AN EXPERIMENT ON THE DETERMINATION OF FORMAL OXIDATION POTENTIALS AUBREY E. HARVEY, JR., and DELMER L. MANNING University of Arkansas, Fayetteville, Arkansas
G.H. WALDEN,L. P. Hammett, and R. P. Chapman' made a study of the ferrous-orthophenanthroline complex (Ferroin) and developed it as an oxidation-reduction indicator with a formal oxidation potential of 1.14 ,~ the volts. D. N. Hume and I. M. K o l t h ~ f f taking formal potential of the ferrous-ferric couple in one molar sulfuric acid to he 0.68 volt, re-evaluated the formal potential of the indicator at 1.06 volts. Other workersa have studied the effects of different acids employed a t different strengths upon the formal potentials of the ferrous-ferric and the ceric-cerous systems as well as the effects of various sulfuric acid strengths upon the formal potentials of Ferroin and substituted Ferroins. The experimental procedures in the above cited articles are in general too involved t o he employed in a laboratory experiment. However, with certain simplifications the study of the formal potentials of these couples can be made the basis of an excellent experiment that can he easily completed in one laboratory period. Good agreement with the accepted values can he obtained.
A convenient quinhydrone reference electrode can bt: prepared by placing a freshly prepared saturated solution of quinhydrone in 1 M sulfuric acid in a salt-bridge tube of the immersion-type calomel electrode. This eliminates the need of a connecting salt bridge. The dip-type quinhydrone electrode is immersed in the solution. Since the sulfuric acid concentration does not chance durine the course of this titration the ~otential of this electrode is constant.
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EXPERIMENTAL
Equimolar amounts of ferrous ions and ferrous-orthophenanthroline complex in 1 M sulfuric acid were prepared by dissolving 0.12 g. of reagent-grade FeS04. 7H20 and 0.12 g. of orthophenanthroline monohydrate in 180 ml. of distilled water. Eleven ml. of concentrated sulfuric acid were added and the total volume of the solution brought to 200 ml. The solution was immediately titrated potentiometrically with 0.01 M ceric ion in 1 M sulfuric acid (prepared by dissolving 3.06 g. ceric ammonium sulfate in 500.0 ml. 1M HzSOP). Potentiometric measurements were made with a type K1 potentiometer, although other types would be satisfactory. A Fisher pointer-type d.-c. galvanometer with a sensitivity of 0.1 microampere per millimeter was used. Reference electrodes used were saturated calomel or quinhydrone (in 1 M H2S01, Au). The indicator electrodes were gold or platinum. Any combination of these will yield good results. WALDEN,G. H., 2
ML of Ceda Ion Added TitrDtion of an Eguimolm* Solution of F-ow Sulfets and Ferrous Ph.n.nthrolin. sulfat. in 1 M Sulfuric Acid with M Caris Sulfot. in 1 M Sulfuric Acid. Using a Saturated Cdomsl Electrode for Reference a d a Pt El.strode as Indicator
0.01
A typical curve obtained by the simplified procedure is shown in the figure. When the quinhydrone electrode is used for a reference, the results are interpreted as in the original article (ref. 1). When the saturated calomel electrode of known potential is used, the formal potentials of the ferric-ferrous, ferric-ferrous orthophenanthroline, and the ceric-cerous electrodes may he calculated from the observed potentials at the points A, C, and E on the titration curve. These points correspond to the potential observed when the ratio of the concentration of the oxidized and reduced forms in the respective couples is unity. The formal potentials are calculated as follo~vs:
+
E& = Es a . E..bad. = Euba
Ealboda
+ 0.2438
JOURNAL O f CHEMICAL EDUCATION
528
Experimental results for various electrode combinations are shown in the table. Eleet~odesvsed As can be seen, the formal potential of the ferric-ferRef. Ind. Couple rous couple (point A) can be determined with reason8.c.e. Pt FeC+ Fe+++ 0.69 0.68 able accuracy even without preliminary reduction pro~e&:+ FeRs+++ 1.06 1.06 cedures if the solutions are prepared from reagent grade Cef++ b e + + + + 1.36 1.44 8.e.e. AO ~ e + Pe+++ + 0.68 0.68 material immediately before titration. With a standFeFb:+FeRs+++ 1.06 1.06 ard reference electrode, the accuracy with which the ce+++ be++++ 1.39 1.44 complex potential is determined is not dependent upon 1.06 Pt ~ e h + ~ F e R , + + + 1.04 H&! 1.06 Au F e h + + : F e ~ a + + + 1.03 HsQ the preliminary reduction to ferrous ions. This suggests that the simplified procedure might he used for the development of other oxidation-reduction indicaIn the original procedure, it was necessary to reduce tors. the iron completely to the ferrous state before starting One precaution is to be observed in the preparation of the titration in order that point A (from which the value the solution for titration. The ferrous-orthophenanof point C is calculated) might be determined accu- throline complex must be formed in the absence of any rately. With a reference electrode of known potential sulfuric acid. If the reagents are dissolved directly in 1 point C can he calculated, as long as enough ferrous ions M sulfuric acid only one break will be observed in the are present to permit the accurate determination of end titration curve. This break corresponds to the oxidapoint B. tion of the ferrous ions to the ferric state. ..
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. .,.
:
, Formal pdentlal (in 1 M HsSO,), volts . Ezperi- Acmenial cepled
