Alternating current chronopotentiometry—III. Electroreduction of silver(I), cadmium(II), and thallium(I) in calcium nitrate tetrahydrate melt

clevrroahl,nica Acta,VoL23 .pp.679-,585.

OD13J686/78/07UI-0679502.

n

Q Peryamon Preys Ltd. 1978 . Printed in Green arilain.

ALTERNATING CURRENT CHRONOPOTENTIOMETRY-III .t ELECTROREDUCTION OF SILVER(I), CADMIUM(II), AND THALLIUM(I) IN CALCIUM NITRATE TETRAHYDRATE MELT NAROTTAM P . BANSAL ° and JAMES A. PLAMUECK

Department of Chemistry, The University of Alberta, Edmonton . Alberta, Canada

(Received 20 May 1977 ; and in revised form 10 August 1977) Abstract -The eleclroreduction of silver([) on a platinum electrode and of cadmium(II) and thallium(l) on a mercury electrode has been studied by alternating current chronnpotentiometry . Expected relations for the diffusion controlled, one-electrondeposition of an insoluble substance are obeyed for silverp) ;but reduction of cadmium(II) on mercury is more complex . Nucleation of metallic thallium is the rate controlling process during the early stages of the reduction of thallium([).

L INTRODUCTION

Alternating current chronopotentiometry is a modification of do chronopotentiometry in which an alternating current of constant small amplitude is superimposed upon the direct electrolysis current and variations in the alternatingcomponent of the working electrode potential are measured against time. The resulting ac chronopotentiogram consists of sharp peaks and it is possible to measure[[-3] the transition time with better precision than in do chronopotentiometry, where the potential-time curves are distorted by electrical double layer charging[4-8] . Theoretical equations of ac chronnpotentiometry for reversible[1,9] and irreversible[10] processes and an electrode process_ preceded by first[11,12] and second[13] order chemical reactions have been reported . We have developed[2] the potential-time equations for the generalized quasi-reversible process involving soluble and insoluble electrode reaction products, of which the equations for fully reversible and totally irreversible electrode processes are limiting cases. Theoretical treatment of the spherical[14] diffusion case has also been reported . Experimental verification of the theoretical equations of ac chronopotentiometry, for the reversible electrode process involving soluble products (the reductions of cadmium(fl) and zinc(H) in 0 .5 M KCI and thallium([) in 0.5 M KNO 3 ) and for the irreversible electrode process (the reductions of nickel(1I) in 0 .5 M KCNS and cobalt(II) in 0 .5 M KCI) have been reported[2, 3] . The results for the stepwise reduction of copper(Il) in 0 .5 M KCl have also been presented[3] . The purpose of the present paper is two-fold ; firstly, to study the reversible process in which the product of electrode reduction is soluble neither in the solution nor in the electrode material and secondly, to apply the technique of ac chronnpotentiometry to electrode reactions in hydrated melts. The reductions of silver([) " On leave of absence from Department of Chemistry, Ramjas College, University ofDethi, Delhi-110007 (INDIA) . t Part I, [2] ; Part If, L3]679

on platinum and cadmium(fl) and thallium([) on mercury, all in calcium nitrate tetrahydrate melt, were chosen as they have also been examined by other electrochemical methods in this melt[I5-20] . 2 EXPERIMENTAL

A double walled, water jacketed Pyrex cell was used for the measurements . Hot water from a constant temperature bath was constantly circulated around the cell. The temperature was 50 .0 ± 0.1 0. The bath stirrer was turned off only during the measurements to avoid mechanical disturbances . An externally driven magnetic stirrer was used for mixing of the viscous calcium nitrate tetrahydrate melt . Calcium nitrate tetrahydrate (B.D.H ., AnalaR), silver nitrate (B.D .H ., AnalaR), cadmium nitrate tetrahydrate (Fisher certified reagent) and thallium nitrate (Alfa Inorganics) were used without further purification. The silver wire used for the silver(l)/silver reference electrode was obtained from Johnson Matthey and Mallory Ltd. About 80 g of calcium nitrate tetrahydrate was used for each run . As the solubility of oxygen is negligible in this melt[l5], no precautions were taken to exclude air from the system, save for tightly capping the cell to avoid loss of water due to evaporation . The density of the melt without solute[21] was used in calculating the molar concentrations as the solute concentrations used were low. The cell used was in a three electrode configuration. For the reduction of silver([) ions the working electrode used was a platinum sphere blown at the end of a platinum wire which emerged from a platinum-glass seal in a piece of soft glass tubing . Cadmium(II) and thailium(1) cannot be reduced at a platinum electrode in calcium nitrate tetrahydrate melts due to the low hydrogen-on-platinum overvoltage . Hence for these two ions a P.A.R. Model 9323 hanging mercury drop electrode (I-IMDE) was employed which was set up using triply distilled mercury (Engelhard) . A fresh mercury drop of constant known area (0 .0351 cm 2 ) was taken for each observation . The auxiliary dec-



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NAROrrAM P. BANSAL AND JAMSs A. PLAMBECX

trode was a platinum flag of large area . The silver(i) in the melt/silver reference electrode was set up in an isolated fritted glass compartment of 10-20p porosity. The experimental set up used for recording the ac chronopotentiograms was essentially the same as described earlier[2] .

with the superimposed alternating current . Under the conditions that 0 cc t c< r and AX 'i 2 /2iw 12 (r'%2 - t'i7 ) and A/i smaller than unity as is usually the case in or chronopotentiometry where A c< i, the value of E,[from (I)] could be written as

RT An" nF 2iw1 ;2

3. RESULTS AND DISCUSSION 3 .1 . Reversible electrode process involving deposition of an insoluble substance In ac chronopotentiometry the potential-time relation for a diffusion controlled process involving an insoluble electrode reduction product is given by[9] : E = E"aF +

RT Am t /2 nF, la L 2iw'i2

sin(wt -,z/4) _(t 112 _ 1 tr2 )

( 1)

where Ek is the potential observed in do chronopotentiometry for the corresponding electrode process, n is the number of electrons consumed in the reduction of each molecule of the electroactive species, i is the direct electrolysis current, A and w are the amplitude and the angular frequency of the superimposed alternating current, r is the transition time, and the other symbols have their usual electrochemical significance . It is clear from (1) that the potential of the working electrode consists of two parts ; one, the first term on the righthand side of (1), E; is the slowly varying do component and another is the ac component, E,,., which consists of the second term on the right-hand side of (1). The alternating component of the electrode potential has the same frequency but is --45° out of phase

sin(wt - n/4) (r r ;r _ t t ;2)

( 2)

Hence, under the specified conditions, the alternating component of the working electrode potential is directly proportional to A and indirectly proportional to i and w as it was for the case of a reversible electrode process involving two soluble speciesA typical ac chronopotentiogram for the reduction of Ag(1) on a spherical platinum electrode in calcium nitrate tetrahydrate at 50' is presented in Fig- 1 . Equation (1) has been derived under the conditions of semi-infinite linear diffusion . The straight line plot in Fig . 2 of t1 ;2 (measured from the ac chronopotentiograms) against l/i indicates that under the conditions of these measurements linear diffusion theory is an adequate approximation to spherical diffusion to the platinum sphere . The difference between planar and spherical diffusion derived theoretically by Smith[14] is small and not apparent in these results . Neglect of higher harmonics in the present investigations is justified by the use of the lock-in amplifier which eliminates them from the potential-time tracings . According to (2), Eq is directly proportional to the amplitude of the superimposed alternating currentFor the reduction of Agp), Ea ,, has been plotted against A in Fig. 3 keeping w and i constant. The plot is a straight line passing through the origin as expected . From equation (1), a plot of E,, us the logarithm of

Fig . 1 . Ac chronopotentiogram for the reduction of 5 .41 x l0 - ' M Ag(I) on a platinum sphere. Direct current, 54 pA ; amplitude of uq 2 .6ƒA (rms) ; frequenc , 100 H .



Alternating current chronopotentiometr

as

1 .0

1 .S

691

2.0

2.5

Fig.2. Sand plots for the reduction of : A, 5 .41 x 10 -3 M Ag(q, amplitude, 2 .6pA (rms) ; 9.1 .135 x 10 -2 M Cd(1f), amplitude, 1 .95 NA (rms) ; frequenc = 100 H .

50

25

20

is 10

5

r

000

5A ALTERNATING

10.0

CURRENT APPLIED TO

CEtt

1S0

tm 1AA md

Fig. 3, Plots of E„ against amplitude of the alternating current for the reduction of : A, 1 .135 x 10 - ' M Cd(11), direct current, 58 A ; B, 5.41 x 10 -3 M Agf ), direct current, 70uA ; frequenc , 100 H . the -term within the square brackets on the right-hand side should ield a straight line having a slope 2.303 RT/nF from which the number of electrons a can be calculated . Such a plot for the reduction of Ag(I) is shown in Fig. 4 ; the experimental slope of 0 .065 V is in excellent agreement with the theoretical value of 0.064 V for a one-electron process at 50 .

3 .2 Reduction ofCadmium(II)andThallium(t)onhntde A t pical ac chronopotentiogram for the reduction of Cd(II) on a hmde in calcium nitrate tetrah drate at 50' is shown in Fig . 5 . For a diffusion controlled electrode process where the product of the electrode reaction is soluble either in the solution or in the electrode material, the ac chronopotentiograms should be in accordance with (13) and (20) of our earlier paper[2] . These equations were obtained under the conditions of semi-infinite linear diffusion rather than spherical diffusion . However, the straight line Sand plot (Fig. 2) for the reduction of Cd(II) indicate that, under the experimental conditions emplo ed in the present work, mass transfer is diffusion controlled and the linear diffusion theor could be taken as an adequate approximation for diffusion towards the hmde. The plot of E„, vs A, for the reduction of Cd(II)

has been shown in Fig . 3 and is seen to be a straight line passing through the origin as expected[2] . A plot of E, against the logarithm of the factor within the square brackets of the second term on the right-hand side of (13) of our earlier paper[2] should ield a straight tine having a slope of 2303 RT/nF from which the value of n, the number of electrons involved in the electrode process, can be evaluated. Two such plots for the reduction of Cd(1I) in calcium nitrate tetrah drate at 50' for two different values of the direct electrol sis current are shown in Fig. 4 . These plots are straight lines but their slopes, 0 .053 V and 0 .042 V, are quite different from the theoreticall expected value of 0.032 V for a two-electron process at 50 . The values of n calculated from these two slopes are 1 .22 and 1 .53 for electrol sis current densities of 1 .709 and 1 .254 mA/cm 2, respectivel . Similar straight line plots with increasing slopes were obtained for ac chronopotentiograms recorded at increasing current densities ; the value of n calculated decreased with increase in current densit . These results indicate a change in mechanism for the reduction ofCd(II) with the current densities used . A change in mechanism from n = 2 at lower current densities to n = 1 at higher eds has also been reported b Lovering[18] who studied this



682

NAaoriAM P . BANSAL

0.005

AND

JAMES A . PLAMBECK

0 .010 0 .015 LOGARITHMIC TERM

0020

0025

Fig . 4. Logarithmic plots for the reductions of Ag(l) on a platinum sphere, 0 ; and Cd(tt) on a horde fur direct electrol sis cd, O . 1709 mA/cm2 , 0, 1 .254 mA/cm2 . For more details see text . s stem using different electrochemical techniques . Lovering and others[18,22-24] have suggested that adsorbed intermediates are involved in the reduction of Cd(II) on mercur . Such intermediates, or alternativel a stable Cd(I) species which is known in other though dissimilar melts[26], would account for our observations . Figure 6 represents a t pical ac chronopotentiogram for the reduction of TI(I) on a Horde in calcium nitrate tetrah drate at 50 . The corresponding do chronopotentiogram is also shown for comparison . The initial potential overshoot present in the E,, - t curve is the nucleation ovcrpotential and shows up as a negative peak in the Ea. - r trace. More detailed investigation of this s stem is reported elsewhere[19] . Mo nihan and Angell[15], who have studied this

s stem b chronopotentiometr , did not report an potential overshoots in their chronopotentiograms ; this work has been critici ed b Lovering[18] .

CONCLUSION

In the case of the reversible deposition of insoluble silver on platinum, the expected ac chronopotentiometric relationships are obe ed In the cases of deposition of presumabl soluble cadmium and thallium on mercur the are not obe ed because the deposition process is not a simple reversible reduction .

Acknowledgements- The authors are grateful to the Nation! Research Council of Canada for support of this work through an Operating Grant (to LA-P .) .

Fig . 5 . Ac and do chronopotentiometric curves for the reduction of 1 .41 x l0 - M Cd(ti). Direct current, 60 A amplitude of ac, 2 .6 A (rrrss)4 frequenc , 100 H .

Alternating current chronopotentiometr

683

Fig . 6 . .4c and do chronopotentiograms for the reduction of 1.02 x 10 -2 M Tl(1). Direct current, 70 uA ; frequenc of ac, 100 H ; amplitude, 2.s 4 (rms).

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