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Influence of mercury on hydrogen overvoltage on solid metal electrodes—I. Stationary polarization curves of hydrogen deposition on pure and poisoned electrodes
Ekctmchimicn
Actrr. 1970. VoI. 15, pp. 1653
to 1665. Pcrmunoa Rem.
Printed in Northern Ireland
INFLUENCE OF MERCURY ON HYDROGEN OVERVOLTAGE ON SOLID METAL ELECTRODES-I. STATIONARY POLARIZATION CURVES OF HYDROGEN DEPOSITION ON PURE AND POISONED ELECTRODES* and J. BALEJ
J. VONDRAK
Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia Abstract-Mercury deposited in amounts of the order of a few monolayers on Pt, Fe, Ni and Co raises the overvoltage of hydrogen evolution in alkaline solution. On Cr it deaeases the overvoltage and on MO it has little influence, but these metals corrode in alkali.
R&mn&Le mercure deposalen quantit&s de l’ordre de quelques couches simples sur Pt, Fe, Ni et Co Cl&e la surtension de l%volution d’hydrogkne en solution alcaliue. Sur Cr, il diminue la surteusion et sur Mo il a peu d’influence, mais ces m&aux se corrodent en solutions alcalines. Zusammenfass~-Die Wasserstoffiiberspannung an Pt, Fe, Ni und Co in alkalischer Lasung wird durch in einigen Monoschichten abgeschiedenes Quecksilber erhtiht. Bei Cr wird die Uberspannung verringert, und bei MO ist der Einfluss gering, diese Mctalle korrodieren jedoch in alkalischer LBsung.
DURING
the production
electrolysis, reaction,
an amalgam
which
of chlorine
and
alkali hydroxides
of the alkali metal
is decomposed
by
water
is formed
by means
as the product
in a separate
device
of amalgam
of the cathodic
to form
the alkali
The energy of this decomposing reaction could be metal hydroxide and hydrogen. utilized in a galvanic cell where sodium from the amalgam would be ionized on the anode and hydrogen would be deposited on the cathode. This process could not be carried out on a commercial scaleuntil now, for reasons discussed in adetaiIed review by Balej .l For the successful operation of the amalgam-hydrogen-decomposition hydrogen electrodes with negligible hydrogen overvoltage and a lifetime
possible are required.
cell,
as long as
As mercury is a component of the amalgam decomposer the has to maintain its favourable properties even in the presence of that metal and its compounds. Mercury and its compounds are rather firmly bound to the surface of electrodes, which causes their poisoning and increase of the hydrogen overvoltage. As a resuIt the lifetime of the decomposer is reduced. Other substances, such as arsenic, sulphides, cyanides etc have a similar influence. The poisoning of platinum electrodes by mercury during the electrodeposition of hydrogen has been described by Slygin and Ershler, 2 Reader and Nilsen and more thoroughly by Khomtshenko.4 According to Reader and Nilsen,s nickel electrodes also are poisoned by mercury. Similar results were reported by 3agotskaya and Oshe.6 These authors also found only a small influence of mercury on iron electrodes, especially in an alkaline medium. The morphology of mercury deposited on spherical platinum single crystals was studied by Kaishev, Toshev and MutaftschieFs and the deposition of mercury on a glassy carbon electrode was investigated by Astley, Harrison and Thirsk.s On both used hydrogen
l
electrode
Manuscript received 2 July 1969. 1653
J. VONDRAK and J. BALEJ
1654
these materials mercury is deposited not as a thin layer, but as small droplets, the number of which-in case of platinum--depends on the orientation of the electrode surface. In most of the above mentioned papers, the results on various materials were not obtained under equivalent conditions. Therefore the subject of the present communication is the measurement of the influence of mercury poisoning of various electrode materials (pure metals and alloys), where the evolution of hydrogen from solutions of alkaline hydroxides proceeds under comparable conditions. The poisoning was measured on technically available pure metals with low hydrogen overvoltage in alkaline medium, ie on iron, nickel and cobalt, and further on the metals of the VIb group, chromium, molybdenum and tungsten. As they do not form amalgamslO they should best resist mercury poisoning. The poisoning action of mercury was investigated moreover on some binary alloys of these metals, which might combine the favourable properties of a low hydrogen overvoltage with resistance to mercury poisoning. For comparison, some experiments were carried out with platinum. EXPERIMENTAL
TECHNIQUE
Electrodes Measurements were performed with smooth electrodes. All electrodes were 1 x 1 cm. Those of Ft, Ni, MO and W were mostly made from foil O-1 mm thick with a welded wire from the same material. Some of the platinum electrodes were prepared from foil O-05 mm thick. The other electrodes were prepared by electrodeposition according to Table 1 on a copper electrode previously electrolytically TABLE 1. Metal
co Fe Cr 70% Ni 3O%W
50% 50%
co w
73% Ni 27% Zn 58% Ni 42% Zn
c4WDlTIONS
Bath composition coso~~7H,o NaCl H&0l FeCI,-4H,O C&la H,O CrOB
504 g/l 17g/l 45 g/l 120 g/l 5Og 3ooml 200 g/l
Na3W0,-2HI0 (NH&SO, NH. (25 %I coso,*7H*o NasWOI-2HI0
17 g/i 150 g/l 60 ml/l 60 g/l 70 g/l 66 g/l
NH,OH to pH7 NiSO&HIO ZnSO,.7H,o HaO. NiSOI*7Hs0 Znso,*7H,o HsBOs
50 g/l 2-9 g/l 80 g/l 50 g/l 2.9 8/l 80 g/l
OF METAL
DEPOSlTION
Temperature “C
Cd mA/cms
20
50
75
15
50
450
75
300
21
80
50
21
20
7,75
20
491
Reference
21
In&xxx
of mercury on hydrogen overvoltage on solid metal electrodes-I
1655
The polished in phosphoric acid. The purity of the metals was at least 99.9%. molybdenum and tungsten electrodes were previously polished in a bath containing 75 % of glycerol and 5 % KOH, at 0.2 A/cm*. Before the measurements the electrodes were degreased in distilled carbon tetrachloride and heated to 400OC in a hydrogen atmosphere for 1-2 h. To. prevent any contact of the electrode with the atmosphere the device represented in Fig. 1 was used. The electrode I was fastened to a chromium coated brass rod 2 tightly passing through a ground glass bearing 3. In the position shown in Fig. 1 the rod passed
FIG. 1. Apparatus for the manipulation of electrodes. 1, electrode; 2, rod; 3 glass bearings; 4, stopcock; 5, ground joint; 6, inlet of inert gas. through the opening in the stopcock 4 into the working space (eg a furnace for reduction in hydrogen or a vessel for the measurement of the electrochemical properties). If the electrode was to be transported from one working space to another, it was pulled upwards above stopcock 4 and this was closed. The small stopcock 6 served as an inlet for the maintenance of a moderate hydrogen pressure. Chemicals All measurements of hydrogen overpotential were carried out in 1 M KOH solution prepared by the decomposition of electrolytically prepared potassium amalgam by means of triply distilled water in a continuously working apparatus. The decomposition of the amalgam was accelerated with an auxiliary platinum cathode for hydrogen evolution, which was connected by an external circuit with the amalgam. The hydroxide solution thus prepared was further purified by pre-electrolysis between platinum electrodes for several days. Its efficiency was checked by measuring the The pure hydroxide solution did not contain decrease of mercury concentration.
1656
J. VONDRAK and J. BALHJ
any analytically detectable amount of mercury, using the method described below. The solution was diluted to the required concentration with triply redistilled water. Apparatus The hydrogen overvoltage was measured in an electrolytic vessel with three compartments separated by sintered glass disks. Both side arms contained auxiliary platinum anodes. The measured electrode was placed into the middle compartment containing also a Haber-Luggin capillary connected with the reference hydrogen electrode. Measurements were performed eitherpotentiostatically or galvanostatically. During the measurements the current or the potential were recorded. Poisoning of the electrodes was performed in a separate vessel by the electrolytic deposition of the required amount of mercury from a solution of potassium hydroxide saturated with mercury oxide (see below). The investigated electrode was used as cathode, the mercury pool served as anode, thus giving a constant Hg2+ concentration in the solution. Measuring procedure
At the beginning of the experiment the electrode was annealed in hydrogen and afterwards put into the vessel for the measurement of hydrogen overvoltage and its polarization curve in the pure solution was measured. A chosen amount of mercury was then deposited on the electrode by the method of Reader and Nilsen.s This method is based on simultaneous electrodeposition of poison and hydrogen from 1 M KOH solution saturated with yellow mercuric oxide. The excess of HgO keeps the mercuric ion concentration in the solution constant. The cd was much greater than the limiting cd of mercury deposition iHe and therefore the Hg2+ ion transport to the electrode was held at a constant rate by the mixing of the solution by simultaneously evolved hydrogen bubbles. It was found that the limiting cd iHg depends on the cd of hydrogen evolution i=, according to an empirical equation iHp = 1.63 x 104. igF8A/cmB.
(1)
This equation is valid for the range of cd 2 x 10es to 10-l A/cm2, independently of the electrode material and the temperature within the range of 20-6O”C, Fig. 2. The poisoned electrode was then rinsed in an inert atmosphere with deoxygenated water, transferred back to the vessel with the pure 1 M KOH solution and the polarization curve was measured. In experiments with platinum, after the completed measurement of the polarization curve the mercury was dissolved from the electrode surface in dilute nitric acid; after rinsing with distilled water and heating in a hydrogen atmosphere the electrode was prepared for next measurements. Repeated measurements on the other metals were always performed with new electrodes. All measurements of polarization curves were made at a temperature of 20 f O.l”C. AnaZyticaimethods
Extraction with a dithizone solution in the medium of an acetate buffer and Chelatone II&l1 which is very selective, was chosen for the determination of the amount of mercury on the electrode. For this purpose the electrode was treated for I min in dilute nitric acid and after neutralization of excess acid with ammonia the solution
Influence of mercury on hydrogen overvoltage on solid metal electrodes-I
1657
IO -
0.01
0.1
hi2 F%G. 2.
I
mA/cm’
Limiting cd of mercury deposition during simultaneous hydrogen development from a 1 N KOH solution saturated with yellow HgO.
was used for mercury determination, performed either by calorimetry or by extractive titration. The relative error of the determination in solutions containing O-l pg Hg was 7.7%. An unspecific but very sensitive catalytic effect of chromium, molybdenum and tungsten ions on the rate of the decomposition of pure sodium or potassium amalgam in solutions of alkali hydroxides or salt solutionP was used for the detection of traces of these ions in the solution. RESULTS
AND
DISCUSSION
The behaviour of various materials poisoned with mercury was usually compared by measuring the polarization curves of the pure electrode and the electrode, on which 11.1 rg/cma of Hg was deposited in the way described above with cd 5 mA/cms. This amount of mercury corresponds approximately to a complete poisoning of 1 ems of smooth electrode area. Figure 3 shows the rise in hydrogen overvoltage on a platinum electrode submerged into a solution saturated with mercuric oxide and &multaneously cathodically- polarized at a cd of 5 mA/cm2. The initial steep rise of the overvoltage becomes slower after 3 mm and after 5 min it has become approximately constant. Platinum Hydrogen overvoltage on pure electrodes and those posioned in the described way is demonstrated on Fig. 4. The overvoltage on a pure electrode can be expressed by a Tafel’s equation 7 = -0*800 - O-182 log i. (2) This result is in agreement with the measurement of Ammar and Darwish13 who found the relation 7 = -0-820 - O-168 log i for the hydrogen overvoltage on platinum previously reduced in a stream of hydrogen, for a O-1 N NaOH solution at 25Y!. For oxidized platinum electrodes, lower values of overvoltage were found,14J6 The overvoltage of hydrogen on an electrode poisoned by a standard amount of mercury can be expressed, for cd lower than 10 mA/cmS, by rl = -
l-316 -
O-194 log i.
(3)
3. VONLJU
1658
and J. BAUTI
Hiit.,4cm’ IO 1
1
1
I
. J .I
I
i =5mA/cm2
t
I 200
loo
I 400
I 300
IO
5
Fra. 3. Increase of overvoltage during mercury poisoning of Pt electrode. Mercury deposition at a total cd of 5 mA/cnP.
600-
500
-
> E
400-
6
300-
I 200
-
IOO-
-02
-I
I 0
t
log
i.
I I
I 2
mA /cm2
FIG. 4. Hydrogen overvoltage on several platinum electrodes. 1, pure electrode; 2, electrode after the deposition of 11-l &cm” Hg.
Influenceof mercuryon hydroen overvoltageon solidmetalelectrodes-1
1659
The Tafel constant a rises appreciably due to mercury poisoning, whereas the constant b remains almost unchanged. At cd > 10 mA/cms no stationary values of overvoltage were found; in the course of several hours the overvoltage slowly decreased, almost to the values found for the pure unpoisoned electrode. A more detailed description of this phenomenon, observed on platinum and other electrode materials, and its analysis, will be given in the next paper. Poisoning occurs also on a previously oxidized surface of the platinum electrode. The hydrogen overvoltage reaches almost the same values as on a poisoned electrode previously annealed in hydrogen; the attainment of a constant value, however, proceeds much more slowly than on unoxidized electrodes and may take several tens of hours. If the oxide layer was removed from the electrode surface by a treating in a 3 N HCl solution,le the hydrogen overvoltage was the same as that of the pure platinum electrodes annealed in hydrogen. Further, hydrogen overvoltage on pure oxidized electrodes was much lower than the value observed on electrodes reduced in hydrogen, in agreement with Ammar and Darwish.13 The electrodes were oxidized by treating in distilled azeotropic nitric acid for several weeks. Platinum electrodes made from foil 0.1 mm thick and heated in hydrogen to 400” for l-2 h could be used several times after cleaning their surface with nitric acid, as mentioned above. On the contrary, electrodes from foil O-05 mm thick and heated to 1100°C for 4-6 h changed after poisoning and removing of mercury by HNO,. Cathodic hydrogen evolution on such a poisoned electrode caused great embrittlement of the metal and a disintegration of the electrode surface; as a result, the amount of mercury required for complete electrode poisoning increased with the number of 30 r
0
I 1
I
I
2
3
I
4
I
5
n
FIG. 5. Influence of repeated poisoning and rinsing with acid on the amount of mercury necessary for complete poisoning. n, seriaI number of experimental runs with the same electrode.
J. VONDRAK and J. BALEJ
1660
done with the same electrode, Fig. 5. The hydrogen overvoltage changed in a similar way. Due to surface distintegration and roughness factor increasing, the hydrogen overvoltage of both pure and poisoned electrodes decrease with the number of experimental runs, Fig. 6. The disintegration of the surface is visible by a naked eye and sometimes proceeds to complete destruction of the electrode. runs
600 -
> E
200
081
IO
I
‘,
100
mA /cm2
FIG.6. lnfiuenceof repeatedcathodicpolarizationof poisonedelectrodeon the oyer-
voltage of hydrogenafter the removal of mercuryby means of nitricacid. 1, new pure electrode; 2, new electrodepoisoned for the Crst time; 3, new electrode poisonedfor thefirsttimeafterstrongcathodicpolarization; 4, pureelectrodeafterfirst poisoning, washingwith HNOl and reductionby hydrogen; 5, the same after second poisoning.
Iron
Figure 7 shows the behaviour of pure and poisoned iron electrodes. Up to ca 1 mA/cm2, the change of overvoltage caused by poisoning is insignificant; this is in agreement with the findings of Bagotskaya and 0she.6 At higher cd, a more pronounced influence of mercury on hydrogen overvoltage was observed. This explains why in the original Castner amalgam-hydrogen decomposition cell the iron cathodes were poisoned very rapidly, because cds were of the order of IO2mA/cm2. Nickei
Figure 8 shows the behaviour of the nickel electrode. The Tafel equation is not valid in the whole range of cd both for pure and poisoned electrodes, as already described.17*ls The constants a and b, presented for several different electrodes in Table 2, are evaluated from the linear part of polarization curves at low cd. Mercury poisoning of nickel electrodes gives an increase of hydrogen overvoltage, although the poisoning of nickel enhances the absolute value of both Tafel constants, b as well as a. The reproducibility of measurements on nickel electrodes was lower than on platinum.
Influence of mercury on hydrop
log
overvolbge 011solid metal ekctrodes-1
i.
1661
mA /cm2
Fro. 7. Polarization curve for iron electrode. Full line, pure electrode; dotted line, poisoned ekctrode. 3
500 -
.
I
2
;;:
//
/’ 0
, -2
I -I log i.
I 0
I I
I 2
mA/cm2
FIG. 8. Polarization curve for nickel electrode.
I,
pure; 2, 3.3 /q$m= Hg; 3,6-6 /q#xn’ I+@; 4, 1 l-1 lug/cm’Hg.
With the nickel electrode, the influence of various amounts of deposited mercury on hydrogen overvoltage was measured. The overvoltage increased with the increasing amount of poison; the initial fast rise asymptotically approached a limiting value corresponding apparently to complete poisoning of the electrode. Figure 9 shows the dependence of the Tafel constant a on the amount of poison on the nickel electrode. The poor reproducibility of the overvoltage on different samples of nickel is caused mainly by different slopes of the dependence of the constant a on the amount of poison, though the shape of the curve remains essentially the same and is similar to the curve in Fig. 3 found for platinum. The shape of these curves indicates that the first portions of poison enhance the overvoltage most effectively; the influence of further additions is less pronounced. Probably centres most active for the develop ment of hydrogen are blocked first. 7
J. VONDR.-~Kand J. BALEI
1662 TABLE
2. INFLUENCE OF MERCURY ON THE OVERVOLTAGE OF HYDROGEN ON NICKEL Tafel constants
Electrode
Amount
of mercury
---a
-b
V
No.
&%I~=
1
0 3.3 6-7 3.3 0
O-720
0.145
O-960 0.975 0.950 o-755
O-168 O-165 0.185 o-145
2:;
0.955 1.070 l-095 l-082 o-710 oa73 0.907 O-976 l-088 0.631 0.771 0.691 0.939 0.702 o-750 0,805 O-978 1.094
0.155 o-170 0.175 0.182 o-144 0.164 o-164 0.179 O-210 0.127 0.127 0.143 0.170 o-145 o-149 0.156 O-188 0.215
2 3
11.1
22.2 0 3.3 6.7 31.1 2232 0 3.3 0 11.1 0 3-3 6-7 11-l 22.2
4
5 6 7
From Fig. 9, it appears that 10-20 pg/cms (geometric surface) of mercury are necessary for complete poisoning. I cm* of the real surface of the nickel contains approximately 2 X 1015 atoms. If 1 atom of mercury is necessary for the poisoning of 1 atom of nickel, then on 1 cm* of the real surface of the poisoned electrode O-69 rug Hg would be present. If we accept a roughness factor ca 4, then 2-8 pg Hg would be necessary for a complete monolayer poisoning, ie 3-4 times lower than the observed value. The deposited mercury on nickel may form a continuous film several
zoot, IO I
I 20 Hg.
I 30
I
40
I
50
pg/cm’
FIG. 9. Influence of the amount of mercury on the values of the Tafel constant the evolution of hydrogen on nickel on several electrodes.
a for
1663
Influence of mercury on hydrogen overvoltage on solid metal electrodes-I
monolayers thick or microscopic aggregates ; the shape of these would depend on interphase forces and thus on the electrode potential.
Cobalt Figure 10 shows results for cobalt; mercury significantly increases the hydrogen overvoltage. The polarization curves for the pure and poisoned electrode correspond very well with the Tafel equation. Hydrogen evolution on pure cobalt can be expressed r] = -0.866
-
0.163 log i
(4)
and on an electrode poisoned by a standard amount of mercury by
(5)
0’
I IO-’
I
I
10-a
10-3
1,
A
/cm2
F&10. Hydrogen evolution on cobalt. 1, pure electrode; 2, poisoned electrode.
Chromium, molybdenum and tungsten The original assumption that these metals should better resist to mercury poisoning then the iron and platinum group metals because of their lower ability to form amalgams1° was proved. The results (Table 3) show that the hydrogen overvoltage on TABLE 3. METAL
Material Cr MO W
POISONING IN THE CHROMIUM
Amount of mercury Puglcn+ 0 11.1 0 11.1 2400 0 11-l
-a V O-707 0.686 0.732 0.749 O-746 o-642 0.608
SUBGROUP Tafel constants
b
0.1 10 0.138 0.142 o-1 17 0.128 0.095
J. VONDRAK and J. BALEJ
1664
chromium with a standard amount of mercury is even somewhat decreased in comparison with the unpoisoned electrode; on molybdenum it is almost the same and on tungsten moderately enhanced. Some corrosion of these metals was observed, although hydrogen was cathodically evolved on them: after finishing the measurements of stationary polarization curves, the catalytic testla showed the presence of their ions in the solution. The presence of molybdenum in the solution was shown even by the usual spot test.la The standard potentials of dissolution of these metals in alkaline medium are all more negative that the standard potential of hydrogen evolution. The presence of chromium, molybdenum and tungsten salts in the amalgam electrolytic cell is very harmful. lags If electrodes from these metals were used in the amalgam-hydrogen-decomposing cell, the possibihty of transfer of corrosion products from decomposer to electrolyser should be considered. However, a method for amalgam-decomposition acceleration, based on the addition of small amount of MO&~ soIution into the horizontal amalgam decomposeP and used for several years in industrial decomposers, shows that such compounds remain entirely in the decomposer and in no case affect the hydrogen evolution in the electrolyser. Alloy electrodes Besides pure metals, electrodeposited binary alloys from metals of both groups, Ni-W with a content of 30% W, Co-W (50% W) and Ni-Zn were studied. The results (Table 4) show that poisoning by a standard amount of mercury leads in both tungsten alloys to a marked increase of the hydrogen overvoltage. TABLE4.
POISONXNC3 OF ALLOY
ELECTRODES
Tafel constants Amount of mercury erg/a
Material 70% Ni, 30% W
50%
co,
50%
w
58 % Ni, 42% Zn
73% Ni, 27% Zn
0 11-l 0 11.1 1Y.l 44.4 222 666 0 11.1 44.4 222
V
-a
0.726 l-247 0565 0.882 0.383 O-363 o-373 0.372 0.615 0.480 0.505 O-606 1.077
b 0.113 0.187 0.121 0.180 0.091 0*090 o-093 ::E 0.131 0.130 o-157 0.144
The alloy Ni-Zn, containing according to the cd 27 or 42% of Zn respectively, belongs to the type of Raney alloys, as the less noble zinc dissolves in alkali hydroxides. Thus an electrode with a large active surface and low hydrogen overvoltage is developed. For example, on an electrode with an original content of 42 ‘A Zn, after a polarization of several hours in 1 N KOH solution at 2.8 mA/cm’ the overvoltage fell to -135 mV from an initial value of -395 mV. The influence of mercury poisoning on these eIectrodes is similar to the results obtained on smooth electrodes; however, due to the large spec~c surface area of the electrode, complete poisoning was attained after the deposition of much larger amounts of mercury than on smooth electrodes.
Influenceof mercuryon hydrogenovervoltageon solid metal electrodes-1
1665
CONCLUSIONS ‘I’be experiments prove that the technologically
important metaIs considered for the design of cathodes in an amalgam-hydrogen-decomposition cell because of their small hydrogen overvoltages are significantly poisoned by the deposited mercury. Hydrogen overvoltage on them increases by several hundreds of mV at room temperature. Metals from group VIb, especially chromium, where the deposited mercury does not enhance the hydrogen overvoltage, are more promising but the effect of their corrosion has to be considered. The hope that alloy electrodes of metals from the VIb group and metals from the iron group might combine their favourable properties was not borne out. The influence of elevated temperature and of higher current densities and pressures on the poisoned solid metal electrodes by mercury will be the subject of another paper. Acknowledgements-The authors wish to expresstheir sinceregratitudeto Frof. Dr. A. Regner for his interestin this investigationand for his valuableadviceand discussion.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Xl. 12. 13. 14. 15. 16. 17. 18. 19. W. 21.
REFERENCES J. BALEJ,Chem. Iisty 58, 1013 (1964). A. SEI_YO~~ and B. ERSHLIXR, Acta Physicochim. U.R.S.S. 11,45 (1939). M. G. E and K. W. Nnsrr~, Norges Tekn. Hisk. .Avhundl. 25,263 (1935). Dissertation,Moscow (1965). G. P. RHOMTSHENKO, I. E. BAGOTSKAYAand A. 1. OSHE,Zh. fiz. Khim. 32,1379 (1958). Efectrochim. Acta 9, 1203 (1964). S. Tosrurvand B. MUT-, R. KAISH~Vand B. MUTAFTSHIEY, Electrochim. Acta 7, 643 (1%2). R. m and B. MUT-, Z. phys. Chem. 204,334 (1955). D. J. m, J. A. HARRISON and H. R. THIRSK, Trans. Fara&y Sot. 64,192 (1968). J. B-, I. PASEKA and V. KOUD~LKA,Chem. p&n. 14,395 (1964). R. F~&~IL,Complexonesin Chemical Analysis, pp. 177,235. CSAV, Fraha (1957). J. Bm, V. ROUDELKA and I. PASEKA,Chem.prim. 14,113 (1964). I. A. AMMARand S. D-WISH, J. phys. Chem. 63,983 (3959). P. IDOLIN and B. ERSHLBR, Acta Physicochim. U.R.S.S. 13,747 (1940). P. Do=. B. E-R and A. N. FRUMKIN. Aeta Physicochim. U.R.S.S. 13,779 (1940). M. W. Baarraaand J. L. WEI?-XIN~BR. J. elecrrochem. Sot. 109,1135 (1962). H. Rrr~ and T. YAMAWU, J. Z&s. Inst. Catalysis Hokkaido Univ. l&10 (1963). A. V. -SXAYA and G. A. TSYC+ANOV. Zzv. Akad. iV&. Uzbek. SSR, 13.27 (1957). N. A. TANAN~, Spot A~lySis of Inorganic Substances SNTL, Praha (1957). M. M. J&c and I. M. &NKA, Electrochem. Technol. 4,49 (1966). A. BANNER,Electrodeposition of Alloys, p. 358. Academic F’ress,New York (1963).
Actrr. 1970. VoI. 15, pp. 1653
to 1665. Pcrmunoa Rem.
Printed in Northern Ireland
INFLUENCE OF MERCURY ON HYDROGEN OVERVOLTAGE ON SOLID METAL ELECTRODES-I. STATIONARY POLARIZATION CURVES OF HYDROGEN DEPOSITION ON PURE AND POISONED ELECTRODES* and J. BALEJ
J. VONDRAK
Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia Abstract-Mercury deposited in amounts of the order of a few monolayers on Pt, Fe, Ni and Co raises the overvoltage of hydrogen evolution in alkaline solution. On Cr it deaeases the overvoltage and on MO it has little influence, but these metals corrode in alkali.
R&mn&Le mercure deposalen quantit&s de l’ordre de quelques couches simples sur Pt, Fe, Ni et Co Cl&e la surtension de l%volution d’hydrogkne en solution alcaliue. Sur Cr, il diminue la surteusion et sur Mo il a peu d’influence, mais ces m&aux se corrodent en solutions alcalines. Zusammenfass~-Die Wasserstoffiiberspannung an Pt, Fe, Ni und Co in alkalischer Lasung wird durch in einigen Monoschichten abgeschiedenes Quecksilber erhtiht. Bei Cr wird die Uberspannung verringert, und bei MO ist der Einfluss gering, diese Mctalle korrodieren jedoch in alkalischer LBsung.
DURING
the production
electrolysis, reaction,
an amalgam
which
of chlorine
and
alkali hydroxides
of the alkali metal
is decomposed
by
water
is formed
by means
as the product
in a separate
device
of amalgam
of the cathodic
to form
the alkali
The energy of this decomposing reaction could be metal hydroxide and hydrogen. utilized in a galvanic cell where sodium from the amalgam would be ionized on the anode and hydrogen would be deposited on the cathode. This process could not be carried out on a commercial scaleuntil now, for reasons discussed in adetaiIed review by Balej .l For the successful operation of the amalgam-hydrogen-decomposition hydrogen electrodes with negligible hydrogen overvoltage and a lifetime
possible are required.
cell,
as long as
As mercury is a component of the amalgam decomposer the has to maintain its favourable properties even in the presence of that metal and its compounds. Mercury and its compounds are rather firmly bound to the surface of electrodes, which causes their poisoning and increase of the hydrogen overvoltage. As a resuIt the lifetime of the decomposer is reduced. Other substances, such as arsenic, sulphides, cyanides etc have a similar influence. The poisoning of platinum electrodes by mercury during the electrodeposition of hydrogen has been described by Slygin and Ershler, 2 Reader and Nilsen and more thoroughly by Khomtshenko.4 According to Reader and Nilsen,s nickel electrodes also are poisoned by mercury. Similar results were reported by 3agotskaya and Oshe.6 These authors also found only a small influence of mercury on iron electrodes, especially in an alkaline medium. The morphology of mercury deposited on spherical platinum single crystals was studied by Kaishev, Toshev and MutaftschieFs and the deposition of mercury on a glassy carbon electrode was investigated by Astley, Harrison and Thirsk.s On both used hydrogen
l
electrode
Manuscript received 2 July 1969. 1653
J. VONDRAK and J. BALEJ
1654
these materials mercury is deposited not as a thin layer, but as small droplets, the number of which-in case of platinum--depends on the orientation of the electrode surface. In most of the above mentioned papers, the results on various materials were not obtained under equivalent conditions. Therefore the subject of the present communication is the measurement of the influence of mercury poisoning of various electrode materials (pure metals and alloys), where the evolution of hydrogen from solutions of alkaline hydroxides proceeds under comparable conditions. The poisoning was measured on technically available pure metals with low hydrogen overvoltage in alkaline medium, ie on iron, nickel and cobalt, and further on the metals of the VIb group, chromium, molybdenum and tungsten. As they do not form amalgamslO they should best resist mercury poisoning. The poisoning action of mercury was investigated moreover on some binary alloys of these metals, which might combine the favourable properties of a low hydrogen overvoltage with resistance to mercury poisoning. For comparison, some experiments were carried out with platinum. EXPERIMENTAL
TECHNIQUE
Electrodes Measurements were performed with smooth electrodes. All electrodes were 1 x 1 cm. Those of Ft, Ni, MO and W were mostly made from foil O-1 mm thick with a welded wire from the same material. Some of the platinum electrodes were prepared from foil O-05 mm thick. The other electrodes were prepared by electrodeposition according to Table 1 on a copper electrode previously electrolytically TABLE 1. Metal
co Fe Cr 70% Ni 3O%W
50% 50%
co w
73% Ni 27% Zn 58% Ni 42% Zn
c4WDlTIONS
Bath composition coso~~7H,o NaCl H&0l FeCI,-4H,O C&la H,O CrOB
504 g/l 17g/l 45 g/l 120 g/l 5Og 3ooml 200 g/l
Na3W0,-2HI0 (NH&SO, NH. (25 %I coso,*7H*o NasWOI-2HI0
17 g/i 150 g/l 60 ml/l 60 g/l 70 g/l 66 g/l
NH,OH to pH7 NiSO&HIO ZnSO,.7H,o HaO. NiSOI*7Hs0 Znso,*7H,o HsBOs
50 g/l 2-9 g/l 80 g/l 50 g/l 2.9 8/l 80 g/l
OF METAL
DEPOSlTION
Temperature “C
Cd mA/cms
20
50
75
15
50
450
75
300
21
80
50
21
20
7,75
20
491
Reference
21
In&xxx
of mercury on hydrogen overvoltage on solid metal electrodes-I
1655
The polished in phosphoric acid. The purity of the metals was at least 99.9%. molybdenum and tungsten electrodes were previously polished in a bath containing 75 % of glycerol and 5 % KOH, at 0.2 A/cm*. Before the measurements the electrodes were degreased in distilled carbon tetrachloride and heated to 400OC in a hydrogen atmosphere for 1-2 h. To. prevent any contact of the electrode with the atmosphere the device represented in Fig. 1 was used. The electrode I was fastened to a chromium coated brass rod 2 tightly passing through a ground glass bearing 3. In the position shown in Fig. 1 the rod passed
FIG. 1. Apparatus for the manipulation of electrodes. 1, electrode; 2, rod; 3 glass bearings; 4, stopcock; 5, ground joint; 6, inlet of inert gas. through the opening in the stopcock 4 into the working space (eg a furnace for reduction in hydrogen or a vessel for the measurement of the electrochemical properties). If the electrode was to be transported from one working space to another, it was pulled upwards above stopcock 4 and this was closed. The small stopcock 6 served as an inlet for the maintenance of a moderate hydrogen pressure. Chemicals All measurements of hydrogen overpotential were carried out in 1 M KOH solution prepared by the decomposition of electrolytically prepared potassium amalgam by means of triply distilled water in a continuously working apparatus. The decomposition of the amalgam was accelerated with an auxiliary platinum cathode for hydrogen evolution, which was connected by an external circuit with the amalgam. The hydroxide solution thus prepared was further purified by pre-electrolysis between platinum electrodes for several days. Its efficiency was checked by measuring the The pure hydroxide solution did not contain decrease of mercury concentration.
1656
J. VONDRAK and J. BALHJ
any analytically detectable amount of mercury, using the method described below. The solution was diluted to the required concentration with triply redistilled water. Apparatus The hydrogen overvoltage was measured in an electrolytic vessel with three compartments separated by sintered glass disks. Both side arms contained auxiliary platinum anodes. The measured electrode was placed into the middle compartment containing also a Haber-Luggin capillary connected with the reference hydrogen electrode. Measurements were performed eitherpotentiostatically or galvanostatically. During the measurements the current or the potential were recorded. Poisoning of the electrodes was performed in a separate vessel by the electrolytic deposition of the required amount of mercury from a solution of potassium hydroxide saturated with mercury oxide (see below). The investigated electrode was used as cathode, the mercury pool served as anode, thus giving a constant Hg2+ concentration in the solution. Measuring procedure
At the beginning of the experiment the electrode was annealed in hydrogen and afterwards put into the vessel for the measurement of hydrogen overvoltage and its polarization curve in the pure solution was measured. A chosen amount of mercury was then deposited on the electrode by the method of Reader and Nilsen.s This method is based on simultaneous electrodeposition of poison and hydrogen from 1 M KOH solution saturated with yellow mercuric oxide. The excess of HgO keeps the mercuric ion concentration in the solution constant. The cd was much greater than the limiting cd of mercury deposition iHe and therefore the Hg2+ ion transport to the electrode was held at a constant rate by the mixing of the solution by simultaneously evolved hydrogen bubbles. It was found that the limiting cd iHg depends on the cd of hydrogen evolution i=, according to an empirical equation iHp = 1.63 x 104. igF8A/cmB.
(1)
This equation is valid for the range of cd 2 x 10es to 10-l A/cm2, independently of the electrode material and the temperature within the range of 20-6O”C, Fig. 2. The poisoned electrode was then rinsed in an inert atmosphere with deoxygenated water, transferred back to the vessel with the pure 1 M KOH solution and the polarization curve was measured. In experiments with platinum, after the completed measurement of the polarization curve the mercury was dissolved from the electrode surface in dilute nitric acid; after rinsing with distilled water and heating in a hydrogen atmosphere the electrode was prepared for next measurements. Repeated measurements on the other metals were always performed with new electrodes. All measurements of polarization curves were made at a temperature of 20 f O.l”C. AnaZyticaimethods
Extraction with a dithizone solution in the medium of an acetate buffer and Chelatone II&l1 which is very selective, was chosen for the determination of the amount of mercury on the electrode. For this purpose the electrode was treated for I min in dilute nitric acid and after neutralization of excess acid with ammonia the solution
Influence of mercury on hydrogen overvoltage on solid metal electrodes-I
1657
IO -
0.01
0.1
hi2 F%G. 2.
I
mA/cm’
Limiting cd of mercury deposition during simultaneous hydrogen development from a 1 N KOH solution saturated with yellow HgO.
was used for mercury determination, performed either by calorimetry or by extractive titration. The relative error of the determination in solutions containing O-l pg Hg was 7.7%. An unspecific but very sensitive catalytic effect of chromium, molybdenum and tungsten ions on the rate of the decomposition of pure sodium or potassium amalgam in solutions of alkali hydroxides or salt solutionP was used for the detection of traces of these ions in the solution. RESULTS
AND
DISCUSSION
The behaviour of various materials poisoned with mercury was usually compared by measuring the polarization curves of the pure electrode and the electrode, on which 11.1 rg/cma of Hg was deposited in the way described above with cd 5 mA/cms. This amount of mercury corresponds approximately to a complete poisoning of 1 ems of smooth electrode area. Figure 3 shows the rise in hydrogen overvoltage on a platinum electrode submerged into a solution saturated with mercuric oxide and &multaneously cathodically- polarized at a cd of 5 mA/cm2. The initial steep rise of the overvoltage becomes slower after 3 mm and after 5 min it has become approximately constant. Platinum Hydrogen overvoltage on pure electrodes and those posioned in the described way is demonstrated on Fig. 4. The overvoltage on a pure electrode can be expressed by a Tafel’s equation 7 = -0*800 - O-182 log i. (2) This result is in agreement with the measurement of Ammar and Darwish13 who found the relation 7 = -0-820 - O-168 log i for the hydrogen overvoltage on platinum previously reduced in a stream of hydrogen, for a O-1 N NaOH solution at 25Y!. For oxidized platinum electrodes, lower values of overvoltage were found,14J6 The overvoltage of hydrogen on an electrode poisoned by a standard amount of mercury can be expressed, for cd lower than 10 mA/cmS, by rl = -
l-316 -
O-194 log i.
(3)
3. VONLJU
1658
and J. BAUTI
Hiit.,4cm’ IO 1
1
1
I
. J .I
I
i =5mA/cm2
t
I 200
loo
I 400
I 300
IO
5
Fra. 3. Increase of overvoltage during mercury poisoning of Pt electrode. Mercury deposition at a total cd of 5 mA/cnP.
600-
500
-
> E
400-
6
300-
I 200
-
IOO-
-02
-I
I 0
t
log
i.
I I
I 2
mA /cm2
FIG. 4. Hydrogen overvoltage on several platinum electrodes. 1, pure electrode; 2, electrode after the deposition of 11-l &cm” Hg.
Influenceof mercuryon hydroen overvoltageon solidmetalelectrodes-1
1659
The Tafel constant a rises appreciably due to mercury poisoning, whereas the constant b remains almost unchanged. At cd > 10 mA/cms no stationary values of overvoltage were found; in the course of several hours the overvoltage slowly decreased, almost to the values found for the pure unpoisoned electrode. A more detailed description of this phenomenon, observed on platinum and other electrode materials, and its analysis, will be given in the next paper. Poisoning occurs also on a previously oxidized surface of the platinum electrode. The hydrogen overvoltage reaches almost the same values as on a poisoned electrode previously annealed in hydrogen; the attainment of a constant value, however, proceeds much more slowly than on unoxidized electrodes and may take several tens of hours. If the oxide layer was removed from the electrode surface by a treating in a 3 N HCl solution,le the hydrogen overvoltage was the same as that of the pure platinum electrodes annealed in hydrogen. Further, hydrogen overvoltage on pure oxidized electrodes was much lower than the value observed on electrodes reduced in hydrogen, in agreement with Ammar and Darwish.13 The electrodes were oxidized by treating in distilled azeotropic nitric acid for several weeks. Platinum electrodes made from foil 0.1 mm thick and heated in hydrogen to 400” for l-2 h could be used several times after cleaning their surface with nitric acid, as mentioned above. On the contrary, electrodes from foil O-05 mm thick and heated to 1100°C for 4-6 h changed after poisoning and removing of mercury by HNO,. Cathodic hydrogen evolution on such a poisoned electrode caused great embrittlement of the metal and a disintegration of the electrode surface; as a result, the amount of mercury required for complete electrode poisoning increased with the number of 30 r
0
I 1
I
I
2
3
I
4
I
5
n
FIG. 5. Influence of repeated poisoning and rinsing with acid on the amount of mercury necessary for complete poisoning. n, seriaI number of experimental runs with the same electrode.
J. VONDRAK and J. BALEJ
1660
done with the same electrode, Fig. 5. The hydrogen overvoltage changed in a similar way. Due to surface distintegration and roughness factor increasing, the hydrogen overvoltage of both pure and poisoned electrodes decrease with the number of experimental runs, Fig. 6. The disintegration of the surface is visible by a naked eye and sometimes proceeds to complete destruction of the electrode. runs
600 -
> E
200
081
IO
I
‘,
100
mA /cm2
FIG.6. lnfiuenceof repeatedcathodicpolarizationof poisonedelectrodeon the oyer-
voltage of hydrogenafter the removal of mercuryby means of nitricacid. 1, new pure electrode; 2, new electrodepoisoned for the Crst time; 3, new electrode poisonedfor thefirsttimeafterstrongcathodicpolarization; 4, pureelectrodeafterfirst poisoning, washingwith HNOl and reductionby hydrogen; 5, the same after second poisoning.
Iron
Figure 7 shows the behaviour of pure and poisoned iron electrodes. Up to ca 1 mA/cm2, the change of overvoltage caused by poisoning is insignificant; this is in agreement with the findings of Bagotskaya and 0she.6 At higher cd, a more pronounced influence of mercury on hydrogen overvoltage was observed. This explains why in the original Castner amalgam-hydrogen decomposition cell the iron cathodes were poisoned very rapidly, because cds were of the order of IO2mA/cm2. Nickei
Figure 8 shows the behaviour of the nickel electrode. The Tafel equation is not valid in the whole range of cd both for pure and poisoned electrodes, as already described.17*ls The constants a and b, presented for several different electrodes in Table 2, are evaluated from the linear part of polarization curves at low cd. Mercury poisoning of nickel electrodes gives an increase of hydrogen overvoltage, although the poisoning of nickel enhances the absolute value of both Tafel constants, b as well as a. The reproducibility of measurements on nickel electrodes was lower than on platinum.
Influence of mercury on hydrop
log
overvolbge 011solid metal ekctrodes-1
i.
1661
mA /cm2
Fro. 7. Polarization curve for iron electrode. Full line, pure electrode; dotted line, poisoned ekctrode. 3
500 -
.
I
2
;;:
//
/’ 0
, -2
I -I log i.
I 0
I I
I 2
mA/cm2
FIG. 8. Polarization curve for nickel electrode.
I,
pure; 2, 3.3 /q$m= Hg; 3,6-6 /q#xn’ I+@; 4, 1 l-1 lug/cm’Hg.
With the nickel electrode, the influence of various amounts of deposited mercury on hydrogen overvoltage was measured. The overvoltage increased with the increasing amount of poison; the initial fast rise asymptotically approached a limiting value corresponding apparently to complete poisoning of the electrode. Figure 9 shows the dependence of the Tafel constant a on the amount of poison on the nickel electrode. The poor reproducibility of the overvoltage on different samples of nickel is caused mainly by different slopes of the dependence of the constant a on the amount of poison, though the shape of the curve remains essentially the same and is similar to the curve in Fig. 3 found for platinum. The shape of these curves indicates that the first portions of poison enhance the overvoltage most effectively; the influence of further additions is less pronounced. Probably centres most active for the develop ment of hydrogen are blocked first. 7
J. VONDR.-~Kand J. BALEI
1662 TABLE
2. INFLUENCE OF MERCURY ON THE OVERVOLTAGE OF HYDROGEN ON NICKEL Tafel constants
Electrode
Amount
of mercury
---a
-b
V
No.
&%I~=
1
0 3.3 6-7 3.3 0
O-720
0.145
O-960 0.975 0.950 o-755
O-168 O-165 0.185 o-145
2:;
0.955 1.070 l-095 l-082 o-710 oa73 0.907 O-976 l-088 0.631 0.771 0.691 0.939 0.702 o-750 0,805 O-978 1.094
0.155 o-170 0.175 0.182 o-144 0.164 o-164 0.179 O-210 0.127 0.127 0.143 0.170 o-145 o-149 0.156 O-188 0.215
2 3
11.1
22.2 0 3.3 6.7 31.1 2232 0 3.3 0 11.1 0 3-3 6-7 11-l 22.2
4
5 6 7
From Fig. 9, it appears that 10-20 pg/cms (geometric surface) of mercury are necessary for complete poisoning. I cm* of the real surface of the nickel contains approximately 2 X 1015 atoms. If 1 atom of mercury is necessary for the poisoning of 1 atom of nickel, then on 1 cm* of the real surface of the poisoned electrode O-69 rug Hg would be present. If we accept a roughness factor ca 4, then 2-8 pg Hg would be necessary for a complete monolayer poisoning, ie 3-4 times lower than the observed value. The deposited mercury on nickel may form a continuous film several
zoot, IO I
I 20 Hg.
I 30
I
40
I
50
pg/cm’
FIG. 9. Influence of the amount of mercury on the values of the Tafel constant the evolution of hydrogen on nickel on several electrodes.
a for
1663
Influence of mercury on hydrogen overvoltage on solid metal electrodes-I
monolayers thick or microscopic aggregates ; the shape of these would depend on interphase forces and thus on the electrode potential.
Cobalt Figure 10 shows results for cobalt; mercury significantly increases the hydrogen overvoltage. The polarization curves for the pure and poisoned electrode correspond very well with the Tafel equation. Hydrogen evolution on pure cobalt can be expressed r] = -0.866
-
0.163 log i
(4)
and on an electrode poisoned by a standard amount of mercury by
(5)
0’
I IO-’
I
I
10-a
10-3
1,
A
/cm2
F&10. Hydrogen evolution on cobalt. 1, pure electrode; 2, poisoned electrode.
Chromium, molybdenum and tungsten The original assumption that these metals should better resist to mercury poisoning then the iron and platinum group metals because of their lower ability to form amalgams1° was proved. The results (Table 3) show that the hydrogen overvoltage on TABLE 3. METAL
Material Cr MO W
POISONING IN THE CHROMIUM
Amount of mercury Puglcn+ 0 11.1 0 11.1 2400 0 11-l
-a V O-707 0.686 0.732 0.749 O-746 o-642 0.608
SUBGROUP Tafel constants
b
0.1 10 0.138 0.142 o-1 17 0.128 0.095
J. VONDRAK and J. BALEJ
1664
chromium with a standard amount of mercury is even somewhat decreased in comparison with the unpoisoned electrode; on molybdenum it is almost the same and on tungsten moderately enhanced. Some corrosion of these metals was observed, although hydrogen was cathodically evolved on them: after finishing the measurements of stationary polarization curves, the catalytic testla showed the presence of their ions in the solution. The presence of molybdenum in the solution was shown even by the usual spot test.la The standard potentials of dissolution of these metals in alkaline medium are all more negative that the standard potential of hydrogen evolution. The presence of chromium, molybdenum and tungsten salts in the amalgam electrolytic cell is very harmful. lags If electrodes from these metals were used in the amalgam-hydrogen-decomposing cell, the possibihty of transfer of corrosion products from decomposer to electrolyser should be considered. However, a method for amalgam-decomposition acceleration, based on the addition of small amount of MO&~ soIution into the horizontal amalgam decomposeP and used for several years in industrial decomposers, shows that such compounds remain entirely in the decomposer and in no case affect the hydrogen evolution in the electrolyser. Alloy electrodes Besides pure metals, electrodeposited binary alloys from metals of both groups, Ni-W with a content of 30% W, Co-W (50% W) and Ni-Zn were studied. The results (Table 4) show that poisoning by a standard amount of mercury leads in both tungsten alloys to a marked increase of the hydrogen overvoltage. TABLE4.
POISONXNC3 OF ALLOY
ELECTRODES
Tafel constants Amount of mercury erg/a
Material 70% Ni, 30% W
50%
co,
50%
w
58 % Ni, 42% Zn
73% Ni, 27% Zn
0 11-l 0 11.1 1Y.l 44.4 222 666 0 11.1 44.4 222
V
-a
0.726 l-247 0565 0.882 0.383 O-363 o-373 0.372 0.615 0.480 0.505 O-606 1.077
b 0.113 0.187 0.121 0.180 0.091 0*090 o-093 ::E 0.131 0.130 o-157 0.144
The alloy Ni-Zn, containing according to the cd 27 or 42% of Zn respectively, belongs to the type of Raney alloys, as the less noble zinc dissolves in alkali hydroxides. Thus an electrode with a large active surface and low hydrogen overvoltage is developed. For example, on an electrode with an original content of 42 ‘A Zn, after a polarization of several hours in 1 N KOH solution at 2.8 mA/cm’ the overvoltage fell to -135 mV from an initial value of -395 mV. The influence of mercury poisoning on these eIectrodes is similar to the results obtained on smooth electrodes; however, due to the large spec~c surface area of the electrode, complete poisoning was attained after the deposition of much larger amounts of mercury than on smooth electrodes.
Influenceof mercuryon hydrogenovervoltageon solid metal electrodes-1
1665
CONCLUSIONS ‘I’be experiments prove that the technologically
important metaIs considered for the design of cathodes in an amalgam-hydrogen-decomposition cell because of their small hydrogen overvoltages are significantly poisoned by the deposited mercury. Hydrogen overvoltage on them increases by several hundreds of mV at room temperature. Metals from group VIb, especially chromium, where the deposited mercury does not enhance the hydrogen overvoltage, are more promising but the effect of their corrosion has to be considered. The hope that alloy electrodes of metals from the VIb group and metals from the iron group might combine their favourable properties was not borne out. The influence of elevated temperature and of higher current densities and pressures on the poisoned solid metal electrodes by mercury will be the subject of another paper. Acknowledgements-The authors wish to expresstheir sinceregratitudeto Frof. Dr. A. Regner for his interestin this investigationand for his valuableadviceand discussion.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Xl. 12. 13. 14. 15. 16. 17. 18. 19. W. 21.
REFERENCES J. BALEJ,Chem. Iisty 58, 1013 (1964). A. SEI_YO~~ and B. ERSHLIXR, Acta Physicochim. U.R.S.S. 11,45 (1939). M. G. E and K. W. Nnsrr~, Norges Tekn. Hisk. .Avhundl. 25,263 (1935). Dissertation,Moscow (1965). G. P. RHOMTSHENKO, I. E. BAGOTSKAYAand A. 1. OSHE,Zh. fiz. Khim. 32,1379 (1958). Efectrochim. Acta 9, 1203 (1964). S. Tosrurvand B. MUT-, R. KAISH~Vand B. MUTAFTSHIEY, Electrochim. Acta 7, 643 (1%2). R. m and B. MUT-, Z. phys. Chem. 204,334 (1955). D. J. m, J. A. HARRISON and H. R. THIRSK, Trans. Fara&y Sot. 64,192 (1968). J. B-, I. PASEKA and V. KOUD~LKA,Chem. p&n. 14,395 (1964). R. F~&~IL,Complexonesin Chemical Analysis, pp. 177,235. CSAV, Fraha (1957). J. Bm, V. ROUDELKA and I. PASEKA,Chem.prim. 14,113 (1964). I. A. AMMARand S. D-WISH, J. phys. Chem. 63,983 (3959). P. IDOLIN and B. ERSHLBR, Acta Physicochim. U.R.S.S. 13,747 (1940). P. Do=. B. E-R and A. N. FRUMKIN. Aeta Physicochim. U.R.S.S. 13,779 (1940). M. W. Baarraaand J. L. WEI?-XIN~BR. J. elecrrochem. Sot. 109,1135 (1962). H. Rrr~ and T. YAMAWU, J. Z&s. Inst. Catalysis Hokkaido Univ. l&10 (1963). A. V. -SXAYA and G. A. TSYC+ANOV. Zzv. Akad. iV&. Uzbek. SSR, 13.27 (1957). N. A. TANAN~, Spot A~lySis of Inorganic Substances SNTL, Praha (1957). M. M. J&c and I. M. &NKA, Electrochem. Technol. 4,49 (1966). A. BANNER,Electrodeposition of Alloys, p. 358. Academic F’ress,New York (1963).