse the reaction, 

 is ]irobably the cause of the very slight replacement of lead by hydro- 

 gen in the salt ]^recipi tated by acetate. 



When the trisodium imidosul])liona.te solution is added by degrees 

 to the lead acetate, a gelatinous jjredpitate like lesid hydroxide forms 

 and then redissolves in the lead acetate. When enough of the 

 sodium salt has been added to cause a permanent jjrecipitate this is 

 somewhat slimy and only slowly becomes pulverulent, while the 

 walls of the vessel get coated with crystalline preci])itate. The mixed 

 ])rt)duct is unfit for analysis. When the lead acetate is added 

 gradually to tlie sodium imidosulphonate, the ])recipitate also remains 

 ^■olu!llillous and somewhat gelatinous, and is jirobablv not a ]3ure 

 |>V()(luct. When after adding not too much sodium salt, tlie liquor is 

 quickly filtered, it slowly yields a few brilliant crystals of what is 



