Voltammetric Behavior of Azoxybenzene, Azobenzene, and Hydrazobenzene at the Graphite Electrode LAURA CHUANG, ILANA FRIED,' and PHILIP J. ELVING The University o f Michigan, Ann Arbor, Mich. In aqueous 50% ethanol solution over the pH range of 1.6 to 12.5, azoxybenzene undergoes a 4e reduction at the pyrolytic graphite electrode to hydrazobenzene, azobenzene a 2e reduction to hydrazobenzene, and hydrazobenzene a 2e oxidation to azobenzene; the half-peak potential varies linearly with pH with a change in slope at about pH 6.5 for azoxybenzene and pH 9 for the other two compounds. The behavior of the three compounds at the D.M.E. has been critically evaluated and compared to their behavior at the graphite electrode. Generally, the azobenzenehydrazobenzene system seems to b e less reversible at the graphite electrode than at mercury. The photoisomerization of trans- to cis-azobenzene would normally have little effect on the observed voltammetric behavior, except in the pH region near 8 to 9.
T
azobenzene-hydrazobenzene system has been repeatedly studied a t the dropping mercury electrode (D.M.E.), but the widely discordant results obtained indicated that it would be of interest to investigate the voltammetric behavior of this system a t the pyrolytic graphite electrode (P.G.E.). The experimental conditions used were selected to be generally comparable to the varied conditions used by previous investigators of the system at the D.M.E., as well as to studies by the present authors on allied compounds at the P.G.E. -e.g., the nitrosobenzenephenylhydroxylamine .system (4). The initial results of the present investigation indicated the necessity of also investigating the azoxybenzenehydrazobenzene system and the effect of photoisomerization of the commonly used trans-azobenzene on the observed polarographic pattern. The behavior observed a t the P.G.E. is compared to that observed at the D.M.E.; the bases for such comparison are indicated in the subsequent discussion. HE
EXPERIMENTAL
Azobenzene (Eastman white label) and azoxybenzene (Paragon) were recrystallized twice from 95% ethanol. Hydrazobenzene (EastReagents.
1528
ANALYTICAL CHEMISTRY
PH Figure 1. Variations with pH of half-peak potentials of azobenzene, azoxybenzene, and hydrazobenzene in 50% aqueous-ethanol buffered solutions
0 C,
0
Reduction waves of azoxybenzene Reduction waves of azobenzene Oxidation waves of hydrazobenzene
man yellow label) was recrystallized each time before use from 95% ethanol, t o which a few dro s of aqueous ammonium sulfide soLtion were added t o avoid air oxidation; the crystals were filtered, washed with water, and dried at reduced pressure in an Abderhalden apparatus under refluxing benzene; the product was pure white. Buffer solutions were prepared from reagent grade chemicals. Nitrogen was purified and equilibrated by bubbling successively through two ammonium vanadate solutions reduced by zinc in hydrochloric acid, water containing calcium hydroxide, and 50% waterethanol. Apparatus. Voltanimograms were recorded on a Leeds & Northrup T y p e E Electro-Chemograph (polarization rate, 200 mv. per minute). The water-jacketed H-cell ( I S ) was maintained a t 25" f 0.1" C.; the saturated calomel reference electrode (S.C.E.) was separated from the electrolysis leg by a sintered disk and an agar plug saturated with potassium chloride. The preparation and method of use of the P.G.E. have been described (4, 6). The p H values reported are those measured by a conventional glass
electrode-S.C.E. pair (Leeds & Northrup Node1 7664 p H meter) on t h e Soy0 aqueous ethanol test solutions of ionic strength ca. 0.5-If. h Fisher controlled potential Electro-Analyzer and a current-measuring system consisting of a standard resistor, a Dymec Model 2210 voltage-to-frequency converter, and a Hewlett-Packard Model 52/AR electronic counter, were used for macro scale electrolysis and coulometry. Spectrophotometric measurements were made with a Beckman Model D B recording spectrophotometer and stoppered 1-cm. quartz cells. Procedures. Stock solutions (5mM) were prepared just before use b y dissolving a weighed amount of sample in 9570 ethanol. Voltammetric test solutions were prepared by pipetting 25 ml. of buffer solution into a 50-ml. volumetric flask, adding the appropriate amount of stock s o h tion, and diluting to the mark with %yoethanol; after mixing, the volume was again brought to the mark with ethanol and remixed. A suitable amount of test solution was transferred Present address, Department of Inorganic and Analytical Chemistry, The Hebrew University, Jerusalem, Israel.
to the H-cell a n d purged with nitrogen for 10 minutes before inserting the polished P.G.E.; purging was then continued for 1 minute. The general voltammetric procedure has been described (4). Solutions for coulometry were similarly prepared, using 100-ml. volumetric flasks. Because of the rapid air oxidation of hydrazobenzene, reagents used for preparing stock and test solutions were predeaerated and the stock solution was not kept longer than 0.5 hour. Solutions for spectrophotometry were similarly prepared; to ensure the absence of air, the quartz cell was filled to the brim with test solution so that on stoppering some solution was squeezed out. VOLTAMMETRIC BEHAVIOR
The voltammetric reduction of azobenzene and azosybenzene, and the oxidation of hydrazobenzene in buffered aqueous 50% ethanol solution at the P.G.E., were examined over the p H range of 1.6 to 12.5. Composition of the buffer solutions used and their useful potential ranges are listed in Table I. Typical data are summarized in Table I1 and Figure 1. For convenience and economy of discussion, the potential-pH patterns of the three compounds are individually discussed, and the concentration relations and coulometry considered for the three as a group. Azoxybenzene. Azoxybenzene gives one reduction wave a t the P.G.E., whose EpiZ varies linearly with a change of slope a t ca. p H 6.5:
Ep,2 = -0.18 to 0.081 p H (pH 1.6 to 6.5)
(1)
Epi2 = -0.46 to 0.037 pH (pH 6.5 to 12.5)
(2)
Although Equation 1 has the same slope as observed by Fredrickson (8) a t the D.M.E. (Table 111),EpiZvalues are generally 0.1 volt more negative than Ellzvalues. This difference is probably due, a t least in part, to the differing sol-
Table
II.
pH
Table 1.
pH and Decomposition Potentials of Buffer Systems at Pyrolytic Graphite Electrode
Buffer composition KC1 HC1 McIlvaine
nHa r--
Single Decomposition potentials, volts strength 50y0 EtOH Cathodic Anodic 1.1 1.4 1.6 -0.35 to -0.42 0.86 2.6 3.3 -0.76 0.81 2.4 2.5 3.1 3.5 4.2 -1.08 to -1.25 0.70 to 0.71 5.1 -1.25 0.60 4.8 4.9 5.4 -0.58 NH4C1 NHs 6.6 -1.30 0.80 7.9 0.54 8.9 8.8 8.6 -0.80 9.1 9.0 -1.35 0.59 KCl KOH 11.0 11.4 -1.40 0.50 12.0 12.5 -1.01 0.52 pH values refer to 1M ionic strength buffer prepared, dilution by equal volume of water, and dilution by equal volume of 95% ethanol as described for preparation of voltammetric test solution. *Potential where current in 50% aqueous ethanol solution deviates by 1 pa. from straight-line portion of background curve is taken as supporting electrolyte decomposition potential. Potentials us. S.C.E., reproducible to h0.05 volt.
+
Double strength
+
+
vent media and liquid junction potentials -e.g., the transition from 30% methanol to 50% ethanol in pH 3.1 buffer causes Epizfor azoxybenzene to be 0.07 volt more negative; an increase from 25 to 50% ethanol made Ep/2 for azobenzene 0.08 volt more negative (14). The difference between Equation 1 and the data reported by Holleck and Holleck (11) for the D.M.E. can also be attributed to the different experimental conditions. Holleck and Holleck reported that maximum suppressors make Eliz more negative and chanBe the E1/2-pHslope, and that a change In concentration from 1 0 - ~to 10-3M makes Ell2about 0.1 volt more negative. Despite the differences reported by different investigators using different conditions, there is general agreement that the reduction potential of azoxybenzene becomes linearly more negative with increasing pH with a decrease in slope between pH 6.5 and 8.5. Azobenkene. EpiZ for the one reduction wave normally observed
for azobenzene a t the P.G.E. varies linearly with p H with a change in slope a t ca. p H 9:
Epiz = 0.112 to 0.079 p H (pH 1.6 to 9)
Epiz
=
(3)
0.231 to 0.021 p H (pH 9 to 12.5) (4)
A prewave, whose magnitude is one fifth or less of that of the regular wave, appears about 0.2 volt before the latter a t p H 8 and 9 (this prewave is discussed below in connection with the effect of light on the voltammetric behavior of azobenzene) . Generally, the potential data for the reduction of azobenzene a t the D.M.E., summarized in Table 111, follow the same broad pattern as at the P.G.E.Le., in the acidic and neutral regions E1,2 varies as 0.06 p H to 0.08 p H with an extrapolated Eliz a t pH 0 of 0.05 to 0.11 volt; above ca. pH 9, the dependency on pH is markedly less. However, there are significant differences between
Variation with pH of Voltammetric Behavior of Azoxybenzene, Azobenzene, and Hydrazobenzene at Pyrolytic Graphite Electrode -EPi2,
0.60
Azoxybenzenea v. i,, pa. Ip/AC
-EplZ,
v.
Azobenzenea ip, pa.
5.1 5.9 1.51 5.4 0.32 2.44 6.6 0.70 7.6 1.94 0.42 3.26 7.9 0.77 5.5 1.40 0 . 5OC 2.85 8.6 0.56 2.88 9.0 0.78 5.7 1.45 0. 6OC 1.60 11.4 0.89 6.1 1.54 0.72 2.37 12.5 0.91 6.4 1.62 0.76 2.12 a A . Electrode surface = 19.6 sq. mm. C. Concentration = 0.2mM. b A . Electrode surface = 12.6 sq. mm. c Extra wave appeared at pH 7.9 (EP/z= -0.32 volt, ip = 0.35 pa.) and
ip/AC 1.22 1.23 1.19
0.62 0.83 0.73 0.73 0.41 0.94 0.84
Hydrazobenzene Concn., mM -Epiz, v. -in, pa. 0.23 No wave 1.71 0.26 0.11 0.30 0.26 0.22 0.30 0.23 0.23 0.30
0.19 0.18 0.29 0.29 0.31 0.34 0.39
1.84 1.71 1.23 1.80 1.10
1.18
1.40
ip/AC
0.51 0.49 0.52 0.44 0.48 0.38 0.41 0.37
a t pH 9.0 ( -0.377 volt, 0.58 pa.).
VOL. 37, NO. 12, NOVEMBER 1965
1529
different investigators, which seem to be due to three principal factors: adsorption of azobenzene on mercury, which results in a dependency of El/z on concentration (10, 16); an increasing difference in the reduction potentials of cis- and trans-azobenzene with increasing pH, and variations in experimental conditions -e.g. , presence and amount of N e O H or E t O H (l4), presence of a surfactant and its type (la), azobenzene concentration (9, 15), .concentration ratio of acid buffer component to azobenzene, and relative rates of electrode process and dissociation of acid buffer component ( 2 ) . Because of the factors mentioned and the resulting confusion in the literature, the salient features of the behavior of azobenzene at the D.M.E. in comparison to that observed at the P.G.E. is discussed below with particular attention to the reversibility of the azobenzene-hydrazobenzene system. Hydrazobenzene. Hydrazobenzene gives a single oxidation wave at t h e P.G.E., whose EpjZ varies linearly with p H with a change in slope at ca. p H 9:
Table 111.
Variation with pH of
Ep/z = 0.044 to 0.041 p H (pH 3.3 to 9)
(5)
Ep/z = -0.154 to 0.018 p H (pH 9 to 12.5)
(6)
No wave was observed at p H 1.6 because of the rapid transformation of hydrazobenzene t o a voltammetrically inactive form -e.g., the ultraviolet spectrum, taken within 5 minutes of solution preparation, showed a complete absence of the hydrazobenzene 290-mp absorption peak. However, EpjZof hydrazobenzene at p H 1.6 can be determined by a reverse polarization technique (3) to be -0.018 volt, which falls on the straight-line plot of Equation 5. EPjzfor azobenzene reduction at p H 1.6 is only 10 mv. more negative than that for hydrazobenzene oxidation; consequently, the electrode process is considered to be essentially reversible, since EpiZobtained through the reverse polarization technique is usually less accurate than Ep,2obtained by normal polarization and the data (Figure 1) indicate increasing reversibility with decreasing pH.
The potentials at the P.G.E. are usually less positive and the dependency on p H is less than those at the D.M.E. (Table 111). Coulometry. Macro scale reductive electrolyses of azobenzene and of azoxybenzene at p H 3.8 proceeded rapidly and yielded hydrazobenzene with coulometric n values of 2 and 4, respectively. Identity of t h e reduction product was established b y comparing the ultraviolet spectra of electrolyzed solutions after the current had dropped to the magnitude of background current t o the spectrum of hydrazobenzene dissolved in the same buffer. Macro scale oxidative electrolysis of hydrazobenzene at p H 3.8 resulted in a 2e process, which yielded azobenzene as a product based on comparison of spectra. Dependence of Peak Current on Concentration. Since t h e cis and trans forms of azobenzene reduce in acidic media a t similar potentials (9, l y ) , the dependence of i, and Ep/Z a t the P.G.E. on concentration was investigated a t p H 3.1 (Table IV).
El,z for Reduction of Azobenzene and Azoxybenzene and Oxidation of Hydrazobenzene at D.M.E.
Medium Reference
Solvent
Electrolyte
pH range
Dependency of EUZon pHa
AZOBENZENE
1-3.5 3.5-9
Holleck et at. (12)
30% MeOH, 0.5mMb
Not given
Wawzonek and Frederickson ( 1 7 ) Castor and Saylor
Buffered 0.08 to 1.4M acetate 0 . 1 M KCIC
Hillson and Birnbaum (9)
30%MeOH 0 . 25mMb 10% EtOH 0.lmMb 2570 EtOH lmM* 30% EtOH 0.02-1.OmMb 90% EtOH 0.2mMb
Fredrickson (8) Holleck and Holleck
30% MeOH 30% MeOH
(2)
Markman and Zinkova (14 ) Nygard (15 )
(11)
Wawzonek and Fredrickson ( 1 7 ) Nygard (15 )
30% MeOH
Holleck et al. (lb)
30% MeOH
a
30% MeOH
In some cases equations for dependency of
* Azobenzene concentration used.
McIlheny buffer 1M KCl 1M HzSOdd
+
0.10-0.059 pH (no additive) 0.087 v./ Ha (no additive) 0.078 v,.AH (0.01% gelatine) Very slight increase with pH 0.054-0.059 pH (trans) 0.063-0.058 pH (cis) 0.060-0.062 pH
Above 9 2-6 2.5-12.5 0.02-12.4 2.9-12.4 0.6-13.8
HOAc-NaOAc 4.8-8.6 10.4-14.8 NaOAc +.MOH (M = Li, Na, K ) AZOXYBENZENE 2-7.5 Buffers 1.2-8 Not given (O.Olyo Above 8 gelatin) HYDRAZOBENZENE Acidic Buffers Alkaline Below 8 1M HzSOtd Above 8
No wave below pH 1.9 0.00-0.042 pH 0.057-0.059 pH (azobenzene = 0.04mM) -0.05-0.060 pH Greater E112 increase with pHe
- 0.075-0.081 pH'
- 0.06-0.091 pH
Only slight variation
0.070-0.060 pH 0.139-0.062 pH 0.0660.059 pH 0.04-0.059 pH (0.02mM hydrazobenzene) 0.10-0.059 pH 1-3.5 Not given 0.107-0.079 pH 3.5-9 Nearly independent Above 9 on pH were calculated from tabular or graphical data given by original authors.
+
+
Also used: HCl + 0.0224 KCl; pH 2.9-7.1, citric acid NazHPOd; pH 8.1-9.4, N&Cl NHs; pH 11.7, 1.OM ethylenediamine; pH 12.2, 1.OM isoamylamine; pH 12.5, 1.OM piperidine. Also used: phosphate and ammonia buffers; 1M NaOH: 0.1M KC1 or KN03. 0.005'% gelatin added: Beyond pH 12.5, Em becomes less negatlve wlth Increasing pH. e Elis is different for cis and trans isomers above pH 8.5. Minus sign before 0.075 omitted in original reference. c
1530
ANALYTICAL CHEMISTRY
Table IV.
Concn., mM 0.2 0.4 0.6 0.8 1.0 1.2
a
Variation with Concentration of Voltammetric Behavior of Azoxybenzene, Azobenzene, and Hydrazobenzene a t Pyrolytic Graphite Electrode"
-EPi2,
0.48 0.50 0.45 0.50 0.48 0.47
Azoxybenzene v. i,, pa.
ip/AC
4.1 7.7 12.2 15.4 19.9 24.4
1.63 1.53 1.61 1.53 1.58 1.62 Av. 1.58 Std. dev. 1 0 . 0 4 4
Azobenzene ~. -Epiz, v. i,, pa. 0.14 0.14 0.14 0.14 0.15 0.17
i,/AC 1.12 0 . 93 0. 88 0. 85 0 . 79 0 . 73 0 . 88 135
0.22 0.36 0.73 1.09 1.46
v. 0.08 0.14 0.07 0.09 0.05
i,,
pa.
= 12.6 sq. mm.
-i,/AC
1.2 2.1 4.4 6.5 8.6
0.45 0.46 0.48 0.47 0.47 0.47 +0.011
=to,
Solution. pH 3.1 McIlvaine buffer. Electrode area
E,,'? for azobenzene becomes slightly more negative with increasing concentration, in agreement with its behavior a t the D.M.E. (16). There is no clear indication as to a shift of EPiz for azoxybenzene with concentration, although becomes more than 0.1 volt more negative when the concentration is increased from 0.1 to 1m M (11). Ep12 for hydrazobenzene tends to become less negative with increasing concentration; Eliz becomes more than 0.1 volt less negative with increase in concentration from 0.02 to 0.2m.W; from 0.2 to 1.0m31 the shift in El,z is less than 0.025 volt per 0.2m J i increase ( 1 5 ) . The peak currents for azoxybenzene and hydrazobenzene vary linearly with concentration. I n a given buffer system, the i/C ratio for azobenzene decreases as concentration increases. The current density (defined as current in microamperes per unit area in square millimeters, and unit concentration in millimoles per liter) was examined to determine its relationship to the number of electrons involved in the faradaic electrode process as determined coulometrically. The average values at p H 3.1 for ttzoxybenzene, azobenzene, and hydrazobenzene are 1.58, 0.89, and 0.47, respectively. The ratio of 1.58 to 0.89 is near that expected on the basis of 4e and 2e processes being involved; however, the value of 0.47 for hydrazobenzene, which undergoes a 2e oxidation process, is considerably less than expected. The calculated current densities of 0.88 and 0.77 for nitrosobenzene and phenylhydroxylamine ( b ) , which undergo 2e processes, are comparable with that of azobenzene. Hydrazobenzene rearranges in 0.1N hydrochloric acid to form benzidine and diphenylene in a 70 t o 30 ratio (1). Absorption maxima [estimated from spectra of Carlin, Nelb, and Odioso ( I ) ] occur at 280 mp for benzidine, 238 mp for diphenylene, and 246 and 290 mp for hydrazobenzene. A stability study of the hydrazobenzene stock solution in 9570 ethanol showed after 85 minutes a 10% decrease in absorbance at 245 mp and n constant absor-
2.8 4.7 6.6 8.6 10.8 11.0
Hvdrazobenzene Concn., mM
i,/AC term in units of pa. liter/sq. mm. mmole.
bance a t 280 to 295 mp, indicating that rearrangement occurred; the decrease in hydrazobenzene absorbance a t 290 mp would be balanced by the increase at 280 mp due to benzidine. The diphenylene absorption a t 238 mp, if any, is masked by the major hydrazobenzene band a t 245 mp, Since the 245-mp absorbance decreases only 10% in 85 minutes, the change due to rearrangement within 30 minutes is likely to be insufficient to affect the voltammetric behavior significantly. The concentration-dependence study of hydrazobenzene a t p H 3.1 was done within 0.5 hour; no significant change was observed in the ultraviolet spectra of the stock solution before and afoer this series of experiments. The current density of hydrazobenzene varied from 0.37 to 0.52 over the p H range from 3.3 to 12.5, which suggests that there may be home loss of hydrazobenzene due to rearrangement or other cause; however, there is no doubt as to the constancy of the i,/C ratio at p H 3 and the fact that the electrochemical reduction of hydrazobenzene involves 2 electrons. PHOTOISOMERIZATION OF AZOBENZENE
The trans- and cis-azobenzene isomers reduce at nearly identical potential in acidic media at the D.il1.E.; in neutral or basic medium, the cis form apparently reduces at about 10 mv. less
negative (15, 17). Since the isomerization of azobenzene is sensitive to both ultraviolet and visible light (7, 18, 19), interconversion under ordinary laboratory conditions could occur. Consequently, the possibility was investigated that the prewave observed at pH 8 and 9 might be due to the cis isomer. trans-Azobenzene absorbs at 318 and 229 mp with a minimum at 250 mp. Since the cis form has a major absorption maximum a t 250 mp (5), the change in ratio of absorbance a t 318 to 250 m+ can be utilized as a n index of cis formation. A 0.2mJi trans-azobenzene solution in p H 7.9 buffer was prepared in near darkness with predeaerated reagents. The initial voltammogram was recorded with the H-cell shielded from light; the ultraviolet spectrum of the original solution, diluted to 0.036mM without exposure to light, was also recorded. The test solution in the H-cell was then exposed to normal laboratory lightingLe., ceiling fluorescent light and daylight from an east window. After 15 minutes, a sample of the test solution was withdrawn, diluted to 0.036mM with 50% ethanolic p H 7.9 buffer solution, and examined spectrophotometrically, while the remaining test solution was polarographed. Similar sets of voltammograms were taken at p H 9.0 and 3.1. Cltraviolet spectra were also recorded at several p H values without
Potential, v. Figure 2. Voltammogram of frans-azobenzene (0.2mM) in pH 7.9, 50% aqueous ethanol
-No exposure to light
- - - - After 15 minutes' exposure to light VOL. 37, NO. 12, NOVEMBER 1965
0
1531
accompanying voltammetric measurement. The results are summarized in Tables V and VI; typical voltammograms of azobenzene solutions before and after light exposure are shown in Figure 2. Transformation to cis-azobenzene was observed over the p H range of 1.6 to 12.5, as well as in the 95% E t O H stock solution-e.g., after 10 to 20 minutes' light exposure a prewave appears in p H 7.9 buffer and the 318 to 250 mp absorbance ratio decreases. Under ordinary experimental conditions an investigator would normally be working with a mixture of cis- and trans-azobenzene because of the time needed-prior to
Table V.
pH 7.9 9.0 3.1
actual recording of a polarogram -to prepare stock and test solutions, and to deaerate the latter. However, in conformity with the behavior of the isomers at the D.M.E, the presence of cis-azobenzene does not appreciably alter the voltammetric behavior of the trans isomer in the acidic region. Exposure to light of a p H 3.1 solution of trans-azobenzene does not change its Ep12 or ip,even though cisazobenzene is present. However, at p H 7.9 and 9.0, cis-azobenzene not only causes a prewave but also shifts EplZ of the trans-azobenzene wave about 20 to 25 mv. more negative. At p H 11.4 and 12.5, no distinct pre-
Effect of Exposure to Light on Voltammetric Behavior of Azobenzene trans-Azobenzene = 0.20 mM Absorbance ratio at Exposure - Prewave Main wave 3 181250 time, min. E,n, v. zp, pa. Ep/2, V. i P I Fa. mp 0 15 0 20
-0.320
0.20
-0.377
0.28
0 20
-0.498 -0.520 -0.575 -0.600 -0.146 -0.148
1.95 1.60 1.96 1.60 2.80 2.80
9.65 5.51
Effect of Exposure to Light on Isomerization of frans-Azobenzene to cis-Azobenzene at 25" C. Absorbance ratio at Exposure Absorbance at 315-20/250 time, Light source 315-20 mp 250 mp min. Solvent a mp None 1.041 0.089 11.70 0 95% ethanol Sunlight 4.35 0.796 0.183 45 1.000 0.100 10.00 None 0 pH 7 . 9 buffer 0,733 0.172 Sun fluorescent 4.26 45 10.03 1.034 0.103 None 0 pH 11.4 buffer 6.06 0.849 0.140 Fluorescent 20 1.000 0.102 9.81 None 0 pH 12.5 buffer 0,880 0.140 6.29 Fluorescent 12 1.040 0,108 9.64 None 0 pH 1.6 buffer 0.911 0.151 6.04 Fluorescent 10 Table VI.
+
a trans-Azobenzene concentration at start of experiment was 0.05 mM solutions used contain 507, ethanol.
All buffered
Reversibility of the Azobenzene-Hydrazobenzene System: Experimental Conditions Used by Different Investigators" Investigator Holleck et Wawzonek and and reference Present study Nygard (16) al. ( 1 8 ) Fredrickson ( 1rj D.M.E. b D.M.E. D .M.E. b P.G.E. Electrode 30% EtOH 30Y0 MeOH 307, MeOH 50y0 EtOH Solvent Acetate Acetate Not given PhosphateBuffer system citrate Ammonia Phosphate Ammonia KOH KCl "3, NHd+ CsHiiN * HNOrCsHiiN 0 . 5 0.25 0.2 0.2 Concn.e, mM None None None 0.005 Gelatin, 70 Table VII.
+
a Differences in half-wave or half-peak potentials of azobenzene and of hydraeobenzene at same pH plotted in Figure 3. D.M.E. = dropping mercury electrode. b P.G.E. = pyrolytic graphite electrode. c Concentration of electroactive species.
1532
0
ANALYTICAL CHEMISTRY
wave is observed, but the background is higher and more poorly defined, which may indicate that under the present experimental conditions the difference between cis- and trans-azobenzene probably decreases as p H increases above p H 9. Hillson and Birnbaum (9) report a maximum difference in Eliz of trans- and cis-azobenzene at p H 12.5, above which the difference decreases and becomes small (ca. 10 mv.) a t p H 14. DISCUSSION
Comparison of the results obtained in the present study using the P.G.E. with those obtained a t the D.M.E. is complicated by the frequently nonconcordant results obtained a t the D.M.E., as well as by possibly significant differences in experimental conditions as summarized in Table VII. I n such a situation, a logical way of examining the over-all agreement of the data is to use the differences found by different investigators between the reduction potential of azobenzene and the oxidation potential of hydrazobenzene (AEll2 or A&2) (Figure 3). The voltammetric behavior of azoxybenzene a t the P.G.E. is comparable to that a t the D.M.E.; Ep12in E t O H is slightly more negative than E112 in MeOH. Macro scale electrolysis indicates that azoxybenzene undergoes a 4e reduction to hydrazobenzene, for which the following over-all equation can be written,
+ 4 H + + 4e =
C6H5--N=N-C8H5
1
0
CeHs-N-N-C6H5
I
H
1
+ HzO
(7)
H
It is obvious from the pH-dependency indicated by Equation 1 that the actual mechanistic path for the electrochemical process is involved with the possibility that intermediate chemical reactions may play an important role. The variations of the pH-dependencies for azobenzene and hydrazobenzene from a simple factor such as 0.059 similarly prevent the postulation of reaction sequences on the basis of present knowledge. Reversibility of Azobenzene-Hydrazobenzene System. T h e azobenzene-hydrazobenzene system is apparently reversible a t t h e P.G.E. only a t p H 1.6; AEDl2increases linearly with p H between p H 3 and 11, but is constant above pH 11; the displaced point at p H 6.6 is probably due to the poor capacity of the buffer used, which would result in the p H a t the electrodesolution interface becoming more alkaline during electrolysis.
The patterns observed a t the D.M.E. are generally similar-Le., small differences in the acidic region, increasing differences in slightly acidic to slightly alkaline solution, and a constant or decreasing difference in the more alkaline region. Thus, Nygard’s results (15) indicate that AE1/2 is constant (about +20 mv.) between p H 3 and 5, increases linearly between p H 5 and 7 . 5 with a maximum a t p H 7.5, and is constant from p H 8 to 12. Since increasing the percentage of E t O H tends to make the reduction E l / 2 (or E p d more negative (14) and addition of gelatin tends to make less negative J / A ( 1 2 ) , the lower ethanol concentration and the use of gelatin may account for the smaller AEl12 values obtained by Xygard as compared to AEp/2 a t the I I I I I I 1 I I I I I 0 2 4 6 8 io 12 P.G.E. .Iceording to Wawzonek and PH Fredrickson’s data (1’7), the system is Figure 3. Variation with pH of difference between half-wave potentials reversible between p H 2 and 6 ; for hydrazobenzene and azobenzene as reported by different investireaches a maximum at p H 7.2. Howgators ever, Holleck et al. (12) find the system Experimental conditions given in Table VI1 to be reversible only between p H 1 and A E 1 / 2 , mv. = E112 for hydrazobenzene - E l / z for azobenzene. Data for graphite electrode 3.5; AEi;z increases linearly between refer to E P / z . p H -1 and 9 and then decreases. The 0 Present study, pyrolytic graphite electrode, 0.20mM concentration, 50% EtOH higher concentration of electroactive 0 Holleck et 01. (121, D.M.E., 0.50mM, no gelatin added, 30% M e O H n Nygard (151, D.M.E., 0.20mM, 0.005% gelatin, 30% EtOH species (0.5mdl) used by Holleck et al. A Wawzonek and Fredrickson (171, D.M.E., 0.25mM, 30% M e O H may account for their larger AEI,~. Hillson and Birnbaum (9) claim the LITERATURE CITED system to be generally irreversibIe. attained in about 2 hours, with t h e Kygard ( 1 5 ) maintains t,hat reversibility equilibrium composition, which is (1) Carlin, R. B., Nelb, R. G., Odioso, can be achieved only when the conindependent of p H , depending on the R. C., J . Am. Chem. SOC.73, 1003 centrations of azobenzene and hydrazo(1951 ). intensity of t h e light source. In (2) Castor, C. R., Saylor, J. H., Ibid., benzene approach zero-e.g., 0.02mLlf. voltammetric test solutions, about 75. 1427 (1953). , (At the P.G.E., EpiZ for both azo70y0 of t h e equilibrium s t a t e is (3) -Chuang, L., Elving, P. J., ANAL. benzene and hydrazobenzene does not reached within 10 minutes. CHEM.,37, 1506 (1965). (4) Chuang, L., Fried, I., Elving, P. J., seem to change significantly with conI n the acidic region, the cis and trans Ibid., 36, 2426 (1964). centration.) forms reduce a t similar potentials. (5) Collins, J. H., Jaffe, H. H., J. Am. The addition of surfactants tends t’o I n the alkaline region, the cis form is Chem. SOC.84, 4708, (1962). make the system less reversible-i.e., reduced a t less negative potential-e.g., (6) Elving, P: J., Fried, I., Turner, W. AE,,,, is increased (,I$’). Castor and R., Proceedings of Third Polarographic Ep12for the cis form is about 0.2 volt Congress, Southampton, 1964. Saylor ( 2 ) claimed the reduction to be less negative a t p H 7.9 and 9.0 than for ( 7 ) Fischer, E., J. Am. Chem. SOC.82, reversible in citrate-phosphate buffer, the trans form. The presence of the cis 3249 (1960). but irreversible in acetate buffer of the form a t lorn concentration in a trans(8) Fredrickson, J. D., Ph.D. thesis, same pH. Universitv of Iowa. 1955. azobenzene solution could change the (9) Hillson,” J. P., ‘ Birnbaum, P. P., Based on analysis o f the wave slopeappearance of the background current Trans. Faraday SOC.48, 478 (1952). Le., the familiar log plot-the reduction preceding $he main wave, thus affecting (10) Holleck, L., Proceedings of Third of azobenzene a t the DA1.E. has been the determination of and i,; an Polarographic Congress, Southampton, claimed to be a reversible 2e process 1964. appreciable concentration of cis would (11) Holleck, L., Holleck, G., Z. Natur(2, 14). appear as a prewave. forsch. 19b, 162 (1964). I n general, it would seem that in the Consequently, the photoisomerization (12) Holleck, L., Shams, El Din, A. M., acidic region a t a given concentration that would have occurred during the Saleh, R. M., Holleck, G., Zbid., 19b, level the system is more reversible in 161 (1964). present study would not appreciably (13) Komyathy, J. C., Malloy, F., Elving, methanolic than in ethanolic medium, affect the EPizresults. However, in the P. J., ANAL.CHEM.24, 431 (1952). increase in concentration of electroalkaline region, the presence of cis(14) Markman, A. L., Zinkova, E. V., active species tends to limit the reazobenzene causes the observed transJ . Gen. Chem., U S S R 29, 3058 (1959). versibility at, lower pH values, and the (15) Nygard, B., Arkiv Kemz 20, 163 azobenzene peak current to be less than (1962’). system is less reversible a t the graphite that corresponding to the trans-azo(16) Nygard, .B., Proceedings of Third than a t the mercury electrode. The benzene taken, based on values observed Polarographic Congress, Southampton, latter result is in contrast to that found in the acidic region. 1 Q64. for the nitrosobenzene-phenylhydroxyl(lijWawzonek, S., Fredrickson, J. D., J . Am. Chem. Sac. 77, 3985 (1955). amine system, which was reversible ACKNOWLEDGMENT (18) Yamashita, S., Ono, H., Toyama, over the observable p H range a t both 0.. Bull. Chem. SOC.Japan 35, 1849 graphite and mercury elect,rodes (4). The authors thank the U. S. Atomic (1962). Photoisomerization of Azobenzene. Energy Commission and the Horace G. (19) Zimmerman, G., Chow, L. Y., Paik, U., J . Am. Chem. SOC.80,3528 (1958). Rackham School of Graduate Studies Ordinary laboratory lighting is suffiof The University of Michigan, which cient to cause isomerization of t8he RECEIVED for review December 28, 1964. helped support the work described. Accepted September 7, 1965. trans t o t h e cis form. Equilibrium is > - -
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