Practical Electrode Potentials of Mercuric-Mercurous Systems in the Presence of Chloride Ions
Koller, James E.
In the Zimmerman-Reinhardt method for the determination of iron, .25 molar mercuric chloride (saturated) is used to oxidize excess stannous ion in a solution of hydrochloric acid leaving the iron in the ferrous state. Rieman, Reuss and Maiman report that after the reaction has proceeded to completion, the oxidation-reduction potential of the solution is -.430 volts and this value appears to be far out of proportion with the accepted value for the standard electrode potential for mercuric-mercurcus systems determined by Popoff and his associates to be -.9050 volts. The potential for this system as roughly calculated from the approximate mercuric and mercurous ion concentrations in analytical solutions would appear to be approximately -1.3 volts. This certainly cannot be the true value for under such conditions ferrous iron would be oxidized to the ferric state as evidenced by the standard electrode potential of -.783 volts for the ferrous-ferric system.1 The purpose of this thesis then, is to determine the practical electrode potentials of mercuric-mercurous systems in the presence of chloride ions and acids and to explain the behavior of these systems. The writer will endeavor to accomplish this by determining the electrode potentials of mercuric chloride solutions of varying concentrations with excess mercurous chloride present, and by investigating the potential values for solutions containing measured quantities of potassium chloride and of hydrogen chloride in addition to the calomel and mercuric chloride. This should show the separate effect of both the excess chloride and the acidity of the Zimmerman- Reinhardt solutions.
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