Excerpt from The Effect of Substitution on the Free Energy of Reduction of Benzoquinone: Including a Study of the Electrometric Titration Method of Measuring Potentials
That the time required 'for the attainment of equilibrium may often be greatly shortened by the addition of... a secondary substance such as KI as a catalyzer or potential intermediate was shown by Haber and Russ11 for the quinone-hydroquinone system and by Loimaranta and Abegg12 for the arsenite-arsenate system. Luther, however, questioned the reliability of all methods that employ extraneous materials lest the potentials so measured be those of the added substance and not those of the specific com ponents desired, and outlined the criteria that must be met in order that potentials measured with inert electrodes such as gold or platinum may be considered to be those for the specific oxidation reduction system under examination. These methods were, there fore, apparently abandoned until 1920 when Clark14 introduced the method of using the secondary material (in this case titanous chloride) as a titrating agent and thereby gave a means of altering the concentration of the components and of calculating directly the concentrations of the oxidized and reduced forms from the volume of the titrating agent used and the volume required for complete reduction. Clark infers that his potentials are the correct ones Since he obtains nearly constant values for different concentrations, but Offers no other proof of the validity of his method of measure ment. It will be shown from the data which follow that the potentials measured by the titration method meet all the criteria proposed by Luther and in addition satisfy the still more rigorous test of approaching the equilibrium from either side.
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