Q QI r r0
R
t T 7
A constant, defined by Equation 22, reflecting the effective electrode sphericity for the second-order, one-half regeneration mechanism. A constant, defined by Equation 39, reflecting the effective electrode sphericity for the first-order, one-half regeneration mechanism. Spherical distance coordinate. Spherical electrode radius in centimeters. Reduced form of the redox couple. Time in seconds. Dimensionless time parameter for the second-order, one-half regeneration mechanism. Dimensionless time parameter for the first-order, one-half regeneration mechanism.
U
v,v X
z
Variable of integration. Dimensionless variables of integration. Dimensionless current function for the first-order, one-half regeneration mechanism. Electroinactive species. ACKNOWLEDGMENT
Helpful discussions of the subject matter with G. L. Booman are gratefully acknowledged. RECEIVED for review October 31, 1968. Accepted February 7, 1969. Work supported by the United States Department of the Interior, Office of Saline Water.
Electrochemistry of 8,8'-Diquinolyldisulfide John J. Donahue and John W. Olver Department of Chemistry, University of Massachusetts, Amherst, Mass.
The electrochemistry of 8,8'-diquinolyldisulfide, RSSR, and mercuric thiooxinate, (RS)2Hg, has been investigated in methanolic-sodium acetate and aqueous 1.OM H2SO4, using conventional polarography, cyclic voltammetry, and controlled potential electrolysis. The data indicate that RSSR is not directly reduced at the DME but is adsorbed and then undergoes a fast precursor chemical reaction to form mercuric thiooxinate, (RS)?Hg, which is electroactive. Controlled potential electrolysis shows that the (RS),Hg formed undergoes a 2e- reduction to give 8-mercaptoquinoline, RSH, as the major product while the oxidation of RSH gives (RS)2Hg. Although the chemical and adsorption equilibria involved in the reduction of RSSR and (RS)Y Hg are complex, the coulometric generation of RSH from either starting material is feasible and should facilitate the use of 8-mercaptoquinoline as an analytical reagent.
8-MERCAPTOQUINOLINE, the sulfur analog O f 8-hydroxyquinoline, was first synthesized by Edinger in 1908 ( I ) , however, specific application of this compound as an analytical reagent was not investigated until 1944 when Taylor (2) reported that 8-mercaptoquinoline was not adaptable to general analytical use because it was readily oxidized by atmospheric oxygen to 8,8-diquinolyldisulfide (2). The work of Bankovskii et al. (3-5) and Freiser and Fernando (6) clearly shows that 8-mercaptoquinoline, RSH, has useful properties as an analytical reagent. In particular, Freiser (6) has shown that certain complexes of 8-mercaptoquinoline are more stable than the corresponding 8-hydroxyquinolates ; however, reagent instability has posed a severe drawback in the cases cited.
(1) A. Edinger, Chem. Ber., 41,937 (1908). (2) J. R. Taylor, Virginia J. Sci., 3,289 (1944). (3) V. I. Kuznetsov, Y.A. Bankovskii, and A. F. Levinsh, J. Anal. Chem. (U.S.S.R.),13,299 (1958). (4) Y. A. Bankovskii, A. F. Levinsh, and Z. E. Liepinya, J. Anal. Cltem. (U.S.S.R.),15, l(1960). (.5 .) Y . A. Bankovskii and A. F. Levinsh. J. Anal. Chem. (U.S.S.R.). 17, 725 (1962). (6) 35.1424 . , A. Corsini. 0.Fernando. and H. Freiser. ANAL.CHEM.. (1963). I
_
We have examined the electrochemical behavior of RSSR by conventional polarography, cyclic voltammetry, and controlled potential electrolysis in order to develop a method for the coulometric generation of 8-mercaptoquinoline, thereby avoiding reagent instability problems. EXPERIMENTAL
Apparatus. Polarograms were obtained using an Indiana Instruments and Chemical Corporation Controlled Potential and Derivative Voltammeter (Model ORNL 1988A) and recorded on a Sargent Model SR recorder. All electrochemical experiments were carried out at 25 i 1 "C. Solutions were deaerated with prepurified tank nitrogen which was presaturated with solvent. The saturated calomel electrode was used as a reference electrode in all cases and the dropping mercury electrode had the following characteristics at open circuit in a solution of 1.OM H2S04, m = 2.04 mg sec-', t = 3.69 sec. Cyclic polarograms at low voltage scan rates were obtained using the Heath Model EUW-401 polarography system (Heath Co., Benton Harbor, Mich.), a conventional threeelectrode cell and a Moseley Autograf Model 135 X-Y recorder. At high sweep rates (>0.2 volt second-') the triangular sweep generator of Weir and Enke (7) was used in conjunction with the Heath Model EUW-19A operational amplifier system. A Tektronix 502A Dual Beam Oscilloscope with attached Tektronix Oscilloscope Camera, Model C-12, was used to record the current-voltage curves. A Jaissle Potentiostat Model lOOOT (Jaissle ElektronikLabor, West Germany) was used to maintain the desired potential values during the controlled potential electrolyses. A 1.0-pf capacitor was inserted in the potentiostatic feedback loop to filter 60-cycle ac noise picked up by the unshielded cell. Current-time curves were recorded using a Sargent Model SR Recorder and integrated electronically using a resistance-capacitance integrator (8). Applied potentiostatic potential and the integrator readout potential were measured using a Honeywell potentiometer Model 2730.
(7) W. D. Weir and C. G . Enke, Rev. Sei. Instr., 35,833 (1964). ( 8 ) H. C. Jones, W. D. Shults, and J. M. Dale, ANAL.CHEM., 37, 680 (1965). VOL. 41, NO. 6, MAY 1969
753
n
RESULTS AND DISCUSSION
20
I - 0.2I
-0.6
-
I
1.0 Ede (volts vs. S.C.E.1
I -1-4
1
Figure 1. Polarogram and electrocapillary curves for 1.0 X 10-3M RSSR in sodium acetate-methanol Upper curve. Without RSSR Lower curve. With RSSR
For controlled potential electrolyses, solution volume was 100 ml and that of the mercury 20 ml. Solutions were magnetically stirred in all cases. Electrolyses were corrected for faradaic impurities, faradaic residual, and charging currents by electrolysis of a blank solution. Ultraviolet and visible spectra were taken on a Perkin-Elmer Model 202 spectrophotometer or on a Beckman Model DB spectrophotometer with a Varicord Model 43 linear-log recorder. Operational pH measurements were taken in methanol witH a Corning Model 112 research pH meter. Phosphate and acetate buffers were used to adjust the pH to the desired value. Reagents. Methanol was purified by reflux and distillation from a magnesium-iodine solution at a high reflux ratio (9). Water was refluxed and distilled from alkaline KMn04. Reagent Grade Sulfuric Acid was used as supporting electrolyte in aqueous solution and was electrochemically clean. Sodium acetate (Fisher Certified) was used as the supporting electrolyte in methanol and was electrochemically clean in the potentia1 range of study. Diquinolyldisulfide-8,8’, RSSR, was prepared by the procedure of Badger and Buttery (10) (melting point 201 “C). When controlled potential electrolyses using 0.01M RSSR were attempted, in 1.OM H2S04, a yellow precipitate formed on the mercury pool at open circuit potentials. The ultra= 370 violet spectrum of this precipitate in methanol, A, = 320 mp, and elemp, differed from that of RSSR,, , ,X mental analysis showed mercuric thiooxinate, (RShHg. Mercuric thiooxinate for subsequent experiments was prepared by the above reaction. The light yellow crystals had a decomposition point of 186 “C. The results of elemental analyses on RSSR and (RS),Hg were: Calcd for RSSR: C, 67.47; H, 3.78; N, 8.75; S, 20.01; Found: C, 67.61; H, 3.82; N, 8.61; S, 19.81. Calcd for (RS)*Hg: C, 41.54; H, 2.32; N, 5.38; S, 12.30; Hg, 38.50; Found: C, 41.60; H, 2.40; N, 5.46; S, 12.60; Hg, 38.26. Solutions. Stock solutions of RSSR were prepared by dilution of accurately weighed RSSR. Stock electrolyte solutions were 0.2M NaOAc in CHoOH and 1.OM aqueous sulfuric acid. Solutions for electrochemical experiments were prepared using aliquots of the RSSR and dilution to the proper volume with stock electrolyte solution.
(9) H. Lund and J. Bjerrum, Ber., 64,210(1931B). (10) G. M. Badger and R. G. Buttery, J. Chem. SOC.,1956,3236. 754
ANALYTICAL CHEMISTRY
A. ELECTROCHEMICAL STUDIESIN METHANOLIC-SODIUM ACETATE.1. Polarography of 8,8 ‘-Diquinolyldisulfide, RSSR. Polarograms for RSSR in methanol with sodium acetate as supporting electrolyte (Figure 1) gave two reduction waves. The half-wave potential for the first wave shifted cathodically roughly 50 mV per 10-fold increase in concentration, while the half-wave potential for the second wave remained relatively constant at - 1.41 volts. The limiting currents of the two waves taken at -0.9 volt and -1.6 volts, respectively, are equal within 5 % while the diffusion currents for both waves vary linearly with RSSR concentration. Electrocapillary curves taken in methanol (Figure 1) for the solvent4ectrolyte system with and without RSSR show a depression due to an adsorption process. The adsorption is terminated in conjunction with the second polarographic wave, and the electrocapillary curve rejoins that for the solvent4ectrolyte system at more negative potentials. The reduction of disulfides has been treated in the literature (11, 12). The rate determining step of the electrochemical reduction is believed to be the cleavage of the -S-S bond in a two-electron step as shown in Equation 1 : RSSR
+ 2e- + 2Hf
-
2RSH
(1)
The equation of the polarographic wave is
where E’ = E” - 0.0591 pH and predicts that a plot of Ed, us. log (id - i)/P should be h e a r with a slope of 29.5 mV. Furthermore, the half-wave potential should shift 59 mV per pH unit cathodically as the pH is increased. Plots of 1og(id - i ) / i z us. Ed, for various concentrations, between 0.1 and 1 mM, of RSSR for the first wave gave slopes of 30-32 mV. A plot of E,/zus. pH for operational pH values from 2.0 to 9.0 was linear with a slope of 56 mV. Equation 2 also predicts that the Ell2 is a function of id or concentration as follows, Ell2 = E’
- 0.0295 log id/2
(3)
The present case does not follow the above relationship, but shows a larger shift in E1/2with increase in concentration. These data would indicate that the process is more complicated than Equation 1 predicts. The dependence of the limiting current on the height of the mercury column was measured well out on the plateau of the wave. The ratio of i/HHgl/zwas constant indicating that the overall electrochemical process is diffusion controlled. 2. Polarography of Mercuric Thiooxinate, (RS)2Hg. Polarograms obtained for 4.00 X 10-4M (RS)*Hg and 4.05 X lO-4M RSSR showed single waves whose half-wave potentials differed by only 20 mV. The wave for (RS)*Hg was anodic of that for RSSR. A mixture of the two compounds gave an intermediate unresolved wave. The (RS)2Hg undergoes the overall electrochemical reaction (RS)*Hg
+ 2e- + 2H+
+
2RSH
+ Hg”
(4)
and the equation of the polarographic wave should be identical with Equation 2 above. While RSSR gave a linear
(11) I. M. Kolthoff and C. Barnum, J. Amer. Clzem. SOC.,63, 520 (1941). (12) M. W. Clark, “Oxidation-Reduction Potentials of Organic Systems,” The Williams and Wilkins Co., Baltimore, Md., 1960.
plot Of Ede 0s. log ( i d - i)/iz with a slope O f 30 mV, (RS)zHg and the mixture gave curved or intersecting straight lines indicating a more complex process. 3. Polarography of (RSl2Hg and RSSR at the Rotated Platinum Electrode (RPE). In their work with the cystinecysteine system, Stricks and Kolthoff (13) proposed that cystine undergoes a chemical reaction at the DME to form mercuric cysteinate which is then reduced via two electrons to cysteine and elemental mercury, Hg". If RSSR is reduced directly at the DME, without undergoing a precursor chemical reaction, it should also reduce at about the same potential at the rotated platinum electrode. Polarograms of RSSR and (RS),Hg in methanol were obtained at a rotated platinum electrode which was pretreated in hot concentrated nitric acid and rinsed with water and methanol. The cathodic potential limit in methanol with NaOAc as supporting electrolyte was -1.1 volts. For solutions of RSSR, no reduction wave was obtained in the potential region 0.0 to -1.1 volts. For (RS)*Hg solutions a single wave at El/2 = -0.75 volt occurred. Reruns on RSSR solution without removing the Hg" film on the platinum electrode gave a reduction wave at the same potential as that for the (RS)2Hg compound. If the electrode was cleaned with concentrated nitric acid, water, and methanol after an (RS),Hg polarogram, an RSSR solution again gave no wave as in the original case. The results at the rotated platinum electrode for RSSR and (RS),Hg show that RSSR must undergo a precursor chemical reaction with Hg" before reducing. When RSSR is run, no reduction occurs unless Hg" is first deposited on the rotated platinum electrode by the reduction of (RS),Hg. If this mercury deposit is not removed with nitric acid prior to running a new RSSR solution, the RSSR reacts with the elemental mercury to form (RS)2Hg which is then reduced, but if the elemental mercury is removed by treatment of the electrode with nitric acid no RSSR reduction occurs. Since ii is linear with concentration of RSSR at the DME, the chemical precursor reaction must be very fast at this electrode. The linear dependence of ii on the square root of the height of the mercury column, HE^"^, also suggests a very fast chemical reaction. 4. Controlled Potential Electrolysis of RSSR. Controlled potential electrolyses were used to qualitatively identify the reduction product of the electrochemical reaction occurring for the first wave in methanol. During the electrolyses, the potential of the mercury pool was held constant at -1.00 volt. When the current had decayed to approximately 0.1 of its initial value, 20-ml aliquots of the electrolyzed solution were withdrawn and added to separate solutions containing copper(I1) and nickel(I1) ions. The uv spectra of' the resulting chelates were obtained in chloroform and agreed exactly with those reported by Bankovskii et al. (5) for the 8-mercaptoquinolates of nickel and copper. The number of electrons, n, involved in the reduction process was determined. Electronic integration of the current-time curves gave an n value of 1.99 Ilt 0.05 for two determinations. The spectral and coulometric data show clearly that 8mercaptoquinoline, RSH, is the overall reduction product of the electrochemical reaction. Controlled potential oxidation of an RSH solution (obtained electrochemically) at a potential of -0.3 volt produced (13) W. Stricks and I. M. Kolthoff, J. Amer. Chem. SOC.,74, 4646 (1952).
B
A
-4,0t - 6 4
tt-
3
Figure 2. Cyclic polarograms in sodium acetate-methanol 6.62 X 10-4M(RS)2Hg
- - - - 6.42 X 10-4M RSSR first scan - . -. -. 6.42 X lO-'M RSSR subsequent scan A. 0.1 volt/second B. 1.0 volt/second
only mercuric thiooxinate. The production of (RS)zHg on oxidation of RSH substantiates that RSSR is not directly reduced but first undergoes a chemical reaction to form (RS),Hg which is the electroactive species. 5. Cyclic Voltammetry of RSSR and (RS)ZHg. Cyclic voltammetric data were obtained at a hanging mercury drop electrode using voltage scan rates from 0.1 volt second-] to 22.0 volts second-'. Figure 2 shows the cyclic polarograms for RSSR and (RS),Hg at 0.1 volt second-' ( 2 4 and 1.0 volt second-' (2B). The dashed curve in Figure 2A represents RSSR at 0.1 volt second-'. On the first scan, a single reduction peak, 2' occurs with En = -0.730 volt, while on the anodic scan a single oxidation peak, 3' occurs with E p = -0.633 volt. On the second and successive scans, a small shoulder, l', appears on the main reduction peak, 2'. For (RS),Hg at 0.1 volt second-' (Figure 2A, solid curve), a single reduction peak, 1, is found with Ep = -0.680 volt. This reduction peak for (RS),Hg is in the same potential region as the shoulder, 1 ', obtained for RSSR after the first scan. The oxidation peak, 3, for the (RS)ZHg system is identical to that in the RSSR case. Cyclic voltammograms in Figure 2 8 were obtained at 10 times the voltage scan rate used in Figure 2A. The dashed curve for RSSR shows that the main reduction peak, 2', has shifted to a more cathodic value, Ep = -0.755 volt, while the original anodic peak, 3' has shifted anodically to Ep = -0.625 volt. This shift in peak potential may be due in part to uncompensated iR drop in the system. A new anodic peak, 4', now appears at approximately Ep = -0.730 volt. As the scan rate is increased further, peak 4' increases in height while 3' decreases until at 22.0 volts second-' only peak 4' is apparent. On subsequent scans, the shoulder 1' again appears for RSSR. The solid curve (Figure 2B) represents a cyclic voltammogram for (RS)tHg at 1.0 volt second-'. Peak 1 has shifted cathodically to -0.695 volt, while peak 3 has shifted slightly anodic to -0.617 volt. A new wave, 2, appears at -0.745 volt, corresponding to the potential of the main reduction peak, 2', for RSSR. As in the case of RSSR, a new anodic peak, 4, appears at -0.730 volt. As the scan rate was increased beyond 1.0 volt second-' peak 2 increased in height while peak 1 decreased. As in the case of RSSR, peak 4 increased in height while peak 3 decreased as scan rate was increased. VOL. 41,NO. 6, MAY 1969
755
Ede (volts vs. S.C.E.1
Figure 3. Normalized polarograms for various concentrations of RSSR in aqueous 1.OM H 8 0 4 A. 4.04 X lO*M RSSR B. 2.02 X 10-4M RSSR c. 4.04 x 1 0 - 4 ~RSSR D. 1.01 X 10-M RSSR
Recent work by Wopschall and Shain (14) suggests that the peaks 2 and 4 for RSSR and (RS)2Hg could arise from strong adsorption of the reactant and product of the corresponding electrochemical reactions. These peaks represent the current necessary to oxidize or reduce enough material to saturate the electrode surface and represent a constant quantity of electroactive material. At low concentrations, most of the electroactive species present is consumed by the adsorption process and the adsorption peak predominates over the normal diffusion peak. At concentrations above that required for monolayer coverage of the drop, the current due to the normal reduction or oxidation process increases while that for the adsorption process remains constant. As a result, at high concentrations the peak for the normal oxidation or reduction process predominates. Cyclic voltammograms were taken at a constant voltage scan rate of 0.2 volt second-1 for different concentrations of RSSR and (RS)*Hg. At 6.0 X 10-5Monly peak 4 appeared; at 3.0 X 10-4M peak 3 appears along with peak 4; and at 6.0 X lO-4M peak 3 predominates. Peak 4 also predominates over peak 3 at high scan rates when the concentration is held constant. These data confirm that peaks 4 and 4' are due to the strong adsorption of the product of the oxidation, (RS)*Hg. The cathodic scans for both compounds are also complicated by adsorption. For (RS)*Hg, the current ratio for peak 2 to peak 1 increases as the scan rate is increased, while it decreases as the concentration is increased. These data indicate that peak 2 is due to the reduction of adsorbed (RS)*Hg while peak 1 is due to the diffusional reduction of (RS)*Hg. The cathodic scans for RSSR are not as easily interpreted since the peak separation for 1' and 2' is such that it is difficult to determine the current ratio of these peaks as a function of scan rate and concentration of RSSR. Since RSSR must react with the mercury drop to form the electroactive species (RS)*Hg, peak 2'-which is the only peak found on the first scan-is most likely due to the reduction of adsorbed (RS)2Hg. The RSH formed is desorbed and subsequently reoxidized to adsorbed (RS)*Hg and (RS)2Hg in (14) R , H. Wopschall and I. Shain, ANAL.CHEM., 39,1514 (1967). 756
ANALYTICAL CHEMISTRY
Figure 4. Normalized current-time curves for various concentrations of RSSR in aqueous 1.OMH B 0 4 Droptime
=
3 seconds
solution, peaks 4' and 3'. On the second and successive scans, the unresolved peak for the reduction of (RS)*Hg in solution, I f , appears along with the peak for adsorbed (RShHg, 2'. To summarize the electrochemical behavior of RSSR and (RS)2Hgat the hanging mercury drop electrode: (RS)2Hg Case
Electrochemical (1) peak 1 Electrochemical (2) peak 2 Electrochemical (3) peak 4 Electrochemical (4) peak 3
+
+ + +
2e2H+ + ZRSH [(RS)Bg]soln Hg" 2H+ [(RS)2Hg]adsorbed 2eZRSH Hg" Hg" + [(RS)2Hg]adsorbed 2RSH 2e2H+ 2RSH Hg" + [(RS)2Hg]soln 2e2H+
+
+ + + + + +
+
RSSR Case
Chemical step (1) Electrochemical (2) peak 2' Electrochemical (3) peak 4' Electrochemical (4) peak 3' Electrochemical (5) peak 1' (shoulder)
-
+
-
RSSR Hg" -, [(RS)nHg]adsorbed [(RS)*Hg]adsorbed 2e2H+ ZRSH Hg" 2RSH Hg" [(RS)2Hg]adsorbed 2e2H+ ZRSH Hg" 4 [(RS)2Hg)lsoln+ 2e2H+ [(RS)nHg]soln 2e2H+ + 2RSH Hg"
+ + + + + +
+
-
+
+
+
+
B. ELECTROCHEMICAL STUDIES ON RSSR IN AQUEOUSSULFURIC ACID. Polarographic curves normalized by plotting ijc us. E for various concentrations of RSSR in aqueous 1.OM H2S04 are shown in Figure 3. Depending on the concentration of RSSR, the polarographic waves may be divided into one or more potential regions where discriminable processes occur. At the lowest concentration, a single process is apparent while at the highest concentration at least three processes are apparent. The same polarographic curves when plotted conventionally show a constant height for the abrupt process near the foot of the wave at about -0.108 V for concentrations of RSSR above 2 x lO-4M.
The vertical dashes indicated on the X axis in Figure 3 mark potentials at which current-time curves shown in Figure 4 were taken. The current-time curves obtained at the DME for various concentrations of RSSR in 1.OM H2S0, at those several potentials reflect the complex nature of the chemical and electrochemical processes occurring at the DME. The kinetics of film formation may explain the currenttime curves in Figure 4. The time, t , at which the surface is completely covered is
rs2
t = 1.82 X lo6-
DC2
where rs is the number of moles of surface-active material per cm* of surface at surface saturation, C is the bulk concentration of surface-active material in moles liter1, and D is the diffusion coefficient in cm* second-’ (15). This equation predicts that at high concentrations of RSSR-i.e., 1.01 X 10-aM-coverage of the drop surface is at least an order of magnitude faster than at a lower concentration-i.e., 4.04 X 10-4M. Since the chemical equilibria at the drop surface are dependent on the rate of drop coverage by RSSR which forms (RS);Hg, at high concentrations of RSSR where the drop is immediately covered, all the equilibria involved are rapidly established early in the drop life. As a result, the spikes due to the chemical and adsorption equilibria are not resolved on the current-time curves. At the lowest concentration, 4.0 X lOw5MRSSR, only the current-time curve at -0.07 volt shows any unusual features. For this concentration at potentials beyond -0.07 volt we are well out on the rising portion of the wave where the adsorption and chemical equilibria become unimportant compared with the rapid rate of depolarizer reduction. Electrocapillary curves (Figure 5 ) with and without RSSR also show an unusual change in the potential region of this first steep current rise on the polarograms. The electrocapillary curve with RSSR exhibits an overall depression attributable to adsorption, while in the potential region of the first wave on the current-voltage curves, a sudden decrease in the surface tension occurs. The further depression of the surface tension at -0.9 volt corresponds to the beginning of another wave on the polarographic curve which is confounded with simultaneous Hf reduction. This process is probably similar to the second wave found in methanol but could not be studied further. Electrocapillary, current-time, and polarographic curves such as these have been attributed to inhibition of the electrochemical reaction by a film formed on the electrode surface by molecules of the depolarizer itself (16). The sudden increase in current over an unusually small potential range is due to the reduction of this film. In the present case, RSSR reacts rapidly with Hg” to form an insoluble film of (RS),Hg which is reduced at cathodic potentials >-0.11 volt. The independence of the wave height from the concentration of RSSR for the first wave indicates an adsorption process (18); however, the current is relatively independent of the mercury column height suggesting a kinetic process (id decreased from 2.25 pA at 100 cm steadily to 2.00 pA at 50 cm). The values of id m. column height obtained are such that adsorption mixed with a kinetic process cannot be ruled out. Apparently, (15) P. Zurnan and I. M. Kolthoff, “Progress in Polarography,” Vol. I, p 99, Interscience Publishers, New York, 1962. (16) E. Laviron and C. Degrand, Bull. SOC.Chim. France, 1966, 2 194.
0.0 -0.2 -0.4 -0.6 -0.8 -LO -1.2 Ede (Volts
VS.
Figure 5. Electrocapillary curves for 1.0 aqueous 1.OM H2S04 A . Without RSSR B. WithRSSR
S.C.E.) x 10TaMRSSR in
the formation of the (RS),Hg film is dependent on the concentration of Hg”-Le., surface area of the drop-which is constant in all cases and not on the concentration of RSSR. After the mercuric-thiooxinate film is reduced at - 0.108 volt, the current rises by a function which roughly corresponds to E L.S. (id - i)/iz. Finally, another sharp increase in current occurs well along the rising portion of the current voltage curve. BrdiEka (17) has attributed similar data for phenosafranine to the reduction of the free form of the depolarizer followed by the reduction of the adsorbed reactant; however, the present data cannot be quantitatively interpreted because of the mixed kinetic and adsorption effects. In summary, although several processes, both chemical and electrochemical, occur at the electrode surface which complicate the interpretation of data, the overall heights of polarographic waves are linear with concentration of RSSR. Moreover, the currents measured on the plateau of the polarographic waves, are proportional to the square root of the height of the mercury column, and the overall electrochemical process is diffusion-controlled. Controlled potential electrolysis clearly gives 8-mercaptoquinoline, RSH, as the reduction product. Coulometric generation of 8mercaptoquinoline in aqueous-sulfuric acid is feasible in spite of the chemical problems encountered in this medium. RECEIVED for review May 29, 1968. Accepted February 25, 1969. Work supported by National Science Foundation Grant GP-3469. ~~~
(17) R. BrdiEka, Collect. Czech. Chem. Commun., 12,523 (1947). VOL. 41, NO. 6, MAY 1969
a
757
