142 bichowsky: electrometric titration of zinc 



zinc solution, the initial e.m.f. of the platinum electrode, instead 

 of having the invariant value of — 0.45 volts can be made to have 

 almost any value between 0.0 and - 0.5 volts, depending on the 

 previous treatment of the electrode. If the electrode has been 

 allowed to stand in contact with a ferrocyanide solution for some 

 time the e.m.f. will be between - 0.1 and - 0.25 volts, depending 

 on the length of time it has been in the solution ; if it is charged 

 to say — 2.0 volts while in contact with a zinc solution the initial 

 e.m.f. of the electrode may reach the reversible value of — 0.45 

 volts, or even higher. 



If the electrode has been adjusted to an e.m.f. of — 0.45 volts 

 and the titration of zinc with ferrocyanide carried through as 

 before, the exact reverse of the previous curve is obtained, the 

 e.m.f. begins at — 0.45 volts, drops sharply at the end point to 

 - 0.10 and continues at nearly that value on further addition of 

 ferrocyanide. In this case, therefore, our initial curve is rever- 

 sible. If, however, the platinum electrode has been treated so 

 that it shows an initial voltage of say — 0.20 volts, on adding 

 the ferrocyanide solution the numerical value of the e.m.f. in- 

 creases slowly, instead of decreasing, this increase continuing 

 until the curve that is obtained by plotting e.m.f. against cc. of 

 ferrocyanide added intercepts the normal curve. At this point 

 the e.m.f. is a maximum. From this point on the e.m.f. curve 

 is entirely normal, the end point being perfectly sharp and un- 

 changed. This phenomenon is similar to the one noted by 

 Forbes and Bartlett 1 for the titration of ferrous iron with dichro- 

 mate, but in our case not only is it possible to avoid the phe- 

 nomenon entirely by the proper treatment of the electrode 

 already indicated, but it is also possible not only to control the 

 initial e.m.f. of the electrode, but also the shape and slope of the 

 ascending (abnormal) part of the curve, thus causing the point 

 of interception of the abnormal and normal curve to be varied. 

 In a well stirred solution the abnormal curve starting at - 0.2 

 volts will almost always intercept the normal curve just before 

 the end point is reached, however, if the stirring is made very 



1 Journ. Amer. Che.m. Soc, 35: 1527. 1913. 



