JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VII MARCH 19, 1917 No. 6 



ELECTROCHEMISTRY.— The electrometric titration of zinc 

 with ferrocyanide. F. Russell v. Bichowsky, Geophysi- 

 cal Laboratory. (Communicated by Arthur L. Day.) 



The purpose of this note is to point out a set of very interest- 

 ing polarization phenomena that occur at a platinum electrode 

 when certain insoluble salts, such as zinc, ferric copper, lead, or 

 manganous ferrocyanide, are being precipitated. Only the case 

 of zinc ferrocyanide will be discussed here. 



If we place a platinum electrode in a solution of potassium 

 ferrocyanide and titrate this solution with a solution of some zinc 

 salt the e.m.f. of the electrode measured against some standard 

 electrode, such as the normal calomel electrode, follows a per- 

 fectly normal curve beginning at about — 0.08 volt (the e.m.f. of 

 the ferrocyanide electrode) and increasing slowly with the addi- 

 tion of the zinc solution until near the end point; on the addi- 

 tion of a drop in excess of the amount of zinc solution necessary 

 to precipitate all the ferrocyanide the e.m.f. increases very sharply 

 to — 0.40 volts. With further excess the e.m.f. stays approxi- 

 mately constant, reaching a maximum value of about — 0.45 volts. 

 The electrometric end point is thus very sharp, and the platinum 

 electrode is by all means the most accurate and convenient 

 device for determining the end point of the zinc ferrocyanide 

 titration. 



However, if instead of adding zinc solution to a ferrocyanide 

 solution we reverse the process and add ferrocyanide to the 



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