The anodic behaviour of thallium amalgams in chloride ion solutions

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

351

THE ANODIC BEHAVIOUR OF THALLIUM AMALGAMS IN CHLORIDE ION SOLUTIONS R. D. ARMSTRONG, W. P. RACE AND H. R. THIRSK Electrochemistry Research Laboratory, Department of Physical Chemistry, School of Chem&try, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU (Great Britain) (Received May 16th, 1969)

SYMBOLS Cdl Cb Cp

Co Ca Do E Erb Erl, Er2

qu ql qmon,1 qmon,2 Rp x1 x2 ]~Tl t7 (.0

double layer capacity (#F cm-2) double layer capacity in the absence of specific adsorption (#F cm- 2) parallel interfacial capacity (pF cm-2) concentration of oxidised species (M) concentration of reduced species (M) diffusion coefficient of oxidised species (cm2 s- 1) electrode potential relative to SCE (V) reversible potential of bulk phase (V, SCE) reversible potential of first (second) monolayer (V, SCE) charge on metal (~tC cm-2) charge due to specifically adsorbed anions (/~C cm-2) charge necessary to form first monolayer charge necessary to form second monolayer parallel interfacial resistance (f~ cm 2) distance from metal to inner Helmholtz plane (cm) distance from metal to outer Helmholtz plane (cm) surface excess of TI species (mol cm-2) = (1000/x/2 ) (R T/n2F2)(1/x/Do) angular frequency (s- 1)

The anodic behaviour of Hg 1, Cd(Hg) 2 and Zn(Hg) a, in alkaline solutions has recently been investigated under conditions where the metal dissolution reaction and anodic film formation interact. It has been found that in all cases, phase monolayers (characterised by the occurrence of a nucleation phenomenon) are formed, although only in the case of mercury was there evidence of considerable specific anion adsorpti6n prior to phase formation. For Cd(Hg) and Zn(Hg) these phase monolayers caused a discontinuous fall in the rate of the dissolution reaction, whilst for mercury no detectable change could be found. Previous work 4. on thallium amalgams in chloride solutions has shown that thallous chloride is formed by the deposition of successive monomolecular layers * The quantitative analysis of this work is incorrect because of iR effects.

J. Electroanal. Chem., 23 (1969) 351-359

352

R . D . ARMSTRONG, W. P. RACE, H. R. THIRSK

followed by multilayer growth in the (100) orientation, and it has subsequently been found 5 that the formation of T1CI on solid thallium follows a similar mechanism. Since thermodynamic data 6 suggest that thallium should dissolve as T1 + and T1C1 in the potential region where the anodic film is formed, the system was investigated in order to obtain further information on the effect of anodic film formation on dissolution reactions. EXPERIMENTAL

The thallium amalgams (40, 5, 1, 0.2 at. ~ ) were prepared by direct combination of the elements (twice-distilled Hg, 5N TI pieces) under a nitrogen atmosphere. The solutions investigated were saturated KC1 and saturated KCI+0.01 M HC1. Measurements using more acid solutions were not successful because of the hydrogen evolution reaction, whilst in more dilute solutions the interfacial impedance could not be determined with sufficient accuracy because of the solution resistance. Saturated calomel reference electrodes were used throughout. Current-voltage curves were obtained under conditions of (a) nitrogen and (b) oxygen stirring using a sitting drop electrode (area ,~ 10-1 cm2). Impedance measurements were made using a conventional Wien 7 bridge with a hanging drop electrode. In order to measure the very large interfacial capacities involved, the procedure described earlier was used 1. The drift in potential was minimised (___1 mV min- 1) by connecting the platinum cylinder to a second reference electrode. Measurements were made of the e.m.f.'s of the cell TI(Hg) IT1C11Satd. KCI[ Hg2C12 [Hg and the results are tabulated in Table 1. All measurements were made at room temperature (25 + 2°C) except where control to 0.1°C was needed when an air thermostat was used. TABLE 1 E.M.F. oF CELL: TI(Hg)[T1CIIsATD. KCIlHg2C12IHg

Tl/at.%

E/mV

0.2 1 5 40

608 676 730 818

RESULTS

(i) Current-voltage curves Figure la shows current-voltage curves obtained with nitrogen stirring, the continuous line shows the average value for many measurements. The current is due to the formation of the solution soluble species Zl(Hg) ~ Zl + + e J. Electroanal. Chem., 23 (1969) 351-359

(1)

353

THALLIUM AMALGAMS IN CHLORIDE ION SOLUTIONS

TI(Hg) + C1- --, T1C1+ e

(2)

TI(Hg) +2 C1- --+ T1CI~ + e

(3)

TI(Hg) + 3 C1- ~ TIC13z - +

(4)

e

An active-passive transition is evident at very near the reversible potential (Erb) of the TI(Hg)/T1C1 electrode. Figure lb shows the current-voltage curve due to the oxygen reduction reaction (with the effects of thallium dissolution subtracted out on the assumption that this was the same as in the absence of oxygen). The diffusion-limited current is found at all potentials negative to Erb.

0 -

0 -o°-~;~,'oo

"e'°'"'"

""~'°

-

~ -

-

-

-b

0

0.5

'Eu <

E

I

I

-700

I

I

-750

-800

E/rnV(SCE) 1

1

o

U I O~~

-1.0 I - 700

I -- 7 5 0 (SCE)

E/mV

I

I - 800

Fig. 1, 5% TI amalgam. (a) Potential-dependence of the anodic dissolution current near the monolayer region, (b) potential-dependence of current due to oxygen reduction, after correction for dissolution current. The arrows indicate the potentials of the 1st monolayer and the region of multilayer phase formation.

(ii) Impedance measurements At E < - 0 . 8 V the electrode impedance was purely capacitative. In the region where the dissolution current was found, a faradaic pseudo-capacity due to the reactions (1)-(4) was observed. This was analysed assuming that a "Randles" circuit with a negligible charge transfer resistance was the appropriate analogue, but making no assumption concerning the magnitude of Cd~8. J. Electroanal. Chem., 23 (1969) 351-359

354

R. D. ARMSTRONG, W. P. RACE, H. R. THIRSK

In this case since CR >>Co Rp = 2a/Coo9 ~

Cp = Cd] + Co/2a~o~ Figure 2a and b, shows tests of these relationships for several potentials, whilst Fig. 3 shows the dependence of the derived values of Co on potential. Since this line shows a 60 mV/decade slope as anticipated for reactions (1)-(4), we can assume that the Randles circuit is the appropriate analogue. Table 2 compares the estimated concentration of TI species with that calculated from stability constants 6. Figure 4 shows the values of Cs at 5 kHz for the amalgams investigated. The values of Cp (~o~ ~ ) are also shown (as a dotted line) where these differ significantly from Cs (5 kHz). The following features of the C-E curves should be noted: 1. Each C-E curve shows a maximum followed by a discontinuity (at a potential g,1); 2. The potential of this discontinuity approaches the reversible Tl(Hg)/T1C1 potential as the Tl activity is increased (Fig. 5a);

I 1ooo! -760

I I 0.01 0,02 C0-112/se¢1/2

I Q03

1

0.01

1' Q 0 2 CO-/2/sec 1/2

0.03

I

Fig. 2. 5~o TI amalgam. Dependence on to -{ of: (a) parallel interfacial capacity, (b) parallel interfacial resistance. Numbers above lines denote potentials (mV, SCE).

3x1(33

~.~i0--~

~o

3 x 10TM 10

I , I 0 - 10 E/mY (reversible

I I . J - 20 -30 -40 5 % TI ( H g ) / T I Cl e l e c t r o d e )

Fig. 3. 5 ~ TI amalgam. Potential-dependence of the conch, of dissolving species. J. Electroanal. Chem., 23 (1969) 351-359

355

THALLIUM AMALGAMS IN CHLORIDE ION SOLUTIONS

3. The magnitude of the discontinuity decreases at high T1 activity (Fig. 5b); 4. At potentials > grb + 20 mV, the capacity fell to a low value (not shown) due to the formation of a thick layer of T1C1 on the electrode, except in the case of the 0.2 at.~ amalgam where a discontinuous rise (at a potential Er2) capacity was found before this. TABLE 2 OBSERVED AND CALCULATED VALUESOF DISSOLVEDZl SPECIESCONCENTRATIONIN EQUILIBRIUMWITH TIC1 (mol l- l) (Do = 1.35 x 10- s cm 2 s - 1),

Observed

Calcd. TI +

2.5x 10 -3

T1Cl

4.5x 10 - s

ref.

6.0x 10 -4

,ooo

* Estimated from value given in

100

10C

&

-7~

3~

30O(

3000

~u~ 100(

1000

U. "~m 300

300

10C

100

-6~o

E/mV(SCE)

8.0x 10 -4

4.4x 10 -4

1.9x 10 -3

100C

30C

3-%00 I -7'5o

Total

300C

'E 300 v LL

-7&o

[TlCla] z-

16.

3000

30-600 -6;o

[TICl2] -

-6~o

1_7;o

-7~

-6~o -6~

-•oo

-&o

.

3-%0 -65o

-,ooo

E/mV (SCE)

Fig. 4. 5 kHz log Cs-E curves for various amalgam concns. Broken lines indicate Cp (m~oo) where significantly different from Cs (5 kHz). Arrows indicate respective bulk reversible potentials.

DISCUSSION

(i) The significance of the C-E curves The discontinuities in the C-E curves are similar to those found previously (Table 3) and are due to the formation of an anodic phase monolayer (T1Cl) on the electrode surface. The thermodynamic treatment of this phenomenon has been given J. Electroanal. Chem., 23 (1969) 351-359

356

R. D. ARMSTRONG, W. P. RACE, H. R. THIRSK I00C

5C

4Q

LL

30

3o0

> 20

L.U 100

10

O log CA

1 log CA

Fig. 5. (a) Dependence of the potential difference between the monolayer phase formation and the bulk reversible potential, on amalgam concn.; (b) dependence of the magnitude of the capacity discontinuity at monolayer formation, as observed at 5 kHz, on amalgam concn. TABLE 3 DOUBLE LAYER CAPACITY VALUES IN THE PRESENCE OF ANODIC MONOLAYER FILMS

Electrode

Solution

Cdl/lAFcm- 2

Ref

Pt *Hg Hg Hg Hg *Cd(Hg) 1% *Zn(Hg) 1~ *TI(Hg) 0.2~

1 M HCIO4 1 M NaOH 1 M HC104+0.7 M H 2 C 2 0 4 1 M KNO3+0.05 M borax+0.01 M butobarbitone 1 M NaHCO 3 +0.5 M Na2S 10 M NaOH (2 monolayers) 3 N NaCI+0.1 M NaOH Satd. KC1 1st 2nd 0.1 M KH2PO4+0.1 M K2HPO4+0.6 M KNO a

150-100 180-220 20-25 3--4 22-25 3-4 5-15 130 450 70-130

12 1 13 9 14 2 3 Present work

Hg

15

* Simultaneous dissolution reaction occurs. elsewhere, where it has been n o t e d that the reversible p o t e n t i a l s of thin films a n d b u l k phases m a y differ significantly. T a k i n g into a c c o u n t the m e t a l - m e t a l i o n diss o l u t i o n r e a c t i o n Cd, has the significance: E < Erl

Cdl = ~

(qM+FFTI)

Er2 > E > E r l

Cdl = ~

(qM+FFT]+qrnon, 1)

E E,2

Cal = ~

(qM + FFT1 + q . . . . l + q . . . . 2)

T h e fact t h a t at E < E,I, Cdl > 2 0 # F c m - 2 is evidence for the specific a d s o r p t i o n of C I - , T1 + o r b o t h , at p o t e n t i a l s negative to those at which the p h a s e m o n o l a y e r

J. Electroanal. Chern.i 23 (1969) 351-359

357

THALLIUM AMALGAMS IN CHLORIDE ION SOLUTIONS

becomes stable. The fact that a maximum is found indicates 1° that approximately half coverage of the electrode by these species is achieved before 2D condensation occurs. This situation has previously been found in the adsorption of S 2- ions on mercury. If only C1- were present in the inner layer we could write ~°

(~qM~

:~ C b -~

[t~qM~ [Oql \

\~E ],,

x2

and use this relationship to evaluate ( x 2 - x ~ ) q / x 2 as a function of E since X2 __ XI

--" X2

qt =

f E

J -co

(Cdt-Cb)dE

These quantities are shown in Fig. 6.

h_ ..~.~60 t~

t

8C

a

h_

p ~4o

6C

4c I._

2O

t~ .._...~! 2C

0 700

-800 E/mY (SCE)

- 900

1

i -900 E/mY (SCE)

i

i -1000

Fig, 6. Potential-dependence off_~ o~(Cdl- Cb)dE. (a) 1% amalgam, (b) 5 ~ amalgam, (assuming C~ = 20 #F cm-

2).

(ii) The value of Cdl in the presence of a phase monolayer In the absence of a dissolution reaction 9 Cdl

(t~qM~

(t3qmon,1~

The term (dqufi3E)qmo" will generally give rise to a "geometric" capacity ~ 20 #F cm- 2. For a value >>20 #F cm-z we must conclude that there is some contribution from the term (gq,,o,,1/dE)q M. Physically this means that with a change in potential the electronic or ionic charge associated With the phase monolayer changes. This could be due to the incorporation of interstitial charge into the film, or to the adsorption of species at the solution-film interface. At the present time, only two phase monolayers have been found with Cai >>20 #F cm-z, in the steady state. These are T1C1 (present work) and HgO 1. It is perhaps not without significance that both these monolayers sustain the O2/H202 reaction and allow metal dissolution to occur

J. Electroanal. Chem.,23 (1969) 351-359

358

R . D . ARMSTRONG, W. P. RACE, H. R. THIRSK

reversibly through them which suggests that there are both electronic and ionic states within the monolayers.

(iii) The passivation of Tl amalgam The exact cause of the passivity of metals has been the subject of much discussion. It has been suggested that passivity is due to: (a) an adsorbed layer of anions; (b) a 2D phase monolayer ; (c) a 3D phase. In the present case where (a), (b) and (c) can all be found, no diminution in the rate of the metal dissolution reaction can be detected before a 3D film is present on the electrode. Lower limits for the exchange currents are given in Table 4. However, we must not conclude from this that adsorbed species and the 2D film have no effect on the reaction, only that the effect cannot be detected because of the high exchange currents involved. TABLE 4

Amalgam concn./at.°//oo

Potential/mV SCE

Minimum exchange current/mA cm- 2

Condition

0.2

- 619 - 610

250 250

1st monolayer 2nd monolayer

1.0

- 712 - 663

50 500

adsorption peak 1st monolayer

5.0

- 760 - 730

50 500

adsorption peak 1st monolayer

- 830 - 815

50 250

adsorption peak 1st monolayer

40

ACKNOWLEDGEMENTS

The authors are grateful to J. D. Milewski for assistance in the experimental work. SUMMARY

The impedance of the TI(Hg)/KC1 aq. interphase has been measured and it has been shown that up to two successive 2D phase monolayers of T1C1 can be present on the electrode surface before a 3D film of T1Cl is formed. At more negative potentials there is evidence for disordered anion and/or cation adsorption. The dissolution of Tl(Hg) as T1 + and T1CI proceeds reversibly in the presence of these species and the phase monolayers, but is prevented by the 3D film. REFERENCES

1 2 3 4

R. D. ARMSTRONG, W. P. RACE AND H. R. THIRSK, J. Eleetroanal. Chem., 19 (1968) 233. R. D. ARMSTRONG,J. D. MILEWSKI, W. P. RACE AND H. R. THIRSK, J. ElectroanaL Chem., 21 (1969) 517. R. D. ARMSTRONG, G. M. BULMAN AND H. R. THmSK, J. Electroanal. Chem., 22 (1969) 55. M. FLE1SCHMANN,J. PATTISON AND H, R. TmRSK, Trans. "Faraday Soc., 61 (1965) 1256.

J. Electroanal. Chem., 23 (1969) 351-359

THALLIUM AMALGAMS IN CHLORIDE ION SOLUTIONS

359

5 R. D. ARMSTRONG,L. J. PEARCEAND H. R. THIRSK, Electroehim. Acta, in the press. 6 Stability Constants of Metal Ion Complexes, Chem. Soc., London, Spec. Publ., No. 17, 1964. 7 R. D. ARMSTRONG,W. P. RACEAND H. R. THIRSK,Electrochim. Acta, 13 (1968) 215. 8 J. H. SLUYTERS,Rec. Tray. Chim., 79 (1960) 1092. 9 R. D. ARMSTRONGAND E. BARR, J. Electroanal. Chem., 20 (1969) 173. l0 R. PARSONSAND J. M. PARRY, Trans. Faraday Soc., 59 (1963) 241. 11 R. D. ARMSTRONG,D. F. PORTERAND H. R. THIRSK, J. Eleetroanal. Chem., 16 (1968) 219. 12 S. GILMANin A. J. BARD (Ed.), Electroanalytical Chemistry, Vol. 2, Marcel Dekker Inc., New York, 13 14 15 16

1967. R. D. ARMSTRONGAND M. FLEISCHMANN,Z. Physik. Chem. Frankfurt, 52 (1967) 131. R. D. ARMSTRONG,D. F. PORTERAND H, R. THIRSK,J. Phys. ~Them., 72 (1968) 2300. R. D. ARMSTRONG, M. FLEISCHMANNAND J. W. OLDFIELD,J. Electroanal. Chem., 14 (1967) 235. J. HEYROVSKqAND J. KI~ITA,Principles of Polarography, Academic Press, New York, 1966, p. 106.

J. Electroanal. Chem., 23 (1969) 351-359