raphic Catalyti sbaIt (I I), 1. M. KQlthQff School of Chemistry, Unioersity of Minnesota, Minneapolis, Minn. 55455 P. Mader Department of Biophysics and Plant Physiology, University of Agriculture, Prague 6, Czechosloaakia When t h e molar ratio of cysteine, cysteine ethyl ester, cysteamine, thioglycolic acid, or thiosalicylic acid to cobalt(l1) in borate, tris, ammonia, or phenolate buffers reaches a certain value 6, which in general i s close to one, the BrdiEka current attains a maximum value il which does not increase with increasing thiol concentration, but is linearly proportional to cobalt(l1) concentration. The value of il i s independent of pH and the Concentration of the alkaline constituent of the buffer. It depends on t h e concentration of the acid constituent of the buffer and decreases when the latter gets small. Under the above conditions the BrdiEka current i s diffusion controlled (Equation I), the trace of cobalt(l1)-cysteinate complex diffusing to the electrode initiating a chain reaction (Equations 2 and 3): Co(ll)RS
+ 2e
-+
Co(0)RS; initiation, diffusion controlled
(1) Co(0)RS
+.
BH -+ Co(0)
T
(2) propagation
that the mechanism and the kinetics which determine the BrdiEka currents vary with experimental conditions, and this situation makes it very difficult, if not impossible, to determine the effect of a single factor on the current. The shape of current-thiol concentration curves at a given cobalt concentration, and of the current-cobalt concentration curves at a given thiol concentration, which resemble a Langmuir adsorption isotherm, has led several authors to the conclusion that a cobalt(I1)-thiolate complex is adsorbed on the surface of the mercury electrode. However, Klumpar (6) and Kuik (7) have shown that for low molecular weight thiols this assumption is consistent neither with theory nor with experimental facts and that the resemblance with Langmuir adsorption isotherms is fortuitous. Recent studies (8-10) have revealed that the characteristics of BrdiEka currents vary with experimental conditions, in particular, with the ratio of concentrations of the active components cobalt(I1) and thiol. When the concentration of the thiol is very low and cobalt is in sufficient excess, the catalytic current is essentially kinetically controlled but becomes diffusion controlled when the ratio of thiol to cobalt concentration exceeds a certain value. The catalytic currents are very much greater than corresponds to the sum of the diffusion currents of the cobalt species present and very much less than the diffusion current of the proton donor in the buffers used. It remains particularly puzzling why at high enough ratios of thiol to cobalt(I1) concentrations, the BrdiEka currents are diffusion controlled, and also do not increase with increasing thiol concentration (vi.). In the present paper a mechanism is proposed which accounts for this unusual behavior. Also, an interpretation is presented for the fact that BrdiEka currents in low molecular weight thiol-cobalt(II1) hexaammine chloride mixtures are so much smaller than those observed with cobalt(I1). In a subsequent paper, we intend to discuss Brdi’cka currents observed at the other extreme conditions, when they are entirely kinetically controlled.
” + e Co(0)RS7 +- ‘/zW, (3) Co(0)RS Co(0) + RS Co(0)amalgam Co,Hg,; Co(0)RSH + .
4
4
-+
termination (4) instead of Co(0)RS the protonated form may disproportionate. The effect of ionic strength on iL has been i n vestigated and interpreted. Surfactants greatly increase the BrdiEka current, which attains a maximum at the potential where the surfactant i s being desorbed, and after passing a minimum it increases again to the “normal” maximum of the BrdiEka current. Thus i n t h e presence of surfactant, two maxima are observed as with proteins. An interpretation is given of the fact that C O ( I ~ I ) ( N H ~yields ) ~ ~ + only extremely small BrdiEka currents with cysteine-like compounds. Cobalt(l1) in alkaline buffers i n t h e presence of excess of cysteine i s air-oxidized instantaneously to a cobait(il1) complex, which yields the same BrdiEka current as cobalt(l1).
THELITERATURE on catalytic hydrogen currents observed at the dropping mercury electrode in cobalt(I1) or nickel(I1) -thiolate mixtures in alkaline buffers, called after their discoverer BrdiEka currents ( I ) , comprises more than 1000 papers. During the last ten years several reviews have been published (2-5). The detailed mechanism of these currents is still incompletely understood and the literature contains many contradictory statements. The main reason for this is (1) R.Brdirka, Collecf. Czech. Chem. Commim., 5, 122, 148 (1933). (2) R. Brdic‘ka,2.Phys. Chem., Sonderheft, 1958,165. (3) 0 . H. Muller, “Methods of Biochemical Analysis,” D. Glick, Ed., Interscience Publishers, Inc., New York, 1963, Vol. 11, p 329. (4) S. G. Mairanovskii, J. Elecfroanal. Chem., 6,77 (1963). ( 5 ) A. Calusaru, ibid., 15,269 (1967). 1762
e
EXPERIMENTAL
L-Cysteine hydrochloride was an Eastman Kodak Co. reagent. L-Cysteine ethyl ester hydrochloride, cysteamine hydrochloride, and thioglycolic acid were products of Nutritional Biochemicals Corporation, Cleveland. Thiosalicylic acid (6) J. Klumpar, Collect. Czech. Chem. Commun., 13, 11 (1948). (7) M. Kuik, ROCZ.Chem., 42, 143 (1968). (8) M. Bfezina and V. Gultjaj, Collect. Czech. Chern. Commun., 28, 181 (1963). (9) M. KfitovB and M. Br‘ezina,ibid., 31,743 (1966). (10) A . Basinski, J. Ceynova, and Z . Gadek, Rocz. Chem., 40, 101 (1966).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
1.0
o.e
Figure 1. Interference by “presodium” current; 0.01M phenolate, 0.03M phenol, 0.09M NaC104 (1) Buffer alone (residual current); (2) plus 1 X 10-4M cysteine ethyl ester (residual and presodium current); (3) as 2 plus 1 X 10-6MCoClI; (4) plot of net Brdirka current cs. potential [curve 3 minus sum of cobalt(X1) diffusion current and curve 21
3.6
;.4
D.2
was an Aldrich Chemical Co., Inc., reagent. All other chemicals were of C.P. quality. All potential values are referred to the saturated calomel electrode (SCE). Two capillaries have been used throughout this work, the characteristics of which, at 50 cm mercury pressure and zero applied voltage, were: a vertical capillary 1, m = 1.08 mg/ sec, t = 6.68 sec; a tilted capillary 2 , m = 0.41 mg/sec, t = 7.60 sec. Unless otherwise stated, experiments have been run with the vertical capillary 1. Current-time curves and a few other types of measurements have been recorded with capillary 2 , which was tilted at an angle (11) of 45” to overcome depletion of the electroactive species in early stages of formation of the drop. Details about various kinds of measurements have been described previously (12). In connection with the sensitivity of organic thiols and of cobalt(I1)-thiolate mixtures toward oxidation by atmospheric oxygen, especially at higher pH, the following procedure was always followed in the preparation of reaction mixtures. An air-free solution of the thiol was added to the previously deaerated buffer solution and upon continued passage of nitrogen, air-free solution of CoC12was added from a microburet. This order of mixing had an additional advantage in that it prevented precipitation of cobalt(I1) hydroxide at higher pH, because of complexation of all cobalt(I1) with an excess of thiol. In every set of experiments under specific conditions polarograms were run in the absence of cobalt(I1). In order to find the value of the BrdiEka current, these blanks composed of the sum of charging and “presodium” currents, were subtracted from the total current. BrdiEka currents exhibit a maximum, and all currents reported in this paper refer to those at the potential of the maximum. RESULTS
Effect of Ratio of Thiol to Cobalt Concentration. This effect has been studied with several low molecular weight thiol compounds (cysteine, its ethyl ester, cysteamine, thioglycolic acid, thiosalicylic acid) and in various buffer media (boraxboric acid, tris-HC1, phenol-sodium phenolate, etc.). In order to avoid large current densities at which hydrogen bub(11) I. Smoler, Collect. Czech. Chem. Commun., 19, 238 (1954). (12) I. M. Kolthoff and P. Mader, ANAL.CHEM., 41, 924 (1969).
bles separate on the surface of the electrode, small concentrations of both cobalt(I1) and thiol have been used. This also avoided the appearance of large kinetic “presodium” currents which are due to hydrogen evolution from the SH group and which are observed both in the absence and in the presence of cobalt(I1) at potentials between those of the maximum of the BrdiEka current and the currents of the reduction of a buffer constituent. Interference by the “presodium” current even at relatively large pH is especially pronounced with cysteine ethyl ester (Figure 1) and cysteamine (see curve 2 in Figure 1). With all thiols used in the present study in which their concentration was relatively large, the interfering “presodium” current limits the pH at the lower level of the pH range covered by the buffer. With all thiol compounds studied the BrdiEka maximum current, i,, at a given cobalt(I1) concentration increased with increasing thiol concentration to a limiting value, il, which did not change upon increase of the thiol concentration. The values of il for a given thiol were found to differ in different buffer types. Examples of the variation of il with cobalt(I1) and cysteine concentrations in a tris bufl‘er are presented in Figure 2. The thiol concentration, [RSHII,,, necessary to attain the limiting current il, was found to increase with increasing concentration of cobalt(II), while the ratio of [RSHII,, to [Co(II)], at which il was attained and which is denoted as limiting ratio L,, was about constant at varying concentrations of both RSH and Co(I1). This is illustrated by the dashed line in Figure 2. The value of L,, was found to depend on the kind and composition of the buffer used, its pH, and also on the kind of the thiol compound studied. In tris buffers at pH between 8.2 and 9.3, the values of L, for all thiol compounds studied were found to vary between about 0.5 and 2. Small L, ratios were found with cysteine in tris buffers of pH 9.05 ( L , CS 0.5, see Figure 2), and in 0.1M NH3, 0.1M ”821 buffer, pH 9.3 (LT E 0.25). In 0.06M borax, pH 9.2, the value of L , for cysteine was close to one. Diffusion Control of Limiting Brdizka Current ir. Under all conditions used, i2 of the above thiols was found to be diffusion controlled. This was evident from the linear increase of i2 with the square root of the height of mercury in the reservoir, and from the shape of instantaneous current us. time
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
176.3
cccI2
(X
10-53
Figure 3. Effect of varying ratio of [Co(II)] to [RSH] on i,; 0.08M tris, 0.02M tris-MC1, pH 9.05 Concn of cysteine: (1) 0.25; (2) 1; ( 3 ) 3 X 10-5M
curves, the latter being monotonous parabolas with values of the exponent x in the expression i = kt“ varying between 0.19 and 0.22 (theoretical value for pure diffusion control is 0.17). Also the temperature coefficient was found normal for diffusion control, this value between 20 and 30°C being found in different buffers equal to f1.5 to +2,0zper degree. Under the specified conditions diffusion control of i 2 was found with all thiols used in tris, ammonia, borate, and phenolate buffers. Effect of Cobalt Concentration on i2. When the ratio of thiol to cobalt Concentration was greater than L,, the diffusion controlled current iz was found to increase linearly with increasing concentration of cobalt(1I) (see curve 3 in Figure 3). When the ratio of [RSH] to [Co(II)] remained considerably less than L,, the line giving the relation between the current 1764
e
i, and cobalt concentration was a parabolic curve, the exponent y in the expression i, = k[C0(11)]~ being less than one (curve 1, Figure 3). Variation of L , and iz with Changing pH, Buffer Capacity, and Ionic Strength. The effect of pH and of concentration of tris at constant tris-H+ concentration of 0.02M and that of cobalt of 3 X 10-5M is illustrated in Figure 4 for cysteine in tris buffers. The lowest value of L , (where i, becomes constant) was observed at pH 8.6 f 0.15, L, increasing with increasing or decreasing pH. Under the conditions in Figure 4, the magnitude of the diffusion controlled current iz was independent of the concentration of the alkaline buffer constituent (tris) and pH. In another set of experiments, carried out at three different ionic strengths, the concentration of tris was kept constant, while the concentration of its hydrochloride was changed. At a given ionic strength, there was again a pH region at which it remained constant. At higher pH, i2 started to decrease, this decrease starting at a lower pH with increasing ionic strength. The value of il in the pH range where it was constant was the same at ionic strengths between 0.1 and 0.3M (Figure 5). The current was still diffusion controlled when it was less than the maximum value of iz. It remained unchanged with increase, of concentration of RSH but, at constant ionic strength, it increased with increasing tris-H+ concentration. For example, when at a constant ionic strength of 0,2M, the concentration of tris-H+ was increased from 0.01 to 0.05M and that of tris in the same ratio, iz increased from 1.35 PA to the maximum value for iz of 1.95 PA. The effect of ionic strength is also clearly illustrated in ammonia buffers by curve b in Figure 6. At point B on curve b, the composition of the solution was the same as at point A on curve a, except that at point B the ionic strength was equal to 0.01 and at point A , 0.1M. Upon increase of the ionic strength by adding potassium chloride without changing the composition of the solution otherwise, the value of il decreased continuously. Curve a shows again a regEon of ammonium concentrations where iz remains constant and independent of ammonia concentration (between 0.05 and 0.2M). Also, on the part of the curve where iL decreases with increasing pH, iz is unaffected by ammonia concentration
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
4
2
6
8
cysteine (x l c - 5 ~ )
Figure 4. Variation of L, with pH at constant tris-HCl concentration; 0.02M tris-HCI, 3 x 10 -jM COClZ Concn of tris and pH: (1) 0.005M,7.50; (2) O.OlA4, 7.80; (3) 0.02M, 8.10; (4) 0.06M, 8.40; (5) 0.09M, 8.75 ; (6) 0.18~V, 9.05; (7) 0.40M, 9.40
1.3
1.6
.
I
-.?
G
I,
tr s-it
2
II
c
L
L-‘_
Figure 5. Variation of il with pH at constant tris concentration and with ionic strength; 1 X 10-5M CoCI,, 1 X 10-4Mcysteine, 0.2M tris, ionic strength was kept constant with NaCl; (a) p 0.1; (b) p 0.2; (c) p 0.3M (between 0.05 and 0.2M in Figure 6) and hence also by pH, but it decreases with decreasing ammonium ion concentration regardless whether or not the pH is altered. Experiments described with cysteine in tris and ammonia buffers were also carried out with cysteine ethyl ester, cysteamine, and thioglycolic acid. Both cysteine ethyl ester and cysteamine exhibit extremely large “presodium” currents, which are much larger in magnitude than those of cysteine in the same concentration. These “presodium” currents overlap with Brdizka currents (see Figure 1). Experiments with these two thiols were limited to the higher pH region. Thioglycolic acid at the lowest values of ionic strength gives an almost negligible “presodium” current. This current, however, increases considerably in magnitude with increase of the ionic strength. The above thiols in ammonia
w-:.
e
1;
’x ir->!)
Figure 6 . Variation of current in ammonia buffers with ammonium concentration and ionic strength; 1 X 10-jM COCI~, 1 x 10-4Mcysteine (a) Concn of NH3: 0,0.05M; @, 0.1M; 0 , 0 . 2 M . Concn of NH,Cl 0.05M, and KCI was changed, but %as0.1M; (b) Concn of “1, concnofKC1waschanged
and tris buffers gave results similar to those obtained with cysteine. In phenol-phenolate and borax-boric acid buffers, the acid constituents are uncharged. Figure 7 shows the effect of phenol concentration and of ionic strength on the limiting Brdizka current, iL, of cysteine (curve 1) and its ethyl ester (curve 2). With both thiols, il again did not increase with increasing phenol concentration after the latter had become 0.03M (curve 2 ) to 0.04M (curve 1, Figure 7). Contrary to the behavior in a buffer. the acid constituent of which is a
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
a
1765
/
0 . 3 ’--
,:I
,:2
.ij
.:L
,j: p!.3?2:
.y
.I:C
.a3
cij;
Figure 7. Effect of phenol concentration on iL; O ~ ~ ~ ~ ~ a ” 1p X~ 10-jM e ~ o COCL~ ~ a t ~ , (1) plus 1 X 10-4M cysteine; (2) plus 3. X 10-4A4 cysteine ethyl ester. 0, ,U = 0.01M; A, 1 = 0.1O.M was ~ a ~ ~ ~with a iNa@lOa ~ e d
univalent cation, the ii cs. uncharged acid concentration curve is not affected by ionic strength. Effect of Trivalent Cobalt. As is well known, trivalent cobalt in the form of cobalt(II1)hexammine chloride instead of cobalt(l1) in the buffer gives rise only to very small BrdiEka currents even at large cysteine concentrations (compare curves 1 and 2 in Figure 8). O n the other hand, trivalent cobalt prepared by direct oxidation of the cysteine-cobalt(I1) mixture in a borate buffer gives exactly the same Brdifka current as divalent cobalt (compare curves 2 and 3 in Figure 8). Solution 3 in Figure S was prepared by passing a stream of air through solution 2 for 10 seconds. During this time all cobalt(I1) was oxidized to cobalt(II1). This was confirmed by the value of the Co(1II) to Co(I1) diffusion current which was one half of that of the Co(I1) to Co(0) wave, taking into account the effect of potential on the drop time. Also, solution 2, which was initially colorless, turned intensely yellow even during the first second of treatment with air. The intensity of the color did not change upon further passage
Figure 8. Polarograms of cysteine with trivalent cobalt; 0.06Mborax9 2 X 1W4Mcysteine, pH 9.2
of oxygen after 10 seconds, while the polarogram obtained after 10 minutes’ oxidation was identical with that observed after 10 seconds (compare curves 3 and 4 in Figure S), Observations similar to those obtained with cysteine in borate buffers were made also with cysteamine and thioglycolic acid, both in borate and in tris buffers. Effect of Surfactants. Presence of small amounts of the surfactants polyvinyl alcohol, polyvinyl pyrrolidone, Triton X-114, and gelatin in the cobalt(1I)-thiolate mixtures brings about an increase of the BrdiEka current at potentials at which the surfactant is adsorbed on mercury. An example with cysteine using polyvinyl alcohol (PVA) in borax buffer is shown in Figure 9. The increase of the BrdiEka current starts at about -1.25 V and yields a maximum value a t potentials near -1.65 V, a t which potential PVA is being desorbed from the mercury surface (13). At higher concentrations of PVA (above 0.004%:) when the coverage of the electrode surface by the surfactant approaches completion, the BrdiEka current becomes suppressed. The same behavior is observed with other surfactants. Gelatin, which desorbs from mercury only a t potentials much more negative than those a t the BrdiEka maximum, brings about a n increase of the BrdiEka current in the entire potential region covered. Effect of PVA on the BrdiEka current of cysteine using cobalt(II1)hexammine chloride is shown in Figure 10. The almost negligible BrdiEka current was found to be considerably increased in the presence of as little as 0.002% PVA. At the same time, a new maximum was observed in the presence of PVA at about -1.3 V, which resembles maximum B described in a previous paper (12). This maximum was suppressed when the PVA concentration exceeded 0.003%, while the Brdiitka maximurn increased in the whole concentration region of PVA tested. DISCUSSION
Some of the important results which need interpretation can be summarized as follows. When the molar ratio of (13) I. M. Kolthoff and I. M. Issa, unpublished results,
2
(1) plus 5 X 1O-’iW @o(IVH&C~,; (2) plus 5 x 1O-6M coc12; (3) the same as 2 after 10 seconds of oxidation with air; (4) the same as 3 after additional BO minutes of oxidation with air
3,4
1.0
1.1
1.2
1.3
-E
1766
o
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14,DECEMBER 1970
(‘!)
1.4
1.5
1.5
1.7
1.2
1.2
1.4
1.5
-E ( V )
Figure 9. Polarograms with PVA; 0.06M borax, 4 X 10-4MCoCl,, 2.3 X IO-4M cysteine, pH 9.2 Concn of PVA: (1) 0 ; (2) 1; (3) 2; (4) 3; (5) 4.8 X loF3%. Tilted capillary, 33 cm Hg
cysteine-like compounds to cobalt(I1) concentration reaches a certain value L,, the BrdiEka current reaches a maximum value il, which does not increase with increasing thiol concentration, but is linearly proportional to the Co(1I) concentration. The value of L, varies somewhat with the thiol used; in general it is equal to or somewhat smaller than one, the lowest value of ‘ 1 4 being found with cysteine in ammonia buffer. Another important result is that, although L, varies with pH, il is constant over a certain p H range. I t is independent of the concentration of the alkaline constituent of the buffer but it starts to decrease when the concentration of the acid constituent becomes less than a certain value (pH at alkaline side of the buffer). Probably a decrease would also be observed when the pH approaches the extreme acid side of the pH range of the buffer. An indication that this is the case has been observed with cysteine; however quite generally the “presodium wave” (of thiol in absence of cobalt) is displaced to less negative potentials with decreasing pH and then overlaps with the BrdiEka current (Figure 1)
when there is excess of RSH over Co(11). It has been found conclusively that under all our experimental conditions il is diffusion controlled. This is most unusual because the diffusion current of a cobalt-thiol complex-the only species which diffuses from the bulk of the solution to the surface of the electrode (G.i.)-is some 50 times smaller than i ~ . This indicates that a chain reaction occurs at the surface of the electrode, with the electroreduction of the reducible cobalt(I1)thiol species as the chain initiating reaction and rate determining step. In the following we will denote the diffusing cobalt species by Co(11)RS. Under certain conditions, ligand L from the bufer or even water may also be present in the inner coordination sphere of the cobalt ion. Furthermore, with increasing value of the ratio [RSH]/[Co(II)] the complex may also contain two or three RS groups. Thus the composition of the complex may be represented by Co(II)(RS),(L),(H~O), in which x may vary between 1 and 3. The overall charge of this complex depends on charge of RS and L and value of
21-
Figure 10. Polarograms with PVA; 5 X 10-jM CO(NH&, 2 X 10-4M cysteine, 0.06M borax, pH 9.2
i ‘
‘t
Concn of PVA: (1) 0 ; (2) 1; (3) 2; (4) 3; (5) 6 ; ( 6 ) 9; (7) 12 X
0
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
e
I767
It has been known for a long time that presence of Co(1I)RS complex is essential for the appearance of BrdiEka currents (1-5? 8-10>. At low pH when all RS is present in the protonated form and none in the form of a complex with Co(II), no BrdiEka current is observed. Also, in agents which strongly bind Co(II), like EDTA, no BrdiEka currents are observed (14, 15). From the fact that il is diffusion controlled and independent of x in Go(II)(RS),(L)y(H20)z it may be concluded that the diffusion current hardly varies with x and that the length of the chain reaction [Equations 2 and 3, (ni.)]at the electrode determines the value of iz. The following mechanism, in which BH i s the acid constituent o i the buffer, is proposed to account for the experimental results x and y .
Co(X1)RS f 2e
-+
Co(0)RS; initiation, diffusion control (1)
+ BH Co(0)RSH + e Co(0)RS
Co(0)RS
-+
-+
+B
-+
1/2Hz Co(0)WSH
--f
Co(0)Hg
Co(0)RSH
+ RS
and/or Co(0)RSH
-+
Co(0)Hg
+ RS + H+
1
propagation
(2)
(3)
HS-R]
+ BH"
7r
-+
HS-R]
[CO(O) c
(I
+B
(2a)
The direct protonation of the central metal atom cannot be ruled out as a possibility. Low-valent transition metals in many complexes are potential Lewis bases in that they contain non-bonding electron pairs. A proton can add directly to the metal to form another type of protonated complex species. Such a complex may be either reduced directly to Co(0)RS and molecular hydrogen, or undergo an electronic transformation to give a coordinated hydrion species, in which cobalt attains back its formal two-valent oxidation state : [co(O)Rs]
+ BH+
4
[A+
Co(0)RS
termination by disproportionation
]+B-+[!+
Co(1I)RS ]+B
(5)
(44
+
(14) V. Chrnelai and J. Nosek, Collect. Czech. Cliem. Commrm., 24,3084 (1959). (15) V. Chmelai, .W. Bfezina. and V. Kalous, /bid., 28, 197 (1963). (16) A. A. VIEek, 2. Elektrochem,, 61, 1014 (1957). (17) I. M. Kolthoff, P. Mader, and S. E. Khalafalla, J. Electroanal. Chem., 18, 315 (1968). e
n-
[CO(O)
(4)
With an excess of RSH, neither rate of formation of a Go (1I)RS complex nor that of its protonation needs to be considered. The formation of a Co(0)RS complex similar to Co(0) complexes (Equation 1) proposed by VlEek in his studies of reduction of cobalt or nickel complexes with N-ligands seems reasonable (16). It is possible that the first reduction product is CO(O)(RS)~(L)~,but apparently this species rapidly disproportionates into Co(0)RS f (x - 1) RS yL. This means that the catalytically active species Go(0)RS is always the same, immaterial what the composition of the complex is which diffuses from the bulk of the solution to the electrode. When the ratio L, of [RSH]/ [Co(II)] is less than one, there is excess of Co(I1)L in the bulk of the solution. The rate of reduction of Co(1I)RS is very much greater than that of Co(I1)L (17). and it occurs at potentials considerably less negative than those at which Co(1I)L reduction occurs. Thus, the excess of Co(I1)L at the electrode reacts very rapidly with RS, which is formed on disproportionation of Co(0)RS at the surface of the electrode (Equation 4). This accounts for the fact that a diffusion controlled current iz is still observed when L, is somewhat smaller than 1. The zero-valent cobalt thiolate complex can be expected to be relatively stable. Transition metals in unusually low oxidation states form more or less stable complexes with two types of ligands-namely, unsaturated organic molecules or ions which form so-called a-complexes in which both donation and back-acceptance of electrons by the ligands are accomplished exclusively by use of ligand a-orbitals, and a variety of ligands which are able to stabilize low oxidation states by sharing electrons from fiiled metal orbitals, to form a type of n-bonding which supplements the u-bonding arising from lone-pair donation. High electron density on the
1768
metal atom-by its nature in a low oxidation state-can thus be delocalized to the ligands. Organic thiols belong to the latter group of ligands, the donor sulfur atom having empty low-lying dn-orbitals which can be used in dr-bonding. The increased electron density on the sulfur atom accounts for its increased basicity and thus for its tendency to be protonated:
This latter species is unstable in water and reacts with protons to give Hz and Co(II)RS, the latter being reduced again to Co(0)RS complex. The actual hydrogen evolution reaction in this case would be of chemical, rather than electrochemical nature, in contrast to the previously discussed mechanism. In other words, instead of reaction 3, the current would be produced by the electrochemical reaction 1, the cobalt(I1) species being reformed in reaction 5. Finally, the direct oxidation of Co(0)RS complex, or even that of the reduction intermediate-Co(1)RS complex-by hydronium ion might also occur if the reducing power of the generated complexes were strong enough. Nothing is known about the acid-base or redox properties of cobalt-thiolate complexes in unusually low ( + I , 0 ) oxidation states. The reason we prefer our interpretation presented in Equations 1-4 is that the rates of reactions 5 should be independent of the electrode potential and thus occur appreciably already at potentials of the cobalt(I1) prewave, at which potentials Co(0)RS is being generated at the electrode surface (17). Instead, the BrdiEka current is observed at potentials as much as 400 mV more negative than at the prewave (see curve 1 in Figure 1). Thus, the species whose reduction accounts for the BrdiEka current cannot be a chemically regenerated Co(1I)RS complex. From the fact that in a given buffer (ammonia, tris) the value of il remains constant when [BH+] is greater than a certain value (Figures 5 and 6) it may be concluded that under these conditions the protonation reaction 2 occurs quantitatively and also that the disproportionation reaction of Co(0) RS (Equation 4a) i s much faster than that of Co(0)RS (Equation 4). This is substantiated by the effect of ionic strength on BrdiEka current. The complex Co(0)RS is always negatively charged, hence in buffers in which the acid constituent is a univalent cation BH+, the rate of the protonation reaction 2 decreases with increasing ionic strength. Consequently, with increasing ionic strength the minimum value of [BH+] at which ic starts to become constant must increase (Figure 5). On the other hand, in buffers, in which the acid constituent BH is uncharged (borate, phenolate buffers), there should not be an effect Qf ionic strength
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
upon the minimum value of BH to attain a constant ii. Actually this is found under specified conditions, when the presodium current is small enough as not to cause too much uncertainty in evaluating the BrdiEka current (Figure 7). The maximum in the BrdiEka current, which with cysteinelike compounds is usually observed at a potential of the order of -1.6 V (~s.SCE) is probably related to adsorption properties of Co(0)RS and/or Co(0)RSH. In spite of the negative charge of the former and the negative charge of the electrode, both species apparently are partly adsorbed on the surface of the mercury. With increasing negative potential, there is increasing desorption of these active compounds. It is fair to assume that the rate of disproportionation (Equation 4) of these active compounds in the non-adsorbed state is much greater than when they are adsorbed on the surface of the electrode. The adsorption may even greatly contribute to their chemical stability. The increasing desorption and subsequent increasing rate of disproportionation tends to decrease the BrdiEka current; on the other hand, increasing negative potential increases the rate of electroreduction (1). The overall effect apparently is a decrease of the BrdiEka current when the potential becomes more negative than it is at the maximum. The first product of the disproportionation reaction (Equation 4 or 4a) is metallic cobalt, Co(O), which is very unstable in the presence of mercury and rapidly yields cobalt amalgam and Co,Hg, (18, 19). Direct formation of Co (0)RS by reaction of Co(0) with RS, if any, would be expected to be very slow and not to occur under our experimental conditions, when the life period of metallic Co(0) is short. This conclusion is substantiated by the fact that with Co(II1) hexammine instead of Co(I1) hardly any BrdiEka current is observed. Co(II1)hexammine is much more stable than Co(1II)RS complexes. The first reduction product, Co(I1) (NH3)e2+ apparently is electroreduced so rapidly to Co(0 that it does not form or forms only a trace of Co(I1)RS complex at the surface of the electrode, whereas the Co(0) does not react with RS- at the electrode surface. This accounts for the fact that extremely small BrdiEka currents are observed with CO(III)(NH&~+,whereas with Co(1II) (RS),, the BrdiEka currents are identical with those observed with Co(II)(RS),. The large increase of the Brdizka current in the presence of small concentrations of surfactants (Figure 9) apparently is related to a stabilization of adsorbed Co(0)RS and/or Co(0) RSH at the surface of the mercury. The presence of adsorbed compounds on the mercury surface can greatly increase the lifetime of freshly deposited cobalt, nickel, or iron atoms, because of the changed physical conditions at the (18) W. Kemula and Z. Galus, Bull. Acad. Pol. Sci.,Ser. Sci., Cliim., Geol., Geograph., 7,729 (1959). (19) B. K. Hovsepian and I. Shain, J. Electronrial. Chem., 12, 397 (1966).
metal-surfactant-mercury interface (20-22). Similar stabilization may occur when the Co(0)RS complex is deposited in the presence of adsorbed surfactant. An increased lifetime of the complex results in an increase in BrdiEka current, which actually is observed (Figure 9). At large concentrations of the surfactant, when the surface coverage by the adsorbed surfactant is close to complete, the BrdiEka current becomes suppressed, the surfactants then replacing the adsorbed complex from the surface of the electrode. At potentials where the surfactant is desorbed, the BrdiEka current decreases, to increase again to its normal maximum value of a potential near - 1.6 V in the absence of surfactant. Thus in the presence of small concentrations of surfactant two maxima in the BrdiEka current are observed, resembling the characteristics of BrdiEka currents in protein solution. These two maxima are not observed in the presence of gelatin which is not desorbed in the potential region where the BrdiEka currents are observed (Figure 10). RECEIVED for review June 26, 1970. Accepted September 4, 1970. This investigation was supported by Public Health Service Grant No. Ca-89723-03 from the National Cancer Institute. (20) V. F. Ivanov and 2. A. Ioffa, J. Plzys. Ckem. USSR, 38, 563 (1964). (21) Zbid., 36, 571 (1962). (22) G. N. Babkin, Izo. Vyssh. Ucheb. Zaaed., Kliim. Khim. Tekhnol., 7,90 (1964).
Corrections Improved Exponential Dilution Flask for Gas Chromat sgra phy In this article by L. J. Lorenz, R. A. Culp, and R. T. Dixon [ANAL.CHEM., 42,1119 (1970)l the height of the Teflon stirrer was in error and should be 40.8 mm. Stirrers much higher than 40.8 mrn are unstable and difficult to balance.
A Complexometric Titration for the Determinationof Sodium Ion In this article by James D. Carr and D. G. Swartzfager [ANAL. CHEM.,42, 1238 (1970)l there is an error in the caption for Figure 3, page 1240. It should read as follows: “Effect of varying potassium ion concentration on the titration curve at a constant pH of 13.00.” The value of pH 12.00 is incorrect and misleading when compared to the correct figure and caption of Figure 1.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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