The reduction of nickel dimethylglyoxime complex preadsorbed on mercury

i ELSEVIER

Journal of Electroanalytical Chemistry 431 (1997) 171-181

The reduction of nickel dimethylglyoxime complex preadsorbed on mercury Silvana Ramirez a, Gabriel J. Gordillo a,,, Dionisio Posadas b a DQIAQF, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellon I!, 1428 Buenos Aires, Argentina b INIFTA, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Sucursal 4, Casilla de Correo 16, 1900 La Plata. Argentina

Received 25 October 1996; revised 26 March 1997

Abstract

The voltammetric reduction of NiU(DMGH)2 complex preadsorbed on mercury both in 1 M NH4CI + NH 3 buffer and in 0.1 M borate buffer solutions was studied. In this work experimental evidence points to Nin(DMGH)2 reduction with simultaneousdecomposition of the ligand DMGH- and reduction of Ni(II) to Ni(O). The experimental sweep rate dependence of peak current and potential is explained in terms of a ErC i surface reaction scheme and a rate expression that takes into account attractive forces between the adsorbed reactant complex. © 1997 Elsevier Science S.A. Keywords: Voltammetric reduction; Nickel dimethylglyoxime complex; Peak current

I. Introduction In recent years differential pulse polarography and adsorptive stripping voltammetry methods for the determination of cobalt and nickel, in the form of their dimethylglyoxime complexes, have been developed and applied to numerous water and biological samples, plating baths, and other materials [1-4]. Although the electrochemical reduction of Ni(II) and Co(H) dimethylglyoxime complexes at mercury electrodes has been widely investigated [ 1,5-9], up to now there is no agreement about the explanation of the nature of the reduction process [8-10]. The first observations of a polarographic wave due to Co(II) in the presence of dimethylglyoxime (DMGH 2) in ammonia buffer solution were made by Stromberg and Zelyanskaya I11]. These authors, taking into account experimental evidence for bubble formation, explained their results by the occurrence of catalytic hydrogen discharge. Vinogradova and Prokhorova [5-7] studied the polarographic behaviour of Ni and Co dimethylglyoximates in a 0.1 M ammonia buffer solution. They observed two cur rent peaks at -1.05 V and - 1 . i 7 V/SCE for each complex (for metal concentrations greater than 2 - l 0 -s

* Corresponding author. E-mail: [email protected] 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 7 ) 0 0 1 8 3 - 6

M), with no adsorption pre-concentration, on a stationary mercury drop electrode. These authors proposed that the observed peaks are catalytic hydrogen waves, acting the complexes as catalyzers, and the following reaction scheme: Me(DMGH)2 4, NH~- ~ Me(DMGH)2 H+ 4- NH 3

(1)

where Me represents either Ni(ll) or Co0I). When an electron is added, an uncharged particle is formed: Me(DMG8)28+ 4. e- ~ Me(DMGH)2H

(2)

which can react bimolecularly: 2Me(DMGH)2H ~ 2Me(DMGH)2 + H2

(3)

In this way the catalyst is regenerated in its basic form, a molecule of hydrogen is released and the cycle is repeated. Niirnberg et al. [1,8] made detailed electrochemical studies of the Nin(DMGH)2 complex in 0.1 M ammonia buffer solution applying different techniques. Strong adsorption of reactant was detected when Ni(II) was added to an ammoniacal solution containing DMGH~, neutral bis(dimethylglyoximate)-nickel(II), Ni~(DMGH)2 being the adsorbed species. The authors observed that adsorption takes place at potentials between 0.05 and - 0 . 9 V (Ag[AgCI), being maximum between - 0.2 and - 0.8 V. Considering that the peak potential shifts linearly with pH with a slope of -0.037 V / p H in the range 7.5 to 9.5,

S. Ramlrez et al. / Journal of Electroanalytical Chemistry 431 (1997) 171-181

172

Umland et al. [9] reported DMGH 2 as being polarographically active, both in acidic and alkaline media, DMGH~ and DMGH- being the reactants, respectively. The authors assigned this behaviour to a conformational orientation of the molecule, that can also be achieved in the presence of Ni(lI) or Co(lI). In a previous work we studied the adsorption equilibrium and kinetics of the Ni(DMGH) 2 complex on mercury in both ammonium and borate buffers [15]. Adsorption voltammetry and differential electrode capacitance measurements were made as a function of time, adsorption potential and complex concentration. The adsorption equilibrium proposed a Fmmkin adso~tion isotherm with an attractive interaction parameter. The adsorption kinetics, for short adsorption times, was described by a general expression like:

which confirms the influence of proton concentration on reaction rate, they proposed the following reaction mechanism:

(4) (5)

Nin(DMGH)2 + Hg ~ Nin(DMGH)2 ads NiU(DMGH)2 ads "4- e -

~

Nit(DMGH'~ ;ads

NII(DMGH)2ads+ H + + e (6)

Ni°(Hg) + DMGH 2 + DMGH-

the last one being the rate-determining step. Due to the reduction of the central ion, the complex is destroyed and the iigands desorbed. The rate expression proposed by the authors is: ! = KCH÷C.i(DMG.) , exp[

-- ( l +/3)

FER-'T-

']

(7)

It was assumed that reaction (5) is at equilibrium and that Langmuir conditions apply. K is "an overall constant for all partial steps including a term 2e- per Ni atom reduced, is the symmetry coefficient" [8] for a le- transfer. The terms F, E, R and T have the usual meanings. Not many studies about the reduction of vic-dioximes, particularly DMGH 2, have been made [12,13]. Vic-dioximes of aliphatic, aryl aliphatic or alicyclic diketones are generally reduced to the corresponding amines in acidic media, either via a single eight electron step or via a six electron intermediate, the acidic species being the electroactive one. Spritzer and Meites [14] studied the reduction of DMGH 2 at pH values between 1 and 3 in citric + citrate buffer solution. These acidic solutions lead to well defined polarographic waves with half wave potentials given by the expression:

E:/2 =

-0.718 - 0.096pH

where F represents the surface concentration of the adsorbed complex; c the complex concentration in solution and t the time. K is a constant that includes potential dependence. The present work is aimed at clarifying the mechanism of Ni(DMGH) 2 reduction at mercury electrodes.

2. Experimental A stock buffer solution of 4 M NHaCI + NH 3 was prepared from both concentrated HCI ( p = 1.18 g c m -3, 37% w / w ) and NH 3 (p = 0.91 g c m -3, 25% w/w). Ni(II) standard solutions were prepared from Ni(NO3)2.6H20, with addition of concentrated HNO 3 up to pH 5, while 0.1 M alcoholic DMGH 2 solutions were obtained dissolving

(8)

leading to 2,3-diaminebutane as reduction product [14]. " '" o

I

'

(9)

r=K.

I

'

.

I

'

-

.

/

~:

¢

-1.o -0.8

-1.OO

ol L I

-OA

-1.50

' -1.1

' .~L]/

I -1.0

SURF.PREP i

i

ADS,

DE

S , Tlld;i rain 1 -O.g

-0.8

ELECTRODE POTENTIAL / V Fig. 1. I / E response to a negative potential scan for different adsorption times, a = 0 s; b = 50 s; c = 60 s. Adsorption potential: - 0 . 5 V. 9 . 9 . 1 0 -6 M Ni(II), 1 • 10 -4 M DMGH 2 in 1 M ammonia + ammonium chloride buffer solution, v = 0.100 V s - i. Inset: Potential program for conditioning electrode.

S. Ramlrez et al./ Journal of Electroanalyticai Chemistry 431 (1997) 171-181

173

-1.00

.c

-2.00

I.-

z

LU

-3.00 0

-4.00

II

-5.00

I -1.5

i

I

I

-1.3

-1.1

-0.9

I

I

I

-0.7

-0.5

-0.3

ELECTRODE POTENTIAL / V

Fig. 2. I / E response to a negative potential scan, showing peaks I, II, III and IV, for the same solution as described in Fig. 1. Adsorption potential: - 0 . 8 V. Adsorption time: 900 s. 1 and 2 indicate the cycle number and the arrows indicate scan direction. : , = 0.100 V - s - i .

Solutions were de-aerated with nitrogen, previously passed through the buffer solution. Gas flow was maintained over the solution to prevent oxygen reabsorption. Voltamperometric and capacitance measurements where made in a thermostatted cell (25.0 ± 0.2°C) with three electrodes. A Beckman 39210 mercury electrode was used as working electrode; a SCE and platinum gauze were used as reference and counter electrodes, respectlvely. Mercury was purified according to [16]. Differential electrode capacitance measurement in 1 M KCI aqueous solution was employed to test the purity of the mercury. Before starting an experiment, the electrode was condi-

appropriate amounts of the solid in absolute ethanol. All chemicals were p.a. grade (Merck or similar) and water was obtained from a Milli-Q Water System. Working solutions were prepared daily, by dilution of appropriate amounts of the stock solutions mentioned above, to the following final concentrations: 1 M NH4CI + N H 3, l • l0 -4 M DMGH 2 and Ni(II) between l • l0 -s and l • l0 -s M. Higher Ni(II) concentrations could not be used due to precipitation of the complex. The solution pH was 9.30 ± 0.02. Although most of the experiments were carded out in ammoniacal media, some of them were done in 0. l M borate buffer solution, pH 9.30. I

I

I

I

I

I

15 b

.(

10

I.-

z uJ Gg m

o x ,I[ uJ o.

8 5

"

0

-

I

I

I

I

I

I

0

0.05

0.t0

0.15

0.20

0.25

SCAN RATE / Vs"1

Fig. 3. Dependence of peak I current on scan rate, for constant adsorption time and potential ( - 0 . 8 V). a = 5 . 1 0 -6 M Ni(ll) and 1 • l0 -4 ~,| DMGH2 in 1 M ammonia + ammonium chloride buffer solution, b = 3.1 • 10 -6 M Ni(II) and 1 • 10 -4 M DMGH 2 in 0.01 M borate buffer solution.

174

S. Ram[fez et al. / Journal of Electroanalytical Chemistry 431 (1997) 171-181

tioned by applying the potential sequence shown in the inset of Fig. 1. Since the complex becomes desorbed approximately at - 1.0 V, the first potential cycles work as cleaning process. The potential is then switched to the adsorption potential, E,, a. After holding potential at E,,a for a certain time, tad, the potential is swept it, ,he negative ~ t i o n . Electrode potential programs were applied with a PAR 273 system. Programmed bulk electrolysis experiments were performed in a cell provided with an isolated compartment for the counter elect_rode to avoid interference of products, and a mercury pool as the working electrode. The Ni(DMGH) 2 complex was adsorbed at - 0 . 8 V for 60 s, and then potential was swept between - 0 . 8 V and - 1.15 V. These two steps were repeated during 8 h. After the electrolysis, total Ni(II) concentrations in solution were measured by AAS in a VARIAN AA5 spectrometer. The equipment is provided with an air-acetylene burner, and measurements of Ni(II) were performed at A = 232.0 nm. Organic compounds were detected by G C - M S analysis. The equipment used was a TRIO-2 VG MASSLAB (Fisons), with a SPB-1 column. Helium was used as carrier gas and the mass scan was performed between 30 and 700.

Table 1 Kinetic parameters for the NiU(DMGH) 2 reduction in different buffer solutions

IpVS. t' Ep vs. log t,

2.

3.

4.

3. R ~

5. The main characteristics of the voltammetric response of the reduction of Ni(DMGH) 2 in ammonium buffer for different adsorption times, sweep rates, adsorption potential and concentration of the complex in solution are as follows (Figs. 1 and 2): 1. After adsorbing at potentials between - 0 . 1 5 V and - 0 . 8 V a current peak, Ipl, is detected at approxi-

6.

1 M ammonia buffer

0.1 M borate butler

2 7 C V -t m -2 - 0.069 V

7 . 1 C V - I m -2 - 0.060 V

mately - 1.0 V. Peak charge and peak potential depend on adsorption time, t~d, adsorption potential, Ead, and complex concentration in solution, c; The peak has an asymmetric shape and there is no anodic peak related to peak I. This is characteristic of an irreversible reduction process; The dependence of peak current and peak potential on sweep rate, v, in the presence of Ni(DMGH) 2 was analyzed at constant tad and Ead in both ammonia and borate buffer. Linear relationships between peak current versus v and peak potential versus log v are observed (Figs. 3 and 4). The corresponding parameters are shown in Table !; For the highest Ni(II) concentrations, a shoulder was detected at potentials more negative than the one for peak I. As tad increases it turns into a well defined peak, peak II (Fig. 2), for tad greater than 10 min. Peak II charge depends on tad and Ead. In Fig. 5 the dependence of peak current on adsorption time is shown; At long adsorption times and scan rates greater than 0.2 V s -~, a redox couple giving rise to peaks I I I / I V can be detected at about - 0 . 5 V (Fig. 2). This pair is generated by peak I, and can be also detected in presence of peak II; In 0.1 M borate buffer only peak I was detected. The couple III/IV is also obtained under the same conditions as in ammonium buffer;

o

-1.02

8

-1.04

-1.06

-1.08

-1.10 I

I

I

-1.5

-1,0

-0.5

log (vNs -1) Fig. 4. Dependence of peak I potential on logarithm of scan rate for the same conditions described in Fig. 3.

S. Ramlrez et ai. / Journal of Electroanalytical Chemistry 431 ( i 997) 171-181

175

4.00 8

0,,"

3.20 ? E <[ I-

" ............

6

i 2.40

Z

j,r

Iz

j,O'

1.60

1; I

w L

I I

0.80

i j

.if~ ,

I

I

,

0

120

I

I

240

360

,

I

I

480

600

T I M E I seconds

Fig. 5. Dependence of peak current on adsorption time, for constant adsorption potential (-0.8 V). a = peak I current; b = peak II current for a solution containing 9.9.10 -° M Ni(II) and i • 1 0 - 4 M DMGH 2 in l M ammonia+ ammonium chloride buffer solution. Scan rate: 0.025 V s- I.

7. Scan rates were increased up to 100 V s -~. No other response was obtained during the reverse scala under these conditions. Voltammetric experiments in the presence of a saturated solution of dimethylglyoxime (approx. 10 -3 M) in l M ammonia buffer, pH 9.30, showed that dimethylglyoxime reduction gives a pre-peak, at a similar potential as the Ni(DMGH) 2 complex, followed by a cathodic faradaic current that on the next positive and negative scans generates a couple III/IV (Fig. 6). Plots of the pre-peak potential against log v lead to a slope of - 0 . 0 2 5 V. Programmed electrolysis experiments were carded out

,

I

,

-

I ~'

in solutions of Ni(DMGH) 2, in 0.1 M borate buffer, of the following concentrations: 4.1 • l 0 -5 M Ni(II), 4 . 4 . l0 -3 M D M G H 2, pH = 9.3. "Ilais solution had the typical red colour of the Ni(DMGH) 2 complex. As electrolysis proceeded, the solution lost its red colour, this fact being clear evidence of a lower concentration of the complex in the solution. At this point, the reasons for this behaviour could be one or a combination of the following: ( l ) a decrease in the pH of the solution, modifying the equilibriums in solution; (2) reduction of Ni(II) of the complex to Ni(0,); (3) decomposition of ~he ligand ( D M G H - ) of the complex. Since the pH had not changed, electrochemical re-

"

'

I

o

E

i

|

I

III

I

~

|

-

-'1.00

<[

~

-2.00

-~.00 - 1,2

- 1.0

-0.8

-0,6

-0.4

ELECTRODEPOTENTIALI V Fig. 6. Voltammogramsof DMGH: saturatedsolution in 1 M ammonia + ammoniumchloride buffer solution.

S. Ramlrez et al. / Journal of Eiectroanalytical Chemistry 431 (1997) 171-181

176

I

I

I

100 88

73

8O apt i-. =E ii,i

,-" (.1 _q

60

_o

4O

53

Z

.J q[ I-

57

o 1-

'ri, ~

41

2O

,.11:s

,6o

s.all I I I

101

"1

9

97

I

30

/02

116 115

I

80

"

iI

, 117 _

,

90

I

120

m/e Fig. 7. Mass spectra of peak at rt = 7.90 s.

duction had lead to a consumption of the complex, so options (2) and (3) were explored. In order to detect the products of electrolysis in the presence of the complex, the resulting solutions were analyzed for Ni(II) and organic compounds. Ni(II) concentration was lower ( 7 . 7 - l 0 -6 M) than in the original solution. Gas chromatography analysis of the resulting organic products shows one defined peak at a retention time (rt) of 7.9 s and several small peaks at approximately rt = 3.4-3.5 s. Mass spectra (Fig. 7) show that the peak at 7.9 s corresponds to dimethylglyoxime. However, mass

spectra for the low rt peaks (Fig. 8) cannot be assigned to only one compound. The greatest M + / e value is 86; even though this number is in agreement with a compound like 2,3-butanedione, peaks observed between M * - 2 and M +14 cannot be considered as mass losses produced by M ÷= 86. It is to be concluded that this peak corresponds to a mixture of substances that could not be solved in the chromatogram, and were not present in the solutic~ before electrolysis. AAS and GC-MS analysis indicate that both Ni(II) and the ligand DMGH- present in the complex are reduced at

I

I

I

100 ~5

44

8O IA

114

.~E

so

u Z

_o ._1

43

40

I-

o

I,-

.

]i]iiils[ 39

48

I0/-

49

20

I

3O

I

I

50

7O

i

role Fig. 8. Mass spectra of peak at rt = 3.47 s.

I go

S. Ram[rez et al. / Journal of Electroanalytical Chemistry 431 (1997) 171-181

the mercury electrode in borate buffer at E < - 1.1 V. No gas evolution was observed either in the voltammetric or in the programmed electrolysis experiments.

4. Discussion

4.1. The reaction As discussed in the Section 1, one of the possible reaction mechanisms has been formulated through the electrocatalytic effect of the adsorbed complex. However, capacitance measurements show that mercury is free from adsorbed compounds when the potential is more negative than the voltammetric peak potential. This suggests that the reactant is either transformed through the reaction or irreversibly desorbed. This experimental evidence is not in agreement with an electrocatalytic mechanism. Another explanation of the nature of the reduction process [8] is based on the reduction of Ni(ll) of the complex to Ni(0). As pointed out in [ 10], the large voltammetric charge observed during the reduction of the complex cannot be explained by a two-electron process alone. In a previous work [15] it has been established that Ni(DMGH) 2 in ammonium buffer is the only adsorbed species in the potential range between - 0 . 2 and - 0 . 9 V (free DMGH 2 is not significantly adsorbed on mercury at the concentration (1 • 10 -4 M) employed in presence of Ni(lI)). The voltammetric experiments with DMGH 2 alone ( 5 . 1 0 -3 M) show a couple as consequence of DMGH 2 reduction (Fig. 6). Since this pair of peaks, III/IV, is also observed during the reduction of the complex (Fig. 2), it can be concluded that the ligand DMGH-, belonging to the adsorbed complex, is being reduced. The concentration of free DMGH z in solution is too low to give any electrochemical response in that potential range. The results of programmed electrolysis experiments indicate as well that both Ni(ll) and the ligand DMGH-, both belonging to the complex, are reduced, the ligand reduction being an important difference with the mechanism proposed in [8]. On the basis of the geometric structure of the complex, the charge corresponding to the maximum coverage can be determined if the number of electrons exchanged is known. The complex molecule can be considered as a prism of sides with dimensions a = 1.668 nm, b = 1.044 nm and c = 0.649 nm [17]. According to these molecular dimensions the minimum surface areas result as: plane ab = 8.70 • 10 -19 m 2, plane a c = 5 . 4 . 10 -19 m 2 and plane bc = 5.39-10 -19 m 2. If the maximum saturation theoretical charge is estimated from the crystallographic data given above, a value of only 1.0 C m -2 can be expected for a 2-electron process. A consistent mechanism for the reduction of the complex must account for the saturation adsorption charge. In previous work [ 15,18] a saturation adsorption charge, S, of

177

about 5.0 C m-2 was reported. If adsorption is supposed to occur by plane bc [9] then the number of electrons transferred would be about 10. On the basis of the results of the reduction of vic-dioximes mentioned in Section 1 [14], it is expected that the number of electrons exchanged should be eight to obtain the corresponding amine. In our case amines were not detected by analysis• The simultaneous reduction of both dimethylglyoxime molecules of the complex to 2,3-butanedione and the Ni(II) would give a total number of 10 electrons exchanged. This is in agreement with the best number necessary to explain the maximum charge mentioned above. This argument shows that a large number of electrons can be exchanged if the ligand belonging to the complex is reduced simultaneously with Ni(II). 4.2. The reaction kinetics and mechanism The current, I, for the irreversible reduction of an adsorbed substance, in the case of a single or multi-stcp reaction, when interactions are not taken into account, may be given by the following expression [19,20]: l=n.F.a.k.F

exp(-ag-f.E)

(10)

where ag is the global transfer coefficient, f ~ F / R T , ,4, the area and E the electrode potential. Since the current is also given by: dF dF 1= nFA dt = nFAVdE

(ll)

a linear dependence between peak potential and the natural logarithm of the scan rate, v, can be predicted [20]: 1

. . . .

in ~. + const

(12)

"f

From the plots of Fig. 4, a value of 1 was obtained for ag. Although different mechanisms have been analyzed, this figure can be explained by a surface ErC i mechanism [21,22], as will be shown later. However, any chemical equilibrium before the rds (i.e., surface ErCrCi or surface CrErC i) will lead to the same figure for ag [21]. A possible course of the reaction can be described in the following scheme: eN i Z Z (DNGH)2

,--,

Ni z ( D N G H ) 2 -

(at) (,,) Tt

(a)

(o) Tt H+

s+

NiII(DHGH)2H+

n*.rn, I P ---') t,-JJ • F P + Ni (0)

e(C)

NiI(DHGH)2H

x-da (a-l) e" (C) ± I P . . . . FP + N i ( O )

(13) where the intermediate products (IP) give Ni(O) and final products (FP) of DMGH- reduction, contributing to the high reduction charge experimentally observed. We have

178

S. Ramfrez et al. / Journal of Electroanalyti cal Chemistry 431 (1997) 171-181

included protons in the mechanism because of the experimental evidence reported in [8]. At pH 9, the, reactant is expected to be the neutral form of the adsorbed complex, therefore there are three possible pathways: BCc, ADo and Aa. However, no predictions on the pathway can be made since it would depend essentially on the values of the surface dissociation constants of reactions B and D [23] (K,, I and K,, 2, respectively). In fact, the square scheme is equivalent to a simple lereaction with a surface apparent rate constant given by [23]: ks,A +

ks,c "CH+' ( Kal" ga2) -!/2

ks'aPP-- [(1 + CH+' Ka,' ) • (1 + CH+"K:'2')] '/2

(14)

where ks,A and k~.c are the surface rate constants of the electrochemical reactions A and C. If the equilibrium B is shifted completely towards the Ni(DMGH) 2 form and the pathway were BCc, kinetic currents would be expected. No experimental evidence has been found to sustain this assumption. Therefore we will not take into account the possible mechanism BCc. Steps A and C could not be the rds since in this case fractional ag values would be expected. Either pathways BCc or ADc (assuming that D is at equilibrium) or Aa, with a or c as rds, lead to C~g= 1 [21]. Both are first-order reactions with respect to H + and, from the experimental evidence collected so far, they are mechanistically indistinguishable. The course of the reaction after the rds cannot be established from mechanistic considerations alone. The irreversible characteristics of the electrochemical response are reflected by the absence of the corresponding cathodic peak, the peak asymmetry and the linear dependence of the peak potential on log v [23,24] (Figs. 3 and 4). Also, at constant v, dependencies of Ep, half peak width, AEp/2 and the ratio IpQp t, on initial coverage, 00, can be observed. As will be shown, these effects can be explained as being due to interactions between adsorbed molecules [24,25]. Since pathways ADc and Aa are indistinguishable we will only analyze Aa. According to the reaction scheme in Eq. (13), the expression for the current can be written as: dO n /-- SAp~ dE

complex and Ni ~ complex in the adsorbed state, respectively. Then: y = Yn = e x p [ ( r l z - r22)On] = exp(w0 n + zOi) Yl exp[( r i , - r2,) 0,]

(17)

where rij is an interaction parameter [25,26]. A further condition is, from the mass balance: d(0iI + 0x) dt = - k , c ~ + 0~

(18)

where c a ÷ is the proton concentration and k a the chemical rate constant of reaction a. The problem is to solve Eqs. (15)-(18) for 0 n. This allows I to be calculated as a function of E and then Ip and Ep to be obtained. This problem cannot be solved analytically unless some simplifying assumptions are made. Laviron [22] has shown that, for a similar reaction scheme and neglecting interaction between adsorbed species, if the sweep rate is such that k a >> v. f then Ep depends linearly on the logarithm of v. This is equivalent to considering that the E °~, of the step A, is much more negative than Ep. Since we observe this sweep rate dependence in our experiments (Fig. 4), we will assume this condition holds in this case. Then, considering that equilibrium A is shifted to the reactants side, an analytical solution can be obtained (see Appendix A, Eq. (A10)):

kaSA " XAYO n

(19)

1 = l + V. X ( l - wO,t)

From the usual considerations Ip is obtained as (see Appendix A, Eq. (AI6)): J'OSAOp v

f A Qp p

,lp = -- 1 -- WOp

1 -- w Q p S - '

(20)

where:

Qp'-S0p----f,

Ip-ldE

/rp

(21)

Equalling Eqs. (19) and (20), Ep results as:

Ep - E 0' = "--f- ! [ln P+ W0p --ln(l--14,'0p) --ln(kgH÷f - 1)]

(15)

(22)

where S - nF/~sat and 0 n =/"Ni//complex/'=satl. Considering lateral interactions between adsorbed complexes and assuming step A is at equilibrium, the coverage for Ni I complex is:

An expression identical to Eq. (19) would be obtained from the analysis of the pathway ADc (surface ErCrC i mechanism). Fitting Eq. (19) to the voltammetric data reproduces the shape of the voltammogram, giving w S - l = 0.435 C -t m 2. Considering the value of S given above, then a value of 2.2 is obtained for w. For w = 0, from Eq. (20), the ratio Ip A-l Qff i v- l should be 38.8 V -~. This condition is fulfilled for small 0 o, where interactions are not important. The value ob-

-- o iYx

(16)

where Fat is supposed to have the same value for the two nickel complexes, X = e x p [ - f . (E - E°')] and Y = ~!I/'~¢I" '~¢11and "~l arc the activity coefficients of the Ni n

S. Ramlrez et aL / Journal of Electroanalytical Chemistry 431 (1997) 171-181 I

I

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I

179

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-1.00

=>

-1.02

_<

pzI11 -1.04

Eadz/-o.V4.&-°'3 '~~.O ,,

0 I1. ~,< -1.06 .1

-0.5

0

-1.08

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•

-0.6

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-0.7

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CHARGEICm

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"=

Fig. 9. Dependence of peak potential on initial coverage. (a) 9 . 9 . 1 0 -6 M Ni(II); (b) 3.1 • 10 -6 M Ni(II). In both cases solution is 1 • 10 -4 M DMGH 2 in 1 M ammonia + ammonium chloride buffer solution. Scan rate: 0.200 V . s - t. Adsorption potentials are indicated in each figure.

tained for Ip A- l Q- i o- ~ is 38.2 V- i at 0 o = 0.04, whereas it is 44.8 _P~ at 00 = 0.12. This shows that the increase in peak current as 0 o increases is due to lateral interactions, as predicted by Eq. (20). Eq. (22) predicts a linear dependence of Ep on log v, at constant initial coverage, as is found experimentally (Fig.

4). At constant v, 0_ is proportional to the initial coverage, then, from Eq. (22~, plots of Ep versus Q at constant 0 ° should give a unique curve independent of the adsorption potential and complex concentration in solution. This behaviour is shown in Fig. 9. The ratio w S-= can also be obtained from the product of the slopes of Ep versus log v and Ip A-IQ~ 1 versus v

(Eqs. (19) and (22)). In the case of 1 M NH4CI/NH 3 buffer solution, w S-! was found to be 0.431, which is very close to the value 0.435 obtained above from the fitting of the experimental voltammogram to Eq. (19). Experimental tendencies are explained under the above considerations (i.e., lateral interaction between adsorbed molecules and decomposition of the ligand accounting for the large reduction charge).

5. Conclusions Experimental evidence allows the conclusion mat during Nin(DMGH) 2 reduction both Ni(II) and DMGH-,

s. Ramlrezet al./ Journalof ElectroanalyticaiChemistry431 (1997) i 71-18!

180

simultaneously belonging to the adsorbed complex, undergo electrochemical reduction. We propose that the central cation promotes the reduction of the ligand of the complex. Then the number of electrons involved must be greater than 2. In order to account for the experimental saturation charge value, as well as for the one obtained from the fitting of the voltammetric data, a value of n = 10 should be considered. No experimental evidence has been found to support an electroeatalytic mechanism. Opposite to the surface ErE i mechanism proposed by previous authors [8], a surface ErC i (or surface ErCrC i) sequence is proposed for the first two steps of the reaction. The resulting expressions for the dependencies of Ip and Ep, considering attractive forces between adsorbed molecules, are consistent with the experimental results.

Acknowledgements We thank the Consejo Nacional de lnvestigaciones Cientfficas y Tecnoltgicas (CONICET) and the Universidad de Buenos Aires (UBA) for financial support. We also thank the reviewer for the valuable comments.

Considering the derivative with respect to X: dY k,, (1 + YX) d[dxOii] + OuY+ Oil X~XX = _~OnY vj

(A6)

and rearranging the former expression we obtain: (1 + YX)

d[ o,,]

dr

+Onx'77"=-(A+l)OnY ux

dx

(g7)

being A = - k a / ( V ' f ) . For an ideal Ere i surface reaction it has been shown [22] that a linear dependence between .Ep and In v is found if Ep is much more negative than Em. This is equivalent to saying that ka > > v . f or A>>l [22]. Under these conditions 0 ! = On. X and near EpOt << 0 n (i.e. if E E °' > 0.1 then 01/0 n << 0.02). From the linear relationship between Ep and in v experimentally found, even for v as high as 100 V . s -I, we infer that Ep<>lzOll. Under this consideration the derivative of Y is:

dY

doll =

(A8)

-wY

dx

dx

and replacing in Eq. (A7) we obtain:

AppendixA

dOll = [1 + YX(I-wOn)] dx - ( A + I)YO,,

In order to find the relationship between the current, I, and the coverage of the adsorbed reactant, 01i, for the proposed mechanism (Eq. (13)) it is necessary to consider three basic equations: (1) the expression for the current (Eq. (15))

Now, from Eq. (A l), and considering that A >> 1, the current may be written as:

don doll l = S v dE = - S v x f d X

If d I / d E = 0 then d l / d x = 0, consequently, from Eq. (Al0):

(AI)

(2) the equilibrium A described by Eq. (16):

(3) the mass balance for step a (rds), Eq. (17): d( 01, + 0!) = -k,,O!

dt

ASvf x YOu I=

(A3)

dx

__ __

d[ On(

= -k~OnYx

where 0p, Yp and Xp are the values of 0 n, Y and X corresponding to the peak coordinates. Rearranging Eq. (A9) leads to: -

,~Yp0p

-

d x [max

2XpYp

(A12)

1+ YpXp(1-WOp)

[

4

(1----w"0p) = 1 + A(I -W0p) 2

] t/2 - 1

(A13)

If."

a[ o.(l + rx)] dx

_

(A4)

For a single cathodic sweep d t = v- t d E -- - (vfX)- i dx. Then Eq. (A4) may be rewritten as:

vfx

(All)

which compared with Eq. (AI 1) yields:

I + YA')] dt

0r

xp[l + XpYp- WOp]

max

d011 [ assuming Fat has the same value for Ni(II) and Ni(1) complexes. Combining Eq. (A2) and Eq. (A3), we have:

(AI0)

1 + YX(1 - wO,,)

d011[

(A2)

Oi = OuYx

(A9)

- k,,OnYx

(A5)

( A//4)( l - WOp)>> 1

(A14)

S. Ramfrez et al. / Journal of Electroanalytical Chemistry 431 (1997) 171-181 the first term in the series expansion of the right hand side o f Eq. (A13) can be written as:

Xp = [ A ( 1 - wOp)Yp] - '

(A15)

The values of w0p obtained from the experiments fulfil the condition of Eq. (A14). Replacing Eq. ( A I 5 ) in the expression for the current Eq. ( A I 0 ) leads to the expression that has been used throughout this work: Ip = (1

-

-

WOp)

(AI6)

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[8] B. Pihlar, P. Valenta, H. Niirnberg, J. Electroanal. Chem. 214 (1986) 157. [9] J. Weinzierl, F. Umland, Fresenius Z. Anal. Chem. 312 (1982) 608. [10] S.G. Malranovskii, G.V. Prokhorova, E.A. Osipova, J. Electroanal. Chem. 266 (1989) 205. [l l] A. Stromberg, A. Zelianskaya, Zh. Obshch. Khim. 15 (1945) 303. [12] M.R. Rift, F.H. Covitz, Introduction to Organic Electrochemistry, Marcel Dekker, New York, 1974. [13] U. Eisner, E. Kirowa Eisner, in: A.J. Bard, H. Lnnd, (Eds.), Encyclopedia of the Electrochemistry of the Elements, vol. XIII, Marcel Dekker, New York, 1979, chap. 6, pp. 245, 318. [14] M. Spritzer, L. MeRes, Anal. Chim. Acta 26 (1962) 58. [15] S. Ramfrez, G.J. Gordillo, D. Posadas, submitted. [16] C. Wilkinson, Chem. Rev. 72 (1972) 6. [17] E. Godicki, R. Rundle, Acta Cryst. 6 (1953) 487. [18] S. Ramfrez, Doctoral Thesis, 1993. [19] S. Srinivasan, E. Gileadi, Electrochim. Acta 11 (1966) 321. [20] E. Laviron, J. Electroanal. Chem. 101 (1979) 19. [21] J.O'M. Bockris, A.K.N. Ready, Modem Electrochemistry, Plenum Press, New York, 1970. [22] E. Laviron, J. Electroanal. Chem. 35 (1972) 333. [23] E. Laviron, J. Electroanai. Chem. 109 (1980) 57. [24] H. Angerstein-Koziowska, J. Klinger, B.E. Conway, J. Electroanal. Chem. 75 (1977) 45. [25] H. Angerstein-Kozlowska, B.E. Conway, J. Electroanal. Chem. 95 (I 979) 1. [26] E. Laviron, J. Electroanal. Chem. 52 (1974) 395.