140 The Philippine Journal of Science me 



Table II. — Removal of zinc from galvanized iron by immersion — Continued. 



Exper- 

 iment 

 No. 



Solution used. 



Weight 



of 

 sample. 



Loss on immer- 

 sion. 



Remarks. 



30 



31 

 32 



Chromium nitrate plus sodium 

 hydroxide. 



Grams. 

 3.684 



4.629 

 4.259 



Grams. 

 0.356 



0.458 

 0.068 



P.ct. 

 9.68 



9.89 

 1.61 



72 hours required for com- 

 pletion; coating is hard and 

 adherent. 



Reaction not completed at 

 end of 10 days. 



Aluminium nitrate plus sodium 

 hydroxide. 



The results in Table II, for the most part, are self-explanatory, 

 yet some of them merit further attention. 



Theoretically the removal of zinc from iron is easy, since the 

 potential of the former is the higher in most solutions. In 

 stripping with the metallic salts, it would seem necessary merely 

 to find the salt of a metal whose potential, in any given solution, 

 is lower than that of zinc and higher than that of iron. Practi- 

 cally, however, the manner in which the potentials of metals 

 fluctuate in different solutions, the character of the metallic 



» 



deposits formed, and other factors make the operation less 

 simple. 



Iron, cathodic to zinc in most acids, does not assume the 

 passive state in contact with zinc; hence galvanized iron is 

 completely destroyed in concentrated nitric acid (No, 6).^" 



The action of lead salts is interesting and peculiar. The 

 precipitated lead from lead acetate in acid solution (No. 18) 

 or in approximately neutral solution (No. 16) is crystalline 

 in appearance, while that from a solution made alkaline with 

 sodium hydroxide (sufficient to dissolve the precipitate formed) 

 (No. 17) is velvety. The deposits obtained from lead nitrate 

 solution (No. 19) are crystalline, but much harder to remove 

 than those from lead acetate solution. The acidified solution 

 of lead acetate (No. 18) did not attack plate so well as a nearly 

 neutral solution (No. 16). A further peculiarity is the fact 

 that a lead acetate or lead nitrate solution to which sodium 

 hydroxide has been added (lead hydroxide) in sufficient quantity 

 to redissolve the precipitate formed (sodium plumbite) will 



^However, the tin of tin plate can be completely removed by concen- 

 trated nitric acid, without aflfecting the iron base, although according to de la 

 Rive [Gore, The electrolytic separation of metals. Van Nostrand & Co., 

 New York (1894), 56], tin is electropositive to both zinc and iron in con- 

 centrated nitric acid. 



