R. D, ARMSTRONG, D. F. PORTER, AND H. R. THIRSK
2300 deviation being approximately 15” for the least favorable 2rientation of the hydrate water molecules. At 2.29 A this bond is 0.07 A less than the shortest bond listed by Pimentel and M~Clellan.~’ The angle formed by the hydrate water molecule with the two underlying surface hydroxyl groups and the length of bond a (Figure 6) can have a wide range of acceptable values. For example, if the angle is loo”, then the
bond length is 2.58 A. Both of these values are roughly in the middle of the observed range.31t83 It is our opinion that the deviations discussed above are not sufficiently large as to warrant rejection of the proposed model. It is hoped that current infrared investigations will provide additional information concerning the structure of the associatively adsorbed water on the surface of thorium oxide.
The Passivation of Mercury in Sulfide Ion Solutions by R. D. Armstrong, D. F. Porter, and H. R. Thirsk Department of Physical Chemistry, University of Newcastle upon T p e , Newcastle u p o n Tyne, England Accepted and Transmitted b y T h e Faraday Society
( J u l y 14, 1967)
The growth of thin anodic films of mercury sulfide on a mercury electrode in sulfide ion solutions and the effect of such films on the anodic dissolution of mercury have been studied using the potentiostatic method, I n the initial stages of film growth, mercury sulfide (metacinnabarite) is deposited on the electrode in the form of two successive monomolecular layers, and the presence of each layer severely inhibits the rate a t which mercury dissolves as the complex species HgSZ2-. The current-potential relationship was found to exhibit discontinuities corresponding to the formation of each HgS monolayer and it is suggested that the passivation process a t some solid metal electrodes may be of a similar nature.
Introduction
Experimental Section
Previous articles have dealt with the anodic-dissolution reaction’ and the specific anion adsorption2 which occur at a mercury electrode polarized anodically in aqueous sulfide ion solutions. It was shown that the mercury dissolves as the complex ion HgS2- and that the sulfide ion is very strongly adsorbed at the electrode over a short potential range, causing the electrocapillary maximum to be found a t relatively negative potentials. The present work investigates the phenomena occuring a t slightly more anodic potentials, where a solidphase film is present on the electrode surface. Zhdanov and Kiselev3 have examined this system using dilute (10-3 M ) solutions of sodium sulfide in 0.1 M KCl and have concluded that mercury sulfide is deposited in the form of successive monomolecular layers on the electrode surface, but these authors did not examine the kinetics of film growth nor did they positively identify the phase formed. I n an attempt to elucidate these factors, the film growth has been studied in a carbonate-buff ered solution of sodium sulfide of pH 9.5. I n this solution the dissolution reaction is considerably reduced,Z facilitating the examination of the film growth alone. The effect of the film on the dissolution reaction in 1 M NazS solution was then examined.
Kinetic and impedance measurements were made using the potentiostatic technique, details of which are given el~ewhere.’,~Films of the anodic product were removed from a mercury-pool anode and were examined by electron diffraction and by electron microscopy4 techniques. Potentials were measured with reference to an Hg-HgO electrode in 1 M NaOH solution via a liquid junction formed at a three-way tap. The solutions used were 0.5 M NazS 1 M NaHC08 (“buffered sulfide solution”) and 1 M NazS, (“alkaline sulfide solution”), prepared in an atmosphere of purified nitrogen from Analar reagents and deoxygenated triply distilled water. Mercury was prepared by chemical cleaning followed by two successive distillations in vacuo. The reversible potential of HgS in 0.5 M NazS 1M IYaHC03 solution was found experimentally by setting
T h e Journal of Physical Chemistry
+
+
(1) R. D.Armstrong, D. F. Porter, and H. R. Thirsk, J . Electroanal. Chem., 14, 17 (1967). (2) R. D. Armstrong, D. F. Porter, and H. R. Thirsk, ibid., 16, 219 (1968). (3) S. I. Zhdanov and B. A. Kiselev, Doh% A k a d . Naulc SSSR, 155, 651 (1964). (4) M. Fleischmann and H. R. Thirsk, Electrochim. Acta, 9, 757 (1964).
PASSIVATION OF Hg IN
S2-
SOLUTIONS
2301
up Hg-HgS electrodes in this solution and by measuring their potentials with respect to Hg-HgO in 1 M NaOH solution via a liquid junction. These potentials were -0.692 V for the black form of HgS (metacinnabarite) and -0.698 V for the red HgS (cinnabar). I n this work we shall designate the reversible potential of black HgS in this solution as E,. All measurements were made at room temperature, 24
f
1”.
Results Bu$ered Sulfide Xolution. The formation of the solid film on the mercury electrode was observed potentiostatically by applying an anodic rectangular pulse of aotential to the electrode, which was initially held at a potential 0.3 V negative to E,. The resultant currenttime transients for various pulse heights are shown in Figure 1. The first feature attributable to the growth of a solid 6 mV, film was found on pulsing to a potential of E , where the current-time transient exhibited a “shoulder” (Figure 1A) which moved to progressively shorter times and higher current densities as the potential was increased (Figure 1B). The area under the shoulder was approximately constant and corresponded to a charge of 179 f 16 pC C M - ~ . On pulsing to potentials of E , = +23 mV and above, a second feature appeared, taking the form of a peak in the current-time transient (Figure lC, and D). The area under this peak was also approximately constant at 228 f 23 pC Reduction of the anodic phase was also observed, a t the end of the formation period (-350 msec), by changing the potential stepwise to E , = - 18 mV. The resultant reduction transients are shown in Figures lE, and F, and graphical integration yielded charges in agreement with those obtained from the formation transients for the same formation potentials. Measurements of the impedance of the electrode were made in the buffered sulfide solution as a function of electrode potential, E (Figure 2). The admittance at potentials negative to that at which a film is formed is principally due to the adsorption of sulfide ions2 and a t low frequencies is purely capacitative. On increasing the electrode potential positive to this adsorption region, a discontinuous fall in the double-layer capacity, Cdl, from -500 to -35 pF cm-2, mas observed at a potential of E , = +6 mV. A second discontinuity occurred at a potential of E , = $23 mV, where the capacity fell to -20 pF cm-2. On subsequently decreasing the electrode potential, there mas only one discontinuous capacity rise which occurred at E = E,, and the capacity reverted to a high value -500 p F em-2, corresponding to the specific anion adsorption. The high capacities of the adsorption region were measured at 15 or 30 cps, low frequencies being necessary at high capacities to reduce the proportion of the measured impedance due to the solution resistance.2
~~‘~~
x)
y q IO
.-$
2
4
6
1
D
’0 i)
L
s u
200
20
I
I
+
Time, msec.
Figure 1. Current-time transients for the formation (A-D) and reduction (E and F) of HgS in 0.5 M NanS 1M NaHC08 solution, at overpotentials of (A) 6 mV, (B) 17 mV, (C) 23 mV, (D) 32 mV, (E) -18 mV, (F) - 18 mV. The reduction transients are for films formed for 350 msec a t 12 mV (E) and 25 mV (F).
+
The impedance of the electrode at potentials positive to the adsorption region was measured at a higher frequency of 1 kcps, in order to reduce effects due to the dissolution reacti0n.I Though greatly reduced in this solution compared with the alkaline sulfide solution, the reaction is still sufficient to produce significant pseudocapacities at low frequencies when the electrode doublelayer capacity falls to values
