can be utilized to elucidate the nature of an electrolysis product-e.g., identification of trans-azobenzene as the sole oxidation product of hydrazobenzene-and to evaluate, a t least qualitatively, the reversibility of the redox couple. Although the accumulation of electrolytic product could be due either to adsorption on the electrode or to slow dispersion and subsequent rediffusion back to the electrode, the wave shapes obtained with reverse polarization a t the D.M.E. and rotating P.G.E. indicate that the second situation prevails in the case of the compounds involved in the present study. EPiZobtained by reverse polarization may be less accurate than EpiZobtained by normal polarization-e.g., if the concentration of the electrode product on reverse polarization is low, the wave may have a drawn-out appearance which would make determination of EPil difficult. Where the reduction and oxidation potentials of irreversible redox couples are far apart-e.g., azoxybenzene (EplZ = -0.9 volt) and hydrazobenzene (Epi2 = -0.4 volt) a t p H 12.5, an improved wave shape can be obtained by briefly applying a potential where reduction occurs-e.g., -1.2 volt for 2 minutes, before starting the scan from -0.5 volt toward less
negative potential to locate the oxidation wave of the electrode reaction product. Without prior knowledge of where the latter electrolyzes, the wave can be located by trial runs. Because the rate of diffusion of the product away from the electrode is generally not known, the magnitude of the peak current of the waves found on reverse polarization cannot be precisely interpreted, although fairly good estimations can be made on the basis of comparative runs. For example, the composite wave of a ferri-ferrocyanide mixture (0.5 m V each) (Table I : C-1,2) has an average cathodic current component of 4.8 pa. The average cathodic current due to 0.5mM ferricyanide is 2.93 pa. (A-1,2,5); that due to reverse polarization of 0.5mX ferrocyanide is 1.56 l a . (B-3,4); the sum, 4.5, is considered close to the value for the mixture, because the continuous transition from anodic to cathodic current in a composite wave does not permit adequate residual current corrections. Similarly, the sum of the expected anodic currents due to ferrocyanide and reverse polarization of ferricyanide is 4.9 pa. (3.00 plus 1.90); the observed anodic current of the composite reduction wave of the mixture is 4.6.
The reverse polarization technic, which is in essence a slow form of cyclic voltammetry accomplished with an ordinary polarograph, has been used to study two reversible and two irreversible redox couples; in every case, the product of the electrode process was successfully identified. In the case of an unstable compound (hydrazobenzene a t pH 1.6), where ordinary voltammetry is too slow to record the behavior of the compound before it disappears, the compound can be studied by reverse polarization of the more stable member of the couple. ACKNOWLEDGMENT
The authors thank Dr. Ilana Fried for stimulating discussion. LITERATURE CITED
(1) Chuang, L., Fried, I., Elving, P. J., ANAL.CHEM.36, 2426 (1964). (2) Xbid., 37, 1528 (1965).
(3) Lord, S. S., Rogers, L. B., Ibid., 26, 284 (1954).
RECEIVEDfor review March 8, 1965. Accepted August 27, 1965. Work supported in part by the U. s. Atomic Energy Commission and the Horace H. Rackham School of Graduate Studies of The University of Michigan.
Polarography of Selected Cations in Fused Sodium Nitrate-Potassium Nitrate Eutectic at 250" C. H. S. SWOFFORD, JR., and CHARLES L. HOLlFlELD Departmenf o f Chemistry, University o f Minnesota, Minneapolis, Minn.
b
The polarographic reductions of
TI(I), Cd(ll), and Pb(l1) are examined in the fused salt mixture. Reversible behavior is observed for both TI(I) and Cd(ll) which have half-wave potentials of -0.828 volt and -0.665 volt vs. the Ag-Ag(I) reference electrode, respectively. Inconsistent with previously reported work in a lower melting nitrate eutectic, the reduction of Pb(ll) appears to proceed irreversibly with a nonconcentration-dependent prewave preceding a diffusion controlled second wave at higher conPossible excentrations of Pb(ll). planations for this phenomenon are discussed in light of the experimental data.
C
URRENT-VOLTAGE STUDIES in
fused salt media have generally been carried out with the use of solid indicat(22). Polaring microe!ectrodes graphy-Le., voltammetry using a dropping mercury electrode (DME)-has
55455
not been employed by a large number of investigators. Two factors are doubtless responsible for its restricted use a t elevated temperatures; an obvious health hazard and a more serious limitation imposed by the necessity of working in media melting a t temperatures sufficiently low so that the volatilization of mercury from the surface of the drop does not interfere with the diffusion and/or electron transfer process. I n fact, Delimarskii and Markov (3) have stated that the D M E is not useful a t temperatures above 215' C.; that this statement is incorrect is evidenced by the work of Swofford and Laitinen (15) and is further challenged by the present tTork. Kachtrieb and Steinberg were the first workers to make use of the DME in fused salt mixtures (9, IO). Their initial study established its use in the ternary eutectic h'H4N03-LiN03-NH4C1 a t 125" C. However, the instability of this mixture led these investigators to a second study employing a ternary mixture composed of 30 mole yoLiN03,
17 mole % NaN03, and 53 mole % KN03 a t 160" C. as a solvent; these authors found the Ilkovic equation to be applicable and reported halfwave potentials for the reversible reductions of Ni(II), Pb(II), Cd(II), and Zn(I1). Colichman (2) has employed the D M E in molten ammonium formate a t 125' C. and has observed reduction waves for a wide variety of compounds. Stability constants for the chloro complexes of Pb(II), Cd(II), and Ni (11) have been evaluated by Christie and Osteryoung (1) using a D N E in fused LiKOs-KaXO3 a t 180' C. Previous to the present work, and the work of Swofford and Laitinen ( I @ , Inman and Bockris (6) are apparently the only workers who have attempted to use metallic mercury for voltammetric investigations a t temperatures in excess of 200' C. These investigators used a hanging mercury drop to determine chronopotentiometrically the association constants for chloro complexes of cadmium in molten KNO3-XaNOs eutectic a t 260" C. VOL. 37, NO. 12, NOVEMBER 1965
1509
40
n
3
2 \ v
;
4
3
'-
2-
5 20
0-1-2-
-3
-
I
I U.0V' .
-1 .uv
-0.SV
-1.3v
Eapp. vs. t h e r e f e r e n c e
Figure 1.
Residual current curve
-01 oov
-05oov
-1.ooov
EOPP
Figure 2. EXPERIMHTAL
RESULTS AND MSCUSSION
Equipment. Temperature in the electrolytic cell was maintained a t 250' i 2' C. by use of a mercuryin-glass thermometer and manual adjustment of the Variac powering a Glas-Col heating mantle. Other than this minor variation, the electrolytic cell and associated equipment were as described by Swofford (13). Currentvoltage curves were recorded with a Sargent Model XV Polarograph (E. H. Sargent and Co., Chicago, Ill.). Electrodes. The construction and use of both the conventional D M E and the Ag-Ag(1) reference electrode have been well characterized in the literature (8, 13, 15) and will not be discussed further here. Reagents. All chemicals used in connection with these studies were reagent grade. The chemicals were anhydrous in all cases and in addition were oven-dried a t 130' C. and stored in a desiccator over magnesium perchlorate until needed. hocedure. The technique used for preparation and purification of the eutectic melt has been described (13). Weighed quantities of the particular cation under study were added directly to the melt as the appropriate salt through a side entry port, and current-voltage curves were recorded in the usual manner.
A trace of a typical residual currentvoltage curve a t the DME [us. Ag-Ag (I)] for a fused KN03-NaN03 eutectic melt a t 250' C. is shown in Figure 1. The useful voltage range available for polarographic investigations covers about 1.5 volts from +0.1 to -1.4 volts. The cathodic limiting reaction begins a t approximately - 1.4 volts and has been described as arising from the reduction of nitrate (15). The anodic limiting process appears to represent a reversible two-electron oxidation of metallic mercury to mercury (11) ( 5 ) . Polarography of Thallium(1) as TlCl. The polarographic reduction of Tl(1) appears to follow the expected path, Hg' T1+ e Tl(Hg)O (1) forming a soluble amalgam; typical current-voltage curves are shown in Figure 2. At the lower concentrations of thallium (curve a) there are well developed waves, while a t the higher thallium concentrations (curve b) maxima formation is observed. These results agree generally with trends followed for maxima formation as a function of concentration in aqueous
>'
+
+
Typical polarogram
a, 4.6 X 10-4M in TIC1 b, 3.27 X 1OTaMin TIC1
media. The differences in the prereduction portions of the currentvoltage curves can be ascribed, for the m'ost part, to difference in chloride concentrations (14). At higher concentrations of chloride ion (curve b) a well developed anodic wave due to the formation of mercury chloride is observed. Three separate Tl(1) reduction waves, corresponding to three different concentrations of thallium, are analyzed in Figure 3 by plotting the applied e.m.f. (Bapp) us. log (id - i)/i. This relationship is linear with a slope of 0.100 volt which compares favorably with the theoretical value of 0.104 volt expected for a reversible one-electron reduction a t 250" C. The Ei,2 value as determined from Figure 3 is -0.828 volt us. the Ag-Ag(1) reference electrode. Addition of NaCl to the Tl(1) solution did not noticeably affect the Eliz value. Hence, the formation of chloro-thallium complexes does not appear significant. A plot of i d in the diffusion limited region of the polarographic wave us. concentration of Tl(1) is linear and passes through the origin. Polarography of Cadmium(I1) as CdC12. Consistent with aqueous PO-
7. d 6
h
-0,800-
v
a
5 .
5W
4.
v
n a
w
a LL
-0.900 -0.500
0.000 LOG
0.500
id
A 4.6 x 0
0
1810
0
-i
1 0 - 4 ~TICI 3.27 X 10e8MTIC1 8.25 X 10%4 TiCi
ANALYTICAL CHEMISTRY
la00
i3
3. 2. 1.
0.
Figure 4. added)
Typical polarogram (7.2
X lO-'M CdClz
-0.640
-
-0.660
-
h
2
a
P
I
.
-a500
E,,
Figure 5.
vs. log
-
i
i
Figure 6.
(7.2 X 10-4M CdCh
larography and previously published work (IO), the reduction of Cd(I1) a t the DME appears to proceed according to the reaction,
+ Cd(I1) + 2e = Cd(Hg)O +
Cd(Hg)O
(2)
+ z(C1-)
$
I
.o.ooo
i
i
Q500
(7.2 X 10-4M CdClz
"Mu + +
(3)
A polarogram of a nitrate eutectic melt, 7.2 X 10-4M in CdC12, is reproduced in Figure 4. A plot of E applied us. log (2, - i)/i for this polarographic wave is given in Figure 5; it is linear with a slope of 0.046 volt as compared to 0.052 volt which is expected theoretically for a reversible two-electron reduction. Inman, Regan, and Girling (7) have recently published a study of chlorocadmium complexes in a fused KN03NaN03 eutectic mixture a t 250' C. These authors report the overall formation constants for CdCl+, CdCl, and CdC13- as 86.7, 1670 and 16,700 moles/kilogram of solvent, respectively. Calculations based on their reported values indicate that the only species formed t o an appreciable extent under the conditions present for the recording of the polarogram shown in Figure 4 is CdCl+, and that this represents only
vs. log
'
[found from Figure 6 by selecting the potential a t which log ( i d - i)/i is 01 as the El12 value for the Cd-Cd(I1) couple. As expected for a diffusion controlled process, the diffusion limited current is linear with concentration of cadmium (11). Polarography of Lead(I1) as Pb(No&. The reduction of Pb(1I) in fused KNOa-NaNOa eutectic might presumably follow the equation:
also forming a soluble amalgam. Any cadmium present as chloro-cadmium complexes should be reduced according to the reaction, CdC1,(2-z)+ HgO 2e- =
+
.
added) 0 O.OM NaCl added (CI- = 1.35 X 10-aM) A 1.09 X 10-aM NaCl added (CI- = 2.37 X IO-aM) 0 2.34 X 10-aM NaCl added (CI- = 3.68 X IO-aM)
added)
HgO
€spp
.
0
-0.500v
-0.700V
-0.9OOV
Pb+2
EaPP.
Figure a, b,
7. Typical polarograms 2.0 X 10-4M in Pb(N0dz 3.2 X 10-3M in Pb(N0a)n
10% complexation of the total cadmium ion added. A series of polarograms were recorded in which excess chloride ion (as NaC1) was added to a eutectic melt 7.2 x 10-4M in CdClz; plots of E applied us. log ( i d - i)/ifor these C1additions are shown in Figure 6. It is evident that changing the chloride ion concentration by a factor of three produces no appreciable shift in the Cd+"Cd(Hg)O half-wave potential. Based on these data it seems reasonable to accept a value of -0.665 volt
HgO
2e-
=
Pb(Hg)
A polarogram of a solution 0.20mM in Pb(X0a)z is shown in Figure 7. A plot of E applied vs. log ( i d - i)/i is given in Figure 8. The wave analysis indicates that the process is not reversible-i.e., the relationship E applied vs. log ( i d - i)/iis not linear-in this solvent
-0,740 I
-0.500
0.000
0.50
LOG I
Figure 8.
vs. log
i 'd (2.0 -X I
10-4M in Pb(N03)z
TiME
Figure 9. Current-time profiles for individual drops Height of mercury column = 20 cm.; concn. Pb(NOa), 3.2 X 10-aMi Fapp -0.660 0
volt
VOL. 37, NO. 12, N3VEMBER 1965
0
1511
at this temperature. Nachtrieb and Steinberg (10) found Pb(I1) to be reversibly reduced in a lower melting ternary fused nitrate eutectic. When the concentration of Pb(N03)2 is increased, two separate waves are developed (see Figure 7). The first wave does not appear to change appreciably as the concentration of Pb(NO3)z is increased further; however, the second wave is concentration dependent. An examination of currenttime profiles during individual drop lives from -0.6 to -0.7 volt (Figure 9) indicates the possibility of a surface controlled process in the region of the apparent prewave. Further credence for this is supplied by a study of the variation of current (i) with height of the mercury column corrected for back pressure (H). A plot of log (i) us. log (H) at -0.66 volt (in the suspected prewave region) is shown in Figure 10. The slope of the plot is 0.76 indicating a substantially surface controlled process. Also shown in Figure 10 is a similar plot taken at -1.00 volt (in the region of the expected diffusion controlled wave). The slope of this plot is 0.53 indicating a shift to a diffusion controlled process. The polarography of Tl(1) has been successfully characterized in this medium and the half potential for the Tl(I)-Tl(Hg)O couple is in the same region as the couple under consideration, No apparent surface controlling complications arise with this reduction. The addition of NaCl to the Pbsolution does not affect the behavior of the Pb(I1) reduction wave. It seems reasonable to assume, therefore, that the phenomenon is characteristic of Pb(I1). Alternative explanations for polarographic behavior of the type exhibited by Pb(I1) in the KhT03--NaN03eutectic mixture at 250” C. have been published by Reilley and Stumm (12) for aqueous polarography. According to these authors the formation of a n insoluble reaction product may inhibit the electrode reaction. This inhibition, usually referred to as passivation, results from the deposition of the insoluble reduction product at the elec-
1512
ANALYTICAL CHEMISTRY
.-
the melt at 250” C. shows marked signs of oxidation after several hours. Also consistent with the polarographic behavior observed for the reduction would be a reduction mechanism involving the specific adsorption of P b (12). Since such processes are usually associated with organic polarography, they seem somewhat less likely in fused salt media. However, the possibility cannot be ruled out on the basis of the available data. Little can be said regarding the value for the reduction of Pb(I1) in the fused KN03s a N 0 3 eutectic mixture a t 250’ c. The E l l P value disregarding the prewave is near -0.85 volt us the reference.
I
i
1.300
1.200/-
LITERATURE CITED
1.100 1.700
Figure 10.
1,8OOLo6 J.900
2000
Log i vs. log H (Pb+Z =
4 X 10-sM) 0
A
= -0.66 volt, slope = 0.76 Espp = - 1 .OO volt, slope = 0.53 E,,,
trode surface. An examination of the variation of current with time for an individual drop in the region of the prewave (,Figure 9) indicates that current is proportional to the rate of growth of the electrode surface. Such behavior is diagnostic of electrode passivation (12). Inhibition of this type has been previously reported by Swofford in this medium. The rapid initial increase in current at the onset of the prewave for the reduction of Pb(I1) a t higher concentrations may be due to maximum formation of the first kind. While it appears reasonable to accept the formation of an insoluble reaction product as being responsible for the polarographic behavior of P b (11), a n examination of the phase diagram for the Pb-Hg system does not suggest what solid might be formed (4). It is possible that an oxide of lead might be formed via a chemical oxidation following the reduction step in this highly oxidizing nitrate medium. Indeed, a lead wire when introduced into
(1) Christie, J. H., Osteryoung, R. A,, J . Am. Chem. SOC.82, 1841 (1960). ( 2 ) Colichman, E., ANAL. CHEM. 27, 1559 (1955).
(3),Delimarskii, K., Markov, Electrochemistry of Fused Sigma Press, U.S.S.R. 1961. ( 4 ) Hans,e,n, M., “Constitution of Alloys, McGraw-Hill, New
B. F., Salts,” Binary York,
1958. (5) Holifield, C. L., R1.S. Thesis, University of hlinnesota, Minneapolis, 1965. ( 6 ) Inman, D., Bockris, J. 0’ M., Trans. Faraday SOC.57, 2308 (1961).
( 7 ) Inman, D., Regan, I., Girling, B, J . Chem. SOC. (London) 1964, 348. ( 8 ) Kolthoff, I. ,,ht., Lingane, J. J., “Polarography, Interscience, New York, 1952. (9) Nachtrieb. N.. Steinberg. M..’ J. ‘ Am. Chem. hoc. 7 0 , 2613 (lg48). (10) Nachtrieb, N., Steinberg, M., Ibid., 72, 3558 (1950). (11) Reddy, T. B., Electrochem. Technol. 1 , 325 (1963). (12) Reilley, C. N., Stumm, W., “Ad-
vances in Polaroera~hv.”Interscience. New York. 1962. (13) Swofford, H. S., Ph.D. Thesis, University of Illinois, Urbana, 1962. (14) Swofford, H. S., Holifield, C. L., University of Minnesota, Minneapolis, unpublished work, 1965. (15) Swofford, H. S., Laitinen, H. A., J. I
I
“ I
Electrochem. SOC.110, 814 (1963).
RECEIVEDfor review May 24, 1965. Accepted September 1, 1965. Work suported by the School of Chemistry, 1;niversity of Minnesota, and E. I. du Pont de Nemours and Co. (Summer Research Fellowship, CLH).
