Oxidation-Reduction and Acid-Base Indicator Properties of Some Substituted Derivatives of Dicyanobis(1,lO-Phenanthroline)lron(ll) Alfred A. Schilt and Jonathon Bacon Department of Chemistry, Northern Illinois University, DeKalb, Ill. 60115
DICYANOBIS( 1,l0-PHENANTHROLINE)IRON(II), commonly referred to as ferrocyphen, has proven to be a versatile indicator. Closely related to the very commonly used redox indicator ferroin, tris(1,lO-phenanthroline)iron(II) sulfate, it may be used to advantage as a reversible oxidation-reduction indicator in strong acid solutions ( I ) . It is unique in serving as a practical internal indicator for sodium nitrite titrations of sulfamates, azides, sulfa drugs, and aromatic amines (1-3). Moreover, and surprisingly, it is useful as an indicator for the titration of weak bases in various nonaqueous solvents (1). The effectiveness of ferrocyphen as a multipurpose indicator has encouraged further studies of analogous complexes in expectation of developing a series of related indicators with graded properties encompassing a broad range of potentials and acidities. Complexes thus far studied include dicyanobis(2,2'-bipyridine)iron(II), the corresponding ruthenium(I1) and osmium(I1) complexes (4, 5 ) and tetracyanomono(1 ,lophenanthroline)ferrate(II) (6). Recently a number of substituted ferrocyphen complexes were prepared for the purpose of investigating the effects of substituents on proton affinities (7). In this note we report the results of a study of their indicator properties and formal redox potentials. EXPERIMENTAL
Samples of the substituted 1,lo-phenanthroline-iron(I1) cyanide complexes were available from a previous study. Details of their preparation and analysis have been reported elsewhere (7). The 5,6-dimethyl-l ,lo-phenanthroline derivative of ferrocyphen was prepared for the present study following the same general procedure. A Beckman Model G pH meter equipped with a saturated calomel electrode and a platinum electrode was employed for the measurement of formal redox potentials. Weighed amounts of the solid complexes were dissolved in known concentrations of sulfuric acid and titrated potentiometrically with standard cerium(1V) sulfate, maintaining the tempera0.5 "C. Experimental details and reagent ture at 25 preparations are described in an earlier, related paper (5). The formal potential was taken as the potential of the platinum electrode at the halfway point in the titration & measured from the potentiometric end point. Acid-base indicator properties were evaluated in various nonaqueous solvents by means of simulated titrations. If soluble, a small quantity (-0.1 mg) of the solid ferrocyphen derivative was dissolved in 5 ml of solvent, and 0.05M perchloric acid in acetic acid solvent was added dropwise, counting the drops necessary to change the color to yellow. The resultant solution was treated next with 0.05M aniline
*
(1) A. A. Schilt, Anal. Chim. Acfa,26, 134 (1962).
(2) A. A. Schilt and J. W. Sutherland, ANAL.CHEM.,36, 1805
(1964). (3) W. M. Banick, Jr., and J. R. Valentine, J. Pharm. Sci., 53, 1242 (1964). (4) A. A. Schilt, J. Amer. Chem. SOC.,85,904 (1963). 35, 1599 (1963). (5) A. A. Schilt, ANAL.CHEM., (6) A. A. Schilt and A. M. Cresswell, Talanra, 13,911 (1966). (7) A. A. Schilt and T. W. Leman, J. Amer. Chem. SOC.,89, 2012 (1967).
1.1 -1
-
f
i-
z 1.0W i-
0
a
J
0.9-
a
I 0.8LL
0.7 -
2
4
a
6 MOLARITY
12
10
H,SO,
Figure 1. Formal potentials of ferroin and ferrocyphen as a function of sulfuric acid concentration (in same solvent as tested), again counting the drops required to change the color, this time from yellow to red or purple. Treatment of the solution alternately with perchloric acid and aniline was repeated several times to ascertain if the color change continued to be sharp and reversible. A color change was considered to be sharp if no more than one drop of the acid solution was necessary initially to produce it and no more than two drops of the base solution was needed to reverse it. For tests with acetic anhydride as solvent, 0.05M sodium acetate in acetic anhydride replaced the aniline titrant. RESULTS AND DISCUSSION
Formal redox potentials for ferrocyphen and the substituted derivatives in various concentrations of sulfuric acid are given in Table I. Data are lacking for certain cases because of solubility limitations. Formal potentials increase with increasing acid concentration, whereas the opposite trend is followed by ferroin, as shown in Figure 1. This remarkable difference between ferroin and its close relative, the ferrocyphens, can be accounted for on the basis of differences in charges, proton affinities, and tendencies toward ion association. Ferrocyphen complexes exhibit dibasic character arising from proton affinities of the cyanide ligands (4, 7):
+ [Fe(X-~hen)~(CN)~l + 2H+
[Fe(X-phen)z(CN)~l H+
[F~(X-~~~II)~(CN)(CNH)I+ [Fe(X-phen)z(CNH)z]*f
With increasing strong acid concentrations, the uncharged species undergo more extensive protonation and take on greater positive charge. Predictably, charge separation or electron removal (oxidation) is discouraged by an increase in the net positive charge, and formal redox potentials grow more positive with increasing acid concentration. In the case of ferroin, a dipositive cation without measurable proton afinity (4), an increase in sulfuric acid concentration promotes ion association, resulting in a decrease in the effective positive charge on the ferroin species. VOL. 41, NO. 12, OCTOBER 1969
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Table I. Formal Redox Potentials in Sulfuric Acid at 25 "0 Ferrocyphen Molarity of H2S04 Ligand derivative 1 6 8 10 12 PKa 5-Nitro0.90 0.93 0.99 1.01 1.04 3.57 5-Chloro0.85 0.88 0.91 0.95 0.98 4.26 ... 0.84 0.86 0.90 0.93 4.76 5-Phenyl... ... ... ... 0.93 4.84 4,7-DiphenylUnsubstituted 0.80 0.84 0.87 0.90 0.93 4.91 5-Methyl... 0.81 0.86 0.89 0.91 5.26 0.88 5.60 5,6-Dimethyl... ... 0.84 0.86 0.79 0.83 0.84 5.94 4,7-Dimethyl... ... a Potentials are expressed in volts us. the standard hydrogen electrode potential. Each value is the average of at least two determinations.
+ n HS04-
[Fe(~hen)~] 2f
([Fe(~hen)~] z+(HS0,-),)2tn
Abstraction of electrons (oxidation) under such conditions is facilitated, and formal potentials decrease. Ion association undoubtedly plays a similar role in the case of ferrocyphen derivatives, but protonation must occur first and its effect should predominate. A direct relationship exists between formal potentials and ligand pKa values of substituted 1,lo-phenanthroline iron(I1) complexes of the ferroin type (8-10). A similar relationship holds for the substituted ferrocyphen derivatives, as shown in Figure 2. Nucleophilic substitutents increase and electrophilic substituents decrease the formal potential. The effect of a given substituent is greater for ferroin than for ferrocyphen type complexes. Approximate slopes, AEO'/ApKa, of formal potential US. ligand p K a plots are -0.077 and -0.12 for substituted ferrocyphen and ferroin derivatives, respectively. Because a ferroin derivative has three substituted phenanthroline ligands per iron atom and a ferrocyphen derivative has two, it was expected that their slopes, AEo/ ApKa, would be in the ratio of 3 to 2. The ratio found is approximately this, suggesting that the substituent effect per ligand is additive. All of the complexes exhibit reversible behavior in oxidation-reduction reactions, both with respect to changes in color and electrode potential. The colors of the iron(I1) complexes (violet, red, orange, or yellow) depend on acid concentration; iron(II1) species are violet for all acid concentrations. Formal potentials of the substituted ferrocyphens cover a span of about 0.2 V for any given sulfuric acid concentration. Each undergoes a change in potential of about 0.13 V on increasing the sulfuric acid concentration from 1 to 12M. Overall, the observed potentials lie between 0.79 and 1.04 V. Unfortunately, the lower potential region and the anticipated advantages of the methyl substituted ferrocyphens could not be realized because of solubility limitations. The minimum concentration of either sulfuric or hydrochloric acid required to dissolve a sufficient amount of the 5-methyl derivative to impart color to the solution is 3M; corresponding concentrations for the 5,6-dimethyl and 4,7-dimethyl derivatives are 6 and 8M, respectively. Use of the substituted derivatives of ferrocyphen as redox indicators affords certain advantages over the use of similarly substituted derivatives of ferroin. The former cover a range of somewhat lower potentials. Also, because their formal potentials increase with increasing acid concentration, they are ideally suited for use in conjunction with oxidants which (8) R. V. G. Ewens, Nature, 155,398 (1945). (9) W. W. Brandt and D. K. Gullstrom, J . Amer. Chem. SOC.,74, 3532 (1952). (10) G. F. Smith and W. M. Banick, Jr., Talanta, 2, 348 (1959). 1670
ANALYTICAL CHEMISTRY
J
a
;1.02
W
IO
a 0.9J
a .z a 0
LL
0.80.7 -
3.0
4.0
5.0
6.0
L I G A N D pKa
Figure 2. Formal potentials of substituted ferrocyphen derivatives U S . ligand p K , for various concentrations of sulfuric acid also exhibit increasing formal potentials, such as vanadate and dichromate. The control of acid concentration to favor coincidence of the indicator transition potential with the equivalence point potential is thus less critical. Ferrocyphen and its derivatives react rapidly and reversibly with strong acids forming both mono- and diprotonated species. Previous work has shown that a linear relationship exists between ligand pKa and logarithm of proton exchange constants in glacial acetic acid (7). Unprotonated derivatives are similar to o-chloroaniline in basicity ; monoprotonated species are much weaker bases, 5 to 12 times weaker than thiourea. The color changes that accompany protonation are pronounced (blue c--f orange tjyellow) and sufficiently sharp in certain nonaqueous solvents for the detection of titration end points. Differences in relative basicities among the ferrocyphen derivatives are not sufficiently large so that one would serve much better than another as an indicator in titrating a given weak base. Differences in solubilities and color intensities, however, are appreciable. Of those studied, the 4,7-diphenyl substituted ferrocyphen proved superior, both for its higher color intensity and for the greater number of solvents in which it is soluble. It performs satisfactorily as an indicator for the titration of aniline or similar weak bases in any of the following solvents: acetic acid, acetic anhydride, acetone, acetonitrile, benzene, chlorobenzene, chloroform, 1,2dichloroethane, diethyl ether, dioxane, ethyl acetate, ethylene glycol monomethyl ether, nitrobenzene, and propylene carbonate. The trivial name bathoferrocyphen is suggested for this compound, and its use as a nonaqueous acid-base indicator is highly recommended. RECEIVED for review June 4, 1969. Accepted August 6, 1969.
