ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

249

Polarographic behaviour of 2-dimethylaminoethanethiolhydrochloride and its oxidised product at the DME

The electrochemical behaviour of many organo-sulphur compounds com- p r i s i n g - S H , - S - S and, SO3H groups, has been studied in the last 2-3 decades a- 9. The catalytic effects of such compounds have also been investigated 1°- t3 but the reasons for the appearance of a catalytic hydrogen wave are still not fully under- stood 14. 2-Dimethylaminoethanethiol hydrochloride (RSH) provides two possible coordination sites, viz. - S H and N(CH3)2 groups, and shows a strong tendency to complex with metal ions. The dissociation constants of its - S H group in different aqueous and aqueou~nonaqueous media are not known and a literature survey reveals that the polarographic behaviour of RSH and its oxidised product also has not been studied so far.

Experimental Polarographic curves were recorded manually with a Cambridge (general

purpose) polarograph using a thermostatted polarographic H-cell coupled with the saturated calomel electrode. The capillary had the following characteristics; m = 1.427 mg s -1, t=3.62 s, and m~t ~= 1.571 mg~s -~ at 0 V (vs. SCE) in 0.1 M KNO3. Purified nitrogen was used for de-aeration of test solutions.

pH measurements were made with a Cambridge bench pattern (null deflec- tion type) pH-meter fitted with a wide range glass electrode and a calomel electrode.

o / Solutions of 2-dimethylaminoethanethiol hydrochloride (98/o, Evan's Cheme- tics, Inc., New York) and AnalaR grade B.D.H. reagents were prepared in doubly distilled air-free conductivity water. The thiol solution is sensitive towards air-oxida-

~ RSSR 0.8

0,8 0.6

/ O 5~1~ A q . - A c e t o n l t r i [e

0.8 0.6 0.4 0°/o Aq, - Ethclnot

ul 0,6 0,4 0°/o Aq . - M e t h a n o l

° t/ m 0.6 0.4 O.g~

0.4 O,2 ~ 1 ~ A q u e o u s >

o , o 6'.o ~'.o , ~ o ' 12.0

I 0.2 6 ; 1'o ;'2

0.2 ~ 4 6 ; ~'o 1~

o ; ; ,o ~? ._ pH >

Fig. 1. Dissociation constants of SH group in aqueous and nonaqueous media. (I) (IV) 1.0 mM RSH and 0.1 M KNO3, (V) 1.0 rnM RSH; in 0.1 M KNO 3 in Britton-Robinson buffer solns.

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250 SHORT COMMUNICATIONS

tion and hence a freshly prepared solution was always used. For every run, the test solution was prepared in the H-cell itself by mixing the individual de-aerated con- stituent solutions; the solution was then de-aerated again for 1(~15 min prior to the running of polarograms.

Resu l t s Anodic waves o f R S H . RSH produces a well-defined anodic wave in Britton

and Robinson buffer solutions ~5 (pH 2.55-11.90) and 0.1 M KNO3. In unbuffered medium and above pH 7.0, rapid decomposition of the compound is noted by a decrease in the diffusion current with respect to time. After 60-70 h the anodic wave disappears completely and a cathodic wave is formed. The E~ of the anodic wave changes linearly with pH up to a certain limit (pH corresponding to pK o f - S H ) and then assumes an almost constant value indicating the dissocation of the -S H group (Fig. 1). Subsequent investigations were carried out at pH 4.40 in order to avoid, as far as possible, the dissociation of the ligand.

The effect of Hg pressure (h~ff~ia) and concentration of depolarizer (ia~c) shows the diffusion controlled nature of the limiting current {Fig. 2, curves I and II). The linearity and slopes (~0.059 V) of the log plots suggest a reversible transfer of one electron at all pH values.

Control led potent ia l e lectrolysis . The solutions (2.5 ml), (a) 0.5 m M in RSH and (b) 1.0 mM in RSH were electrolysed using the dme (at -0 .05 V vs. SCE) and stationary platinum electrodes (at + 0.6 V), respectively. After 30~40~0 conversion had been achieved by electrolysis (which required about 3-4 h) the electrolysed solu- tions were polarographed and showed (c f ref. 22) (a) a composite wave of charac-

4,0 10 -~

: :

Fig. 2. (I) and (III) i a vs. concn, for RSH and RSSR, respectively, (II) and (IV) i a vs. h~r r for RSH and RSSR, respectively.

Fig. 3. Effect of HgC12 on anodic and cathodic waves. A. (I) 1.0 mM RSH in 0.1 M KNO3 at pH 4.40, (II) 1.0 mM RSH +0.25 mM HgC12 in 0.1 M KNOa at pH 4,40, (III) 1.0 mM RSH + 0.5 mM HgC12 in 0.1 M KNO 3 at pH 4,40. B. (I) 1.0 mM RSSR in 0.1 M KNO3, 0.001 ~,,; Triton X-100 (pH 6.64), (II) soln. (I) + 0.25 mM HgC12, (III) soln. (I)+0.50 mM HgCI 2, (IV) soln. (I)+ 1.00 mM HgCI 2.

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SHORT COMMUNICATIONS 251

teristics similar to the anodic wave (E~= -0.218 V and slope=0.063 V) and (b) a two-step polarogram, the first step having E~ and slope equal to those of the main anodic wave and the second a purely cathodic step with a more negative half-wave potential.

Effect o f H y C l 2 on the anodic wave. The solutions containing RSH and HgCI2 in the proportions, 1:0, 1:0.25, 1:0.5 were polarographed (Fig. 3A)and produced (a) an anodic wave, (b) an anodic-cathodic wave, i.e. composite wave and (c) a cathodic wave, respectively, of almost similar characteristics.

Cathodic wave o f RSSR. It was observed during investigations of the anodic waves of RSH that RSH is very sensitive towards oxidation by atmospheric and dis- solved air, a common difficulty associated with most thiols because of the low strength of the S-H bond (83 kcal*). Mild oxidising agents, e. 9. atmospheric oxygen, halogens, sulphuric acid, etc., oxidise thiols into disulphides by way of thioxy radicals, and in some cases through sulphenic acid derivatives. The unstable nature of RSH solutions and their susceptibility towards air oxidation led us to study their polarographic behaviour. In order to obtain RSSR, a RSH solution was oxidised by bubbling puri- fied air for 6-7 h at 35-40°C. The effect of variation in pH, concentration of RSSR and Hg pressure have been investigated and controlled potential electrolysis and mercuric chloride experiments have been carried out to elucidate the electrode reac- tion mechanism.

Preliminary experiments have shown (at pH 9.58) that the cathodic limiting current is diffusion controlled (heffC~id; Fig. 2, curve IV) and independent of RSSR concentration (/deC; Fig. 2, cur-ee III). With increase in pH, the E~_ of the cathodic wave shifts to more negative potentials (Fig. 1, curve V). At lower pH (3 5) more drawn out waves were observed and well defined waves were obtained in the pH range 6.0-10.75. The plots of log [i/(i d - i)] vs. Ed.e. are linear having variable slopes (0.07-0.125 V) indicating the irreversible nature of the cathodic wave; this is also substantiated by the variation of E~ with the concentration of RSSR.

The controlled potential electrolysis at the DME and mercuric chloride experiments were performed in a manner similar to that adopted for RSH. When the electrolysed solution (for 3-4 h) was polarographed, a cathodic wave with more po- sitive E~ and reduced height (approximately half of the original) was obtained. Logarithmic analysis of this wave gave a plot of slope 0.059 V corresponding to a reversible one-electron transfer process. It is worthwhile mentioning that the elec- trolysed solution did not produce an anodic wave as expected from the reduced product of RSSR.

The solutions composed of RSSR and HgC12 in molar ratios, 1:0, 1:0.25, 1:0.5 and 1:1 produced (Fig. 3B), (a) a cathodic wave, (b) a cathodic wave associated with a pre-wave, (c) a doublet with almost equal heights and (d) a cathodic wave (with a small post wave) of reduced height (approximately half of (a)). These curves show that the E~ values of the second wave of (b) and (c) coincide with the E~ of (a), and the E~ values of the first wave of (b) and (c) with E~ of (d).

Discussion Analysis o f the anodic wave. Kolthoff and his collaborators a6-18 have consid-

* 1 ca1-=4.184 J.

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252 SHORT COMMUNICATIONS

ered the following reaction for the electro-oxidation of the mercaptan:

2RSH ~ RSSR+2H + +2e (i)

2RSH + Hg ~ Hg2(RS)2 + 2H + + 2e (ii)

RSH + Hg ,~- RSHg + H + + e (iii)

They showed that a plot of Ea.~. vs. log [( ia- i)2/i~ (at 25°C) should yield a straight line with slope (0.059/2) V if reactions (i) and (ii) are involved. On the other hand if reaction (iii) is involved the plot of Ea.e. t,s. log [( ia- i)/i] should yield a straight line with a slope of 0.059 V. From Fig. 4 it is clear that only the analysis corresponding to to eqn. (iii) (curve I) yields a straight line and that this line has a slope of 0.06 V. Further, it was found that the half-wave potential remains constant when the RSH concentration is varied from 0.25 to 2.5 raM. This constancy of E~_ is characteristic of reaction (iii) but not of (i) and (ii). It can, therefore, be concluded that the anodic wave of RSH is due to the formation of mercurous mercaptide according to eqn. (iii). If this reaction is reversible then mercury in RSHg should be reversibly reducible at the DME. Experiments with RSH and mercuric chloride (Fig. 3A) indicate that the anodic wave height decreases to half of its original value when 0.25 mol of Hg 2 +/tool of thiol was added (curve II), and disappears totally in presence of 0.5 mol of Hg 2+ (curve III). A composite wave (curve II) showing no inflection at the galvanometer zero line in the first case and in the second case a purely cathodic wave (curve III) having identical characteristics, E~--0.215 V and slope 0.06 V, were obtained. These experiments show that the cathodic wave results fi'om the reduction of the (RS)zHg and corresponds to the reduction of univalent mercury.

The electrolysed solution of RSH produced a composite (anodic-cathodic) wave having again the same characteristics as shown by the anodic wave. On passing H2S gas, the electrolysed solution yielded a black turbidity showing the presence of soluble mercury. The absence of any inflection at the zero current line in the composite waves given by the electrolysed solution and solution (b) (RSH:HgCI2---1.0:0.25, Fig. 3A, curve II) and the identical characteristics of the anodic wave given by RSH (Fig. 3A, curve I) and the cathodic wave given by (RS)aHg (RSH :HgClz= 1.0:0.5, Fig. 3A, curve III) suggest that the anodic wave produced by RSH is reversible and that the ultimate oxidation product is (RS)zHg. The controlled potential electrolysis study indicates that the primary electrolysis product is quickly transferred into (e.s)2Hg.

When the solution electrolysed at platinum electrodes is polarographed at the DME a two-step polarographic wave is obtained. The anodic portion corresponds to the anodic wave and the cathodic to the reduction of the electro-oxidised product. The results of the electrolysis at the platinum electrode are quite different from the phenomenon observed at the DME and suggest that RSH is oxidised to RSSR at the platinum electrode. Thus, RSH is not oxidised to RSSR at the DME but depo- larizes the mercury with the formation of unstable RSHg which changes rapidly into (RS)2Hg. The wave due to (RS)zI-Ig corresponds to the reduction of univalent mer- cury; apparently the reaction

(RS)zHg+Hg ~ 2 RSHg (iv) is very rapid. This is in accordance with the findings of Kolthoff and co-workers (loc cit.). However, the disulphide is formed at the platinum electrode.

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SHORT COMMUNICATIONS 253

A n a l y s i s o f the c a t h o d i c wave . The cathodic wave given by the oxidised product of RSH has been found to be irreversible and diffusion controlled.

The reduction of the disulphide may proceed, (i) in one step involving a two- electron transfer or (ii) in two different steps each corresponding to a one-electron transfer reaction;

R S S R +2e ~ 2RS" or

(v)

RRSSR + e ~ RSSR ~ (vi) SRS- + e ~ 2RS" J (vii)

If the reduction takes place through reaction (V) then the plot of log [i2/(id - i)] vs. Ea.e. should yield a straight line of slope (0.059/2) V. However, this plot gave a curved line (Fig. 4, curve III); on the other hand, a plot of log [i /( i a - i)] was linear (Fig. 4, curve IV). Further, if reduction occurs in two different steps then two cathodic waves of different half-wave potentials should occur ; this has not been observed in the present case. It is plausible therefore, that the electron transfer process takes place in two coinciding steps. In the first step only one molecule of the electroactive sub- stance RSSR- is formed (eqn. (vi)) ; this is then immediately followed by another fast reaction (eqn. (vii)). The first step is of slower rate, and potential determining. How- ever, the total current is controlled by both steps.

Assuming that the reduction of RSSR is controlled by one rate determining step and neglecting the backward reaction, the heterogeneous rate constant for the forward reaction has been calculated from the relation given by Koutecky 2° as extended by Meites and Israel 21. The values are summarised in Table 1.

-0.23 ' ~ . 1 ~ (For RSH) o#

I 0 1 1 9

0 +0.2 4"0,4 -0.86 tog(ia-i)2/; ond Iog(ia-i)/i

(For RSSR) -0.8, " ~ ~ ] 2 E

-0,82-

-0'.8 -0'.4 0 +0.4 +0.8 log i2/(ia-i) and log i / ( id- i ) Fig. 4. (I) Plot of log [(ia-Off] vs. Ed.e. for 1.0 mM RSH (pH 4.4), (II) plot of log [(id--i)2/i] VS. Ea.e. for 1.0 mM RSH (pH 4.4), (III) plot of log [i/(i d - i ) ] vs. Ed.e. for 1.0 mM RSSR (pH 9.58), (IV)plot of log [i2/(ia- i)] vs. Ed.e. for 1.0 mM RSSR (pH 9.58).

TABLE 1 KINETIC PARAMETERS AND DIFFUSION COEFFICIENTS (AT 26°C)

pH - E J V vs. 106 D°/em 2 s-1 k°fh./cm s-1 an SCE

7.88 0.702 2.56 5.57 × 10-1 x 0.844 8.85 0.780 1.87 1.06 × 10 xz 0.803

10.74 0.885 1.68 0.95 × 10-13 0.840 11.27 0.913 1.48 1.12:,< 10 -1'* 0.804

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254. S H O R T C O M M U N I C A T I O N S

The controlled potential electrolysis and mercuric chloride experiments in this case provide interesting information regarding the nature of the ultimate reduction product. On polarographic analysis the electrolysed solution produced a one-electron reduction wave of reduced height and having a half-wave potential different from that of the main cathodic wave. This new wave was found to correspond practically with the cathodic wave given by (RS)2Hg. The c - v curves of RSSR in the presence of different concentrations of HgC12 indicate that as the concentration of HgC12 in- creases, another cathodic wave begins to appear (Fig. 3 B). When HgC12 is present in the molar ratio RSSR:HgC12 = 1:1, the main cathodic wave disappears almost completely and a new wave of different characteristics is formed (Fig. 3B, curve IV), whose height is practically half that of the main cathodic wave. This unique obser- vation in both cases, i.e., controlled potential electrolysis and mercuric chloride ex- periments, shows that the current governed by two steps, suggested for the reduction of disulphide, is now governed by only one step corresponding to a one-electron reduction process. Further, it is also indicated that the reduced product obtained from the controlled potential electrolysis and the compound formed by the interaction of RSSR and HgC12 are the same.

Thus, the thioxy radicals (2RS') produced as a result of the reduction of RSSR do not form RSH but change immediately into a mercuric complex (RS)2Hg, which on reduction gives a cathodic wave corresponding to a one-electron reduction, i.e., due to the reduction of RSHg as suggested previously in the case of thiol (RSH) (eqn. (v)).

Thioamine and diphenyl disulphide produce cathodic waves at more positive potentials. The pre-wave does not appear in the present case as reported for dithioldi- glycollic acid. The reduction mechanism proposed is slightly different and the ultimate reduction product is (RS)zHg and not RSH as reported by earlier workers.

Dissociat ion constants o f the sulphydryl group. It is apparent from the preceding pH study that the E~ of the anodic wave of RSH is pH dependent (Fig. 1). Using the expressions developed by Stricks and Kolthoff xg, the dissociation constant (pK) of the -SH group has been evaluated in different media, viz. aqueous, 50 o/~ aqueous methanol, 50°~ aqueous-ethanol and 25~ aqueous-acetonitrile. The results are given in Table 2 together with the diffusion coefficients.

TABLE 2

DISSOCIATION CONSTANTS OF - S H GROUP AND DIFFUSION COEFFICIENTS (AT 26°C)

Media pK 106 D°/cm z s- 1 pH at which D O has been evaluated

Aqueous 10.25 2.62 4.40 Aqueous-50% methanol 10.20 2.89 6.48 Aqueous-50~/o ethanol 9.80 0.74 5.42 Aqueous-25~ acetonitrile 9.70 2.89 7.18 Aqueous (determined from RSSR) 9.70 1.87 8.85

A c k n o w l e d g e m e n t The authors wish to express their thanks to Professors R. M. Advani, and

R. S. Saxena for providing research facilities. We are also indebted to the Ministry

J. Electroanal. Chem., 36 (1972)

SHORT COMMUNICATIONS 255

of Education, Government of India for the award of scholarship to one of us (A.V.P.), and Evan's Chemetics, Inc., New York for the gift of thiols.

Department of Chemistry, Malav(va Engineerin9 College, Jaipur (India)

M. L. Mittal A. V. Pandey

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10 V. F. TOROPORA, L. A. ANISIMOVA, D. I. BRAIMA AND L. A. PAVILICHENKO, Zh. Obshch. Khim., 38 0968) 1360.

11 M. BREZINA, Abh. Akad. Wiss. Ki. Chem. Geol. Biol., (1964) 198. 12 M. BREZINA AND V. GULTJAJ, Collect. Czech. Chem. Commun., 28 (1963) 181. 13 I. M. KOLTHOFE AND P. MADER, Anal. Chem., 41 (1969) 924. 14 S. G. MAIRANOVSKIL J. Electroanal. Chem., 6 0963) 77. 15 M. BREZINA AND P. ZUMAN, Polarography h7 Medicine, Biochemistry and Pharmacy, Interscience,

New York, 1958. 16 I. M. KOLTHOFF AND C. J. BARNUM, J. Amer. Chem. Soc., 62 (1940) 3061. 17 I. M. KOLTHOFF AND C. J. BARNUM, J. Amer. Chem. Soc., 63 (1941) 520. 18 D. L. LEUSSING AND I. M. KOLTHOFF, J. Electrochem. Sot., 100 (1953) 344. 19 W. STRICKS AND I. M. KOLTnOFF, J. Amer. Chem. Sot., 74 (1952) 4646. 20 J. KOUTECKV, Collect. Czech. Chem. Commun., 18 (1953) 597. 21 L. MEITES AND ISRAEL, J. Amer. Chem. Soc., 83 (1961) 4903. 22 J. TACHI AND S. KOIDE, Proceedings 1st International Congress on Polaroyraphy, 1951.

Received 9th August 1971 ; in revised form 28th September 1971

J. Electroanal. Chem., 36 (1972)