- Email: [email protected]
The specific adsorption of cations on electrodes—I. The adsorption of thallium on platinum at controlled potentials
Electrochimica Acta, 1965, Vol. 10, pp. 717 to 729.
Pergamon Press Ltd.
Printed in Northern Ireland
THE SPECIFIC ADSORPTION OF CATIONS ON ELECTRODES-I. THE ADSORPTION OF THALLIUM ON PLATINUM AT CONTROLLED POTENTIALS * B. J. BOWLES Atomic Energy Research Establishment,
Harwell, England
Abstract-The specific adsorption of thallous ion on to platinum at controlled potential from very dilute solutions in 0.1 M HCI has been studied by means of a radioactive tracer technique. It is shown that adsorption of up to a monolayer of thallium takes place at potentials far removed from the potential for bulk deposition of thallium metal, and that two different adsorption mechanisms appear to be present. An explanation is advanced for the change in apparent saturation surface coverage with applied potential. The integral adsorption coefficient is presented as a function of potential and surface coverage. Resume-L’adsorption specifique de l’ion Thalleux sur Pt a potentiel controle et a partir de solutions diluees dans HCl.O,l M est etudiee au moyen de traceurs radioactifs. Des des potentiels notablement differents de celui correspondant au depart de Tl mttallique massif, l’adsorption se manifeste au dela dune couche monomoleculaire et il semble que deux mecanismes d’adsorption se conjuguent. Une interpretation est suggeree pour rendre compte du changement de saturation apparente du recouvrement superficiel selon la tension appliqde. Le coefficient integral d’absorption est exprime en fonction du potentiel et du recouvrement superticiel. Zusammenfassung-Die spezifische Adsorption von Thallium Ionen bei geregelten Potentiellen von sehr verdtinnten Losungen in 0.1 M HCI, wurde mit Hilfe einer Radioaktiven Technik untersucht. Es wird gezeigt, das die Adsorption bis zu einer Einzelschicht von Thallium durch Potentiellen stattfindet, welche sehr weit von der Potentiellen ftir Massabsetzung von Thalliummetall entfernt sind, und das zwei verschiedene Absorption Mechanismus anwesend zu sein scheinen. Eine Erklarung ist erbracht fiir Anderung in den scheinbaren Slttigungsoberfllchen Abdeckungen mit angewandten Potentiell. Der Integrale Absorption Koeffizient ist als eine Funktion von potentialer und Oberflachen Abdeckung dargestellt. THE phenomenon of adsorption of material at a metal/solution interface as a consequence of the electrical and chemical forces acting at a boundary has been known
for many years. 1-4 Most of the results which have been reported in the literature have been obtained by electrocapillarity and capacitance measurements on liquid metal surfaces in contact with aqueous solutions, the metal most commonly used being mercury. In recent years a number of investigations involving solid electrodes has been made, such studies being almost entirely confined to platinum in both the suitable for such investigasmooth5 and platinized 5s6forms. Platinum is particularly tions as its passivity makes it possible to study adsorption over a large range of potential. Adsorption of neutral substances has been known for many years1,2 and lately the specific adsorption of anions has been investigated. Some adsorption of cations was noted and it was at first assumed that this occurred at the outer Helmholtz layer by the electrostatic attraction of excess adsorbed anions.5 More recently however it has been shown by Obrucheva that cations can exhibit marked specific adsorption on platinum electrodes from fairly concentrated solutions.6 In the work described here the concentrations investigated have been of the order of 1 ,uM. * Manuscript received 22 July 1964. 1
717
B. J. BOWLES
718
The present paper is devoted to the adsorption of thallium on platinum from a solution in hydrochloric acid. The system platinum-electrode/thallium-ion-adsorbent was chosen as it was known from previous work that thallium is particularly strongly adsorbed on platinum. 7 In addition, the isotope 204T1can be obtained with very high specific activity and hence its concentration can be observed to very low levels. EXPERIMENTAL
PROCEDURE
204T1 with a specific activity of about 200 me/g was obtained from the Radiochemical Centre at Amersham. It was purified and taken into O-1 M HCl as thallous WORKING ELECTRODE
/
-1: L.
i
CONTROLLINt ELECTRODE-
2
c,
.G.M. COUNTER
TO
GLASS St NTER
iuvlP
FIG. 1. Apparatus.
chloride. The concentration of thallous ion was determined by the method of standard additions using a square-wave polarograph, these determinations being repeated at The hydrochloric acid used was intervals to check on any changes in concentration. highly purified and contained, for instance, less than 0.005 ppm of lead. All solutions were made up with triple-distilled water. The platinum was of 99.999 per cent purity in the form of wire O-005 in. dia., and was cut into billets of approximately 5 mm in length. Its surface area calculated from the dimensions of the wire billets was 407 cm2. It was cleaned by boiling in fuming nitric acid for several days prior to use. The apparatus is shown in Fig. 1. 100 ml of solution could be passed continuously
Specific adsorption of cations on electrodes--I.
Tlf on Pt
719
round the apparatus via the platinum billets and the immersion Geiger counter by means of the lift pump. The lift pump was operated by argon purified by passing over “Hopcalite” and heated Cu/CuO. This argon also served to remove oxygen from the solution. It was passed through a gas bubbler containing the base solution (HCl) prior to the lift pump to prevent evaporation of the solution in the apparatus. All surfaces in contact with the solution were double sealed to prevent capillary rise and evaporation leading to loss of solution. Electrical connexions were made to the platinum bed, to the reference electrode which connected to the solution at the surface of the bed via the Luggin capillary, and to the controlling electrode, which was also connected to the cell via the crescent-bore stopcock. The apparatus was constructed from Pyrex, since it was found that soft glass contains readily leachable constituents that adsorb on platinum (eg Cu, Pb). The potential of the platinum could be held steady by means of a potentiostat connected to the reference electrode, the controlling electrode and to the platinum bed or column. It was possible to change the potential of the platinum rapidly to a new value and the charge required to attain the new potential could be monitored; hence charging curves could be constructed. All potentials are referred to the reversible hydrogen potential in the same solution. The concentration of thallium in the solution was followed by observing the tube, which was immersed in p-activity of the 204T1 by means of the Geiger-counter the solution (Fig. 1). The efficiency of the counter was quite low due to the relatively large proportion of the total volume of solution contained in parts of the apparatus not in the immediate vicinity of the counter. This fact, in combination with the low energy of the p-particle emitted from 204T1(0.76 MeV), reduced the efficiency of the counter to the region of t-1 per cent. The background to the counter was about 0.7 count/s and an increase of 0.2 count/s could be observed; hence the system was sensitive to the release of 3 x lo-l1 moles of thallium from the electrode into a pure solution of the supporting electrolyte. (204T1 has a half-life of 4.1 y and hence its decay is negligible in a time comparable to that of an experimental run.) Prior to each run, the platinum bed was taken through several potential excursions over the range 0.12 to 1.02 V in the supporting electrolyte only, with the argon-lift pump operating at a standard flow-rate, until reproducible charging curves were obtained. As a check on the state of the platinum surface the characteristics of the charging curve were compared with those preceding other runs. As the platinum was always in contact with some of the solution whereas the solution was out of contact with the platinum for most of the time, it was expected that equilibrium would be established much more rapidly if the starting point of the experiment was with the thallium adsorbed on the platinum rather than in solution. A simplified mathematical solution of this problem assuming no turbulent mixing and a single theoretical plate was obtained from the Mercury computer. This confirmed that equilibrium was established much more rapidly if the desorption rather than the adsorption was studied, and this procedure was adopted. After it had been established that the platinum surface had not altered in character and that the supporting electrolyte in the cell was free of contaminants, an aliquot of radioactive thallium solution was added. The potential of the bed was then fixed at a low value (say O-120 V) until no more thallium was adsorbed. This point was indicated by the activity of the solution becoming constant or zero. The potential
720
B. J. BOWLES
of the platinum was then altered to another value (say 0.270 V) and the activity of the solution was observed until it again became constant. The difference in activities gave the total amount of thallium that had been desorbed by the platinum. The process was repeated stepwise until a potential of 1.02 V was reached, where essentially all the thallium was in the solution. Potentials of greater than l-02 V were employed only rarely, since prolonged immersion in HCl at such potentials tended to result in some dissolution of the platinum and hence a change in the characteristics of the surface.
’
I 0.0
I o-I
I 0.2 Em>
I
0.3
I
I
0.4
05
V
FIG. 2. Variation of strongly adsorbed thallium with potential.
A further set of results was then obtained with a higher initial concentration of thallium, the procedure being repeated. The maximum amount of thallium that could be adsorbed at any one potential was fairly well defined. For instance, the amount that could be adsorbed at O-97 V was about 8 x lo-lo mole/cm2 of platinum: further additions of thallium tended to remain in the solution, and not to be adsorbed. It was observed that a portion of the thallium adsorbed on to the platinum became bound quite firmly and was extremely slow to reappear in the solution, even at high potentials. Figure 2 shows the extent of this strong adsorption. The amount of thallium added to the solution was just sufficient to be totally adsorbed at 0.0 V. The potential was altered to a set value for 5 or 10 min, during which adsorption occurred. The potential was then returned to 1.02 V and the activity release to the solution was followed with time. The rate of reappearance of the activity could be resolved into two components, one of which was very much slower than the other. The release of the more rapidly
Specific adsorption
of cations on electrodes--I.
Tlf on Pt
721
desorbed material appeared to be essentially complete within about 2 min. The slow desorption often continued for several hours before the solution activity became constant. An extrapolation was made of the amount of slowly desorbed material present on the platinum at the time the potential was returned to 1.02 V; the result is expressed in Fig. 2 as a percentage of the total thallium present. No further strong adsorption occurred when the potential was maintained at the set value for much longer periods, and the value of the total amount of strongly adsorbed material was reproduced on repetition of the potential variations.
Argon
Argon
Platinum
controllmg
Reference ___ electrode Platinum column __(0. I mmdia beads)
Ll
---Drop
collecror
FIG. 3. Elution apparatus.
This strong adsorption is possibly due to the penetration of thallium ions into fissures and cracks of atomic dimensions, as proposed for oxygen by 0brucheva.8 It gives rise to an apparent difference in the amount of adsorbed thallium depending on whether the potential is approached from the positive or the negative side. Some confirmation that the strong adsorption component could indeed be due to the trapping of adsorbed material in fissures and cracks of molecular dimensions was obtained by adsorbing active thallium on to a platinum electrode whilst it was held at a potential where strong adsorption was known to take place. The potential was then returned to 1.02 V and the electrode was washed thoroughly with 0.1 M HCl before being removed from the solution, disconnected and dried. The activity of this platinum showed a considerable retention of thallium of the order of the strong adsorption component. A few experiments were performed with spongy platinum granules in an apparatus very similar to an ion-exchange resin column, as shown in Fig. 3. In these experiments the eluant (which could be degassed with argon) passed through the column and on
722
emerging collector.
B. J. BOWLES
from the bottom
proceeded RESULTS
via a Geiger AND
counter
and drop
counter
to a
DISCUSSION
The method employing the column of spongy platinum granules was soon abandoned, for the release of thallium at low potentials was very slow due to the strength of the adsorption. The amount of active component appearing in the eluant at reproducible flow rates was often completely obscured by the background activity due to cosmic radiation etc. In addition, the system suffered from the drawback that it was prone to channelling effects. The few results which were obtained by this method are included with the results obtained by the circulatory or closed loop method.
E Ptr
v
FIG. 4. Variation of adsorption
with potential.
Figure 4 shows typical adsorption/potential curves obtained mainly from the It is apparent that, for low coverages, the thallium is adsorbed circulatory apparatus. quite strongly on the platinum and that up to a potential of about O-75 V virtually The adsorption would appear to be quite strong no thallium appears in the solution. at lower potentials and to extend in some degree to a potential of greater than 1-O V. It is also apparent that the amount of thallium that can be adsorbed increases rapidly (For all curves except the uppermost no as the potential is made more negative. detectable amounts of thallium remain in the solution at an electrode potential of 0.57 V.) The adsorption isotherms at constant potential are shown in Fig. 5. The general trend is for adsorption to take place strongly at low solution concentrations and for the adsorbed material to approach a saturation value as the concentration is increased. Both the adsorption coefficient and the apparent saturation coverage increase as the potential of the platinum is made more negative.
Specific adsorption
of cations on electrodes-I.
TIC on Pt
723
ln Fig. 6 the data are presented in the classical way in the form of a log/log plot. A iinear dependence is apparent and the relationship may be likened to the classical or Freundlich adsorption isotherm. As is generally the case for this isotherm, the value of the slope of the log/log plot is less than unity. It should be noted that the value of the surface area used in the calculations is the geometric area derived from the dimensions of the platinum. It is not possible
c
l.oy/z .I
/
*I
/’
%--
L&c
0.0 0.0
_
.-. 3x-“-
“_-LX--
L
10
-1 -~
20
PO
I 4-o
I
5.0
E,
FIG. 5. Variation of adsorption,
“-PC
I 6.0
*
1
?
.’
0.97v I 7.0
I 8.0
I 9.0
I 10 0
I Il.0
I 12.0
mole/mL x 10m3
(I, with bulk concentration,
2, at different potentials.
to measure the total area available for the adsorption of thallium, though the indications are that the true area is greater than the apparent. A typical value for the amount of adsorbed hydrogen extrapolated to a potential of O-0 V was that equivalent to 4.4 x lo-* C/cm2, ie 2.75 x 1015 atom/cm2; this indicates that the true surface area was about twice the apparent geometric area. Alternatively the adsorption isotherms may be analysed on the basis of the Langmuir equation (j _
klC 1 + k,k,F ’
where q is the amount of thallium adsorbed per cm” of platinum, 2 the concentration of thallium, k, the adsorption coefficient and l/k2 the saturation monolayer capacity. The data given in Fig, 5 may be used to determine the values of k, and k2 at each potential by the usual method of measuring the slope and intercept of linear plots. As expected, these graphs show some departure from linearity at high coverage in the direction expected from repuIsion interactions between particles on adjacent sites and from non-homogeneity of the surface, the more active centres being occupied first. The apparent adsorption coefficient and the apparent saturation coverage as functions of potential are shown respectively in Figs. 7 and 8.
724
B.
J.
BOWLES
E,
M
FIG. 6. Log/log plot of Fig. 5 data. 25-
0
From
column
expts.
zoE ” I x E .o z
15-
8 ._s
\
‘; b
IO-
x
z 4 \ “\ 5x1X \ k xk7 I!
Oo-I
I 02
, o-3
,
;
,
/
04
05
0.6
0.7
EP,
a
kx
, 08
%%-
V
FIG. 7. Variation of adsorption
coefficient with potential.
Also included in these graphs are values derived from the column experiments. These were obtained from values of the solution activity, the volume of eluant passed and the breakthrough point, as shown by Glueckaufg using the equation
where b-1]; is the amount left on the column when eluant concentration is E, 2 the surface area of the column, u the volume of eluant to C and a the pore space.
Specific adsorption
of cations on electrodes-I.
T1+ on Pt
725
The points due to these experiments are shown by the squares. They have been obtained only for high potentials where the adsorption coefficient is small. From Fig. 7 it can be seen that the apparent adsorption coefficient is quite small at 0.95 V but that it increases rapidly as the potential of the platinum is made more negative, reaching a value of 30 cm for a potential of about 0.27 V. This value is indicative of the very strong adsorption that is obtained in this system. Figure 8 shows the apparent saturation coverage for platinum by thallium as a function of potential. It is to be expected that above a potential of about 0.65-0.75 V 50-
q
From
column
expts
40-
30-
20-
IO-
00
FIG. 8. Variation of apparent saturation coverage with potential.
that there will be competition for the adsorption sites between the thallium and oxygen, which is known to be adsorbed at these potentials.lO The change of slope of Fig. 6 at about 0.72 to 0.77 V indicates that such competition might exist, but it seems unlikely that this competition can wholly account for the rapid decrease in the saturation coverage seen at potentials more positive than 0.62 V in Fig. 8. Neither can it explain the variation in apparent saturation surface coverage at more negative potentials. One explanation that could account for this change in apparent saturation coverage with applied potential is that it is connected with the non-uniformity of the surface, with the free energy of adsorption varying considerably for different sites. of cations is due to the It has been suggested by Frumkin l1 that the adsorption formation of intermetallic surface compounds, eg Pt + Tl+ + 12e $ Pt-TI, where n is the number of electrons (G 1) transferred to the electrode ion is transferred from the solution to the electrode.
when one thallium
B. J. BOWLES
716
If the adsorption
takes place on a uniform
surface we can write
where E is the potential, E,, some defined potential, B a constant depending on the number of electrons involved, Mthe fractional surface coverage and c the concentration of adsorbent relative to some standard concentration c,,. If c,, is defined for a solution of unit activity, E, is the potential of a half-covered electrode in such a solution. In this case as E is varied the fractional coverage a varies in a sigmoid manner for constant c due to the nature of equation 2. The value of u will approach unity as E is made very negative and zero as E is made very positive, and will vary most rapidly with potential when tc has a value of 0.5.
FIG. 9. Scheme for non-uniform
absorber.
A non-uniform adsorber may be considered as a number of strips of uniform adsorber arranged adjacent to each other in order of ascending E,,, Fig. 9. In this case the sigmoid distribution of IX will appear as the strip is traversed due to the variation of the left hand side of equation (2) by altering the value of E,,, but keeping If E is now E constant (the cross-hatched portion represents the adsorbed material). altered, the sigmoid distribution will take up a new position as shown by the dotted line, and the amount of adsorbent shown by diagonal shading will be adsorbed from the solution. This simplified picture is complicated by the variation in concentration of a solution of finite extent due to the adsorption/desorption phenomenon, which imposes a skewness on the curves of Fig. 9. In addition, Fig. 9 shows a linear distribution of sites with E,, which is not possible for a surface of finite area. The variation of apparent saturation coverage with applied potential may be it corresponds to the unshaded portions readily understood from the above discussion; above the sigmoid curves of Fig. 9. Since the surface has a finite area, the total amount of material that can be adsorbed is limited, and hence there must be upper and lower limits of E,, beyond which sites are virtually non-existent. The existence of the positive limit can be seen in Fig. 8, where the apparent saturation coverage approaches zero. The frequency
Specific adsorption
of cations on electrodes-I.
Tl+ on Pt
727
distribution of sites with E,, is probably of the form of one or more Gaussian curves superimposed. However, the straight slope of the portion of Fig. 8 that is uncomplicated by oxygen adsorption seems to indicate that the distribution of adsorption sites with different values of E, is approximately constant over the range available for study with the potentials and concentrations used here. Nevertheless, the idea of a constant distribution of adsorption sites should not be taken too seriously, as a large number of types of distribution lead to the same sort of behaviour within experimental error over much of their range.
FIG. 10. Thallium concentration
vs potential, at constant coverage.
The relationship between the values of E, and E may be derived from equation (2) but the value of the constant B is unknown. Preliminary experiments on the charge excess during adsorption indicate that the charge transfer involved is of the order of one electron per thallium atom. However, Fig. 10, log (thallium concentration) US electrode potential at constant coverage, shows slopes equivalent to charge transference of between O-25 and 0.6 electrons per atom adsorbed depending on the surface coverage, but there are doubts as to whether their values are a true representation of the actual average number of electrons transferred. This will be discussed more fully in a later paper. If the graph of saturation coverage us potential, Fig. 8, is extrapolated linearly to zero coverage it is found that the adsorption phenomenon ceases at about I.02 V. This is in rough agreement with the value obtained by extrapolation of the adsorption coefficient to zero in Fig. 7. This value of I.02 V where adsorption just ceases is about 1.3 V removed from the standard potential for the thallium/thallous reaction, and is still further removed by another 0.35 V from the deposition potential of thallium from solutions of micromolar concentrations such as used here. This difference is connected with the difference in binding energy of thallium to the most active centres
B. J. BOWLES
728
on the platinum and the binding energy of subsequent atoms of thallium to thallium atoms already deposited. It is expected that the adsorption behaviour of this system will be of some use in the rigorous purification of solutions from contaminants present in very small concentrations. In fact, the properties of the system given above have been used in this laboratory to remove traces of thallous and other metallic ions from solutions in HCI prior to polarographic determinations sensitive to thallium of about IOU7 M.
FIG. 11. Integral adsorption
coefficient as function of potential for various coverages.
As a guide to the capabilities of the adsorption, Fig. 11 gives the integral adsorption coefficient as a function of potential for various coverages of the geometric surface of smooth platinum. This investigation has revealed that the adsorption of cations on an electrode takes place to a greater extent than has been realized before. The strength of the adsorption at low electrode potentials and the gradual decrease of the adsorption coefficient to zero as the potential is increased encourages the belief that the phenomeSuch applications could be (a) the purificanon has practical analytical applications. tion of solutions from certain contaminants to concentrations of less than 10PIO M (b) the concentration of certain cations from solutions of very low concentration and (c) the separation of cations. REFERENCES 1. G. GOUY, Ann. Chim. Phys. 29, 145 (1903); 9, 75 (1906). 2. G. GOUY, Ann. Phys. 6, 5 (1916); 7, 129 (1917). 3. A. FRUMKINand A. D. OBRUCHEVA, Z. anor
Specific absorption
of cations on electrodes-I.
TI+ on Pt
5. N. A. BALASCHOVA, 2. phys. Chem. 207,340 (1957). 6. A. D. OBRUCHEVA,Dokl. Akad. Nauk SSSR 120,431 (1958). 7. G. C. BARKER,private communication. 8. A. D. OBRUCHEVA,Zh. Fiz. Khim. 26,1148 (1952). 9. E. GLUECKAUF,Nature, Lond. 156, 748 (1945). 10. M. W. BREITER,J. Electrochem. Sot. 109, 425 (1962). 11. A. FRUMKIN,Symposium on Electrode Processes (ed. E. Yeager), Philadelphia (1959).
729
Pergamon Press Ltd.
Printed in Northern Ireland
THE SPECIFIC ADSORPTION OF CATIONS ON ELECTRODES-I. THE ADSORPTION OF THALLIUM ON PLATINUM AT CONTROLLED POTENTIALS * B. J. BOWLES Atomic Energy Research Establishment,
Harwell, England
Abstract-The specific adsorption of thallous ion on to platinum at controlled potential from very dilute solutions in 0.1 M HCI has been studied by means of a radioactive tracer technique. It is shown that adsorption of up to a monolayer of thallium takes place at potentials far removed from the potential for bulk deposition of thallium metal, and that two different adsorption mechanisms appear to be present. An explanation is advanced for the change in apparent saturation surface coverage with applied potential. The integral adsorption coefficient is presented as a function of potential and surface coverage. Resume-L’adsorption specifique de l’ion Thalleux sur Pt a potentiel controle et a partir de solutions diluees dans HCl.O,l M est etudiee au moyen de traceurs radioactifs. Des des potentiels notablement differents de celui correspondant au depart de Tl mttallique massif, l’adsorption se manifeste au dela dune couche monomoleculaire et il semble que deux mecanismes d’adsorption se conjuguent. Une interpretation est suggeree pour rendre compte du changement de saturation apparente du recouvrement superficiel selon la tension appliqde. Le coefficient integral d’absorption est exprime en fonction du potentiel et du recouvrement superticiel. Zusammenfassung-Die spezifische Adsorption von Thallium Ionen bei geregelten Potentiellen von sehr verdtinnten Losungen in 0.1 M HCI, wurde mit Hilfe einer Radioaktiven Technik untersucht. Es wird gezeigt, das die Adsorption bis zu einer Einzelschicht von Thallium durch Potentiellen stattfindet, welche sehr weit von der Potentiellen ftir Massabsetzung von Thalliummetall entfernt sind, und das zwei verschiedene Absorption Mechanismus anwesend zu sein scheinen. Eine Erklarung ist erbracht fiir Anderung in den scheinbaren Slttigungsoberfllchen Abdeckungen mit angewandten Potentiell. Der Integrale Absorption Koeffizient ist als eine Funktion von potentialer und Oberflachen Abdeckung dargestellt. THE phenomenon of adsorption of material at a metal/solution interface as a consequence of the electrical and chemical forces acting at a boundary has been known
for many years. 1-4 Most of the results which have been reported in the literature have been obtained by electrocapillarity and capacitance measurements on liquid metal surfaces in contact with aqueous solutions, the metal most commonly used being mercury. In recent years a number of investigations involving solid electrodes has been made, such studies being almost entirely confined to platinum in both the suitable for such investigasmooth5 and platinized 5s6forms. Platinum is particularly tions as its passivity makes it possible to study adsorption over a large range of potential. Adsorption of neutral substances has been known for many years1,2 and lately the specific adsorption of anions has been investigated. Some adsorption of cations was noted and it was at first assumed that this occurred at the outer Helmholtz layer by the electrostatic attraction of excess adsorbed anions.5 More recently however it has been shown by Obrucheva that cations can exhibit marked specific adsorption on platinum electrodes from fairly concentrated solutions.6 In the work described here the concentrations investigated have been of the order of 1 ,uM. * Manuscript received 22 July 1964. 1
717
B. J. BOWLES
718
The present paper is devoted to the adsorption of thallium on platinum from a solution in hydrochloric acid. The system platinum-electrode/thallium-ion-adsorbent was chosen as it was known from previous work that thallium is particularly strongly adsorbed on platinum. 7 In addition, the isotope 204T1can be obtained with very high specific activity and hence its concentration can be observed to very low levels. EXPERIMENTAL
PROCEDURE
204T1 with a specific activity of about 200 me/g was obtained from the Radiochemical Centre at Amersham. It was purified and taken into O-1 M HCl as thallous WORKING ELECTRODE
/
-1: L.
i
CONTROLLINt ELECTRODE-
2
c,
.G.M. COUNTER
TO
GLASS St NTER
iuvlP
FIG. 1. Apparatus.
chloride. The concentration of thallous ion was determined by the method of standard additions using a square-wave polarograph, these determinations being repeated at The hydrochloric acid used was intervals to check on any changes in concentration. highly purified and contained, for instance, less than 0.005 ppm of lead. All solutions were made up with triple-distilled water. The platinum was of 99.999 per cent purity in the form of wire O-005 in. dia., and was cut into billets of approximately 5 mm in length. Its surface area calculated from the dimensions of the wire billets was 407 cm2. It was cleaned by boiling in fuming nitric acid for several days prior to use. The apparatus is shown in Fig. 1. 100 ml of solution could be passed continuously
Specific adsorption of cations on electrodes--I.
Tlf on Pt
719
round the apparatus via the platinum billets and the immersion Geiger counter by means of the lift pump. The lift pump was operated by argon purified by passing over “Hopcalite” and heated Cu/CuO. This argon also served to remove oxygen from the solution. It was passed through a gas bubbler containing the base solution (HCl) prior to the lift pump to prevent evaporation of the solution in the apparatus. All surfaces in contact with the solution were double sealed to prevent capillary rise and evaporation leading to loss of solution. Electrical connexions were made to the platinum bed, to the reference electrode which connected to the solution at the surface of the bed via the Luggin capillary, and to the controlling electrode, which was also connected to the cell via the crescent-bore stopcock. The apparatus was constructed from Pyrex, since it was found that soft glass contains readily leachable constituents that adsorb on platinum (eg Cu, Pb). The potential of the platinum could be held steady by means of a potentiostat connected to the reference electrode, the controlling electrode and to the platinum bed or column. It was possible to change the potential of the platinum rapidly to a new value and the charge required to attain the new potential could be monitored; hence charging curves could be constructed. All potentials are referred to the reversible hydrogen potential in the same solution. The concentration of thallium in the solution was followed by observing the tube, which was immersed in p-activity of the 204T1 by means of the Geiger-counter the solution (Fig. 1). The efficiency of the counter was quite low due to the relatively large proportion of the total volume of solution contained in parts of the apparatus not in the immediate vicinity of the counter. This fact, in combination with the low energy of the p-particle emitted from 204T1(0.76 MeV), reduced the efficiency of the counter to the region of t-1 per cent. The background to the counter was about 0.7 count/s and an increase of 0.2 count/s could be observed; hence the system was sensitive to the release of 3 x lo-l1 moles of thallium from the electrode into a pure solution of the supporting electrolyte. (204T1 has a half-life of 4.1 y and hence its decay is negligible in a time comparable to that of an experimental run.) Prior to each run, the platinum bed was taken through several potential excursions over the range 0.12 to 1.02 V in the supporting electrolyte only, with the argon-lift pump operating at a standard flow-rate, until reproducible charging curves were obtained. As a check on the state of the platinum surface the characteristics of the charging curve were compared with those preceding other runs. As the platinum was always in contact with some of the solution whereas the solution was out of contact with the platinum for most of the time, it was expected that equilibrium would be established much more rapidly if the starting point of the experiment was with the thallium adsorbed on the platinum rather than in solution. A simplified mathematical solution of this problem assuming no turbulent mixing and a single theoretical plate was obtained from the Mercury computer. This confirmed that equilibrium was established much more rapidly if the desorption rather than the adsorption was studied, and this procedure was adopted. After it had been established that the platinum surface had not altered in character and that the supporting electrolyte in the cell was free of contaminants, an aliquot of radioactive thallium solution was added. The potential of the bed was then fixed at a low value (say O-120 V) until no more thallium was adsorbed. This point was indicated by the activity of the solution becoming constant or zero. The potential
720
B. J. BOWLES
of the platinum was then altered to another value (say 0.270 V) and the activity of the solution was observed until it again became constant. The difference in activities gave the total amount of thallium that had been desorbed by the platinum. The process was repeated stepwise until a potential of 1.02 V was reached, where essentially all the thallium was in the solution. Potentials of greater than l-02 V were employed only rarely, since prolonged immersion in HCl at such potentials tended to result in some dissolution of the platinum and hence a change in the characteristics of the surface.
’
I 0.0
I o-I
I 0.2 Em>
I
0.3
I
I
0.4
05
V
FIG. 2. Variation of strongly adsorbed thallium with potential.
A further set of results was then obtained with a higher initial concentration of thallium, the procedure being repeated. The maximum amount of thallium that could be adsorbed at any one potential was fairly well defined. For instance, the amount that could be adsorbed at O-97 V was about 8 x lo-lo mole/cm2 of platinum: further additions of thallium tended to remain in the solution, and not to be adsorbed. It was observed that a portion of the thallium adsorbed on to the platinum became bound quite firmly and was extremely slow to reappear in the solution, even at high potentials. Figure 2 shows the extent of this strong adsorption. The amount of thallium added to the solution was just sufficient to be totally adsorbed at 0.0 V. The potential was altered to a set value for 5 or 10 min, during which adsorption occurred. The potential was then returned to 1.02 V and the activity release to the solution was followed with time. The rate of reappearance of the activity could be resolved into two components, one of which was very much slower than the other. The release of the more rapidly
Specific adsorption
of cations on electrodes--I.
Tlf on Pt
721
desorbed material appeared to be essentially complete within about 2 min. The slow desorption often continued for several hours before the solution activity became constant. An extrapolation was made of the amount of slowly desorbed material present on the platinum at the time the potential was returned to 1.02 V; the result is expressed in Fig. 2 as a percentage of the total thallium present. No further strong adsorption occurred when the potential was maintained at the set value for much longer periods, and the value of the total amount of strongly adsorbed material was reproduced on repetition of the potential variations.
Argon
Argon
Platinum
controllmg
Reference ___ electrode Platinum column __(0. I mmdia beads)
Ll
---Drop
collecror
FIG. 3. Elution apparatus.
This strong adsorption is possibly due to the penetration of thallium ions into fissures and cracks of atomic dimensions, as proposed for oxygen by 0brucheva.8 It gives rise to an apparent difference in the amount of adsorbed thallium depending on whether the potential is approached from the positive or the negative side. Some confirmation that the strong adsorption component could indeed be due to the trapping of adsorbed material in fissures and cracks of molecular dimensions was obtained by adsorbing active thallium on to a platinum electrode whilst it was held at a potential where strong adsorption was known to take place. The potential was then returned to 1.02 V and the electrode was washed thoroughly with 0.1 M HCl before being removed from the solution, disconnected and dried. The activity of this platinum showed a considerable retention of thallium of the order of the strong adsorption component. A few experiments were performed with spongy platinum granules in an apparatus very similar to an ion-exchange resin column, as shown in Fig. 3. In these experiments the eluant (which could be degassed with argon) passed through the column and on
722
emerging collector.
B. J. BOWLES
from the bottom
proceeded RESULTS
via a Geiger AND
counter
and drop
counter
to a
DISCUSSION
The method employing the column of spongy platinum granules was soon abandoned, for the release of thallium at low potentials was very slow due to the strength of the adsorption. The amount of active component appearing in the eluant at reproducible flow rates was often completely obscured by the background activity due to cosmic radiation etc. In addition, the system suffered from the drawback that it was prone to channelling effects. The few results which were obtained by this method are included with the results obtained by the circulatory or closed loop method.
E Ptr
v
FIG. 4. Variation of adsorption
with potential.
Figure 4 shows typical adsorption/potential curves obtained mainly from the It is apparent that, for low coverages, the thallium is adsorbed circulatory apparatus. quite strongly on the platinum and that up to a potential of about O-75 V virtually The adsorption would appear to be quite strong no thallium appears in the solution. at lower potentials and to extend in some degree to a potential of greater than 1-O V. It is also apparent that the amount of thallium that can be adsorbed increases rapidly (For all curves except the uppermost no as the potential is made more negative. detectable amounts of thallium remain in the solution at an electrode potential of 0.57 V.) The adsorption isotherms at constant potential are shown in Fig. 5. The general trend is for adsorption to take place strongly at low solution concentrations and for the adsorbed material to approach a saturation value as the concentration is increased. Both the adsorption coefficient and the apparent saturation coverage increase as the potential of the platinum is made more negative.
Specific adsorption
of cations on electrodes-I.
TIC on Pt
723
ln Fig. 6 the data are presented in the classical way in the form of a log/log plot. A iinear dependence is apparent and the relationship may be likened to the classical or Freundlich adsorption isotherm. As is generally the case for this isotherm, the value of the slope of the log/log plot is less than unity. It should be noted that the value of the surface area used in the calculations is the geometric area derived from the dimensions of the platinum. It is not possible
c
l.oy/z .I
/
*I
/’
%--
L&c
0.0 0.0
_
.-. 3x-“-
“_-LX--
L
10
-1 -~
20
PO
I 4-o
I
5.0
E,
FIG. 5. Variation of adsorption,
“-PC
I 6.0
*
1
?
.’
0.97v I 7.0
I 8.0
I 9.0
I 10 0
I Il.0
I 12.0
mole/mL x 10m3
(I, with bulk concentration,
2, at different potentials.
to measure the total area available for the adsorption of thallium, though the indications are that the true area is greater than the apparent. A typical value for the amount of adsorbed hydrogen extrapolated to a potential of O-0 V was that equivalent to 4.4 x lo-* C/cm2, ie 2.75 x 1015 atom/cm2; this indicates that the true surface area was about twice the apparent geometric area. Alternatively the adsorption isotherms may be analysed on the basis of the Langmuir equation (j _
klC 1 + k,k,F ’
where q is the amount of thallium adsorbed per cm” of platinum, 2 the concentration of thallium, k, the adsorption coefficient and l/k2 the saturation monolayer capacity. The data given in Fig, 5 may be used to determine the values of k, and k2 at each potential by the usual method of measuring the slope and intercept of linear plots. As expected, these graphs show some departure from linearity at high coverage in the direction expected from repuIsion interactions between particles on adjacent sites and from non-homogeneity of the surface, the more active centres being occupied first. The apparent adsorption coefficient and the apparent saturation coverage as functions of potential are shown respectively in Figs. 7 and 8.
724
B.
J.
BOWLES
E,
M
FIG. 6. Log/log plot of Fig. 5 data. 25-
0
From
column
expts.
zoE ” I x E .o z
15-
8 ._s
\
‘; b
IO-
x
z 4 \ “\ 5x1X \ k xk7 I!
Oo-I
I 02
, o-3
,
;
,
/
04
05
0.6
0.7
EP,
a
kx
, 08
%%-
V
FIG. 7. Variation of adsorption
coefficient with potential.
Also included in these graphs are values derived from the column experiments. These were obtained from values of the solution activity, the volume of eluant passed and the breakthrough point, as shown by Glueckaufg using the equation
where b-1]; is the amount left on the column when eluant concentration is E, 2 the surface area of the column, u the volume of eluant to C and a the pore space.
Specific adsorption
of cations on electrodes-I.
T1+ on Pt
725
The points due to these experiments are shown by the squares. They have been obtained only for high potentials where the adsorption coefficient is small. From Fig. 7 it can be seen that the apparent adsorption coefficient is quite small at 0.95 V but that it increases rapidly as the potential of the platinum is made more negative, reaching a value of 30 cm for a potential of about 0.27 V. This value is indicative of the very strong adsorption that is obtained in this system. Figure 8 shows the apparent saturation coverage for platinum by thallium as a function of potential. It is to be expected that above a potential of about 0.65-0.75 V 50-
q
From
column
expts
40-
30-
20-
IO-
00
FIG. 8. Variation of apparent saturation coverage with potential.
that there will be competition for the adsorption sites between the thallium and oxygen, which is known to be adsorbed at these potentials.lO The change of slope of Fig. 6 at about 0.72 to 0.77 V indicates that such competition might exist, but it seems unlikely that this competition can wholly account for the rapid decrease in the saturation coverage seen at potentials more positive than 0.62 V in Fig. 8. Neither can it explain the variation in apparent saturation surface coverage at more negative potentials. One explanation that could account for this change in apparent saturation coverage with applied potential is that it is connected with the non-uniformity of the surface, with the free energy of adsorption varying considerably for different sites. of cations is due to the It has been suggested by Frumkin l1 that the adsorption formation of intermetallic surface compounds, eg Pt + Tl+ + 12e $ Pt-TI, where n is the number of electrons (G 1) transferred to the electrode ion is transferred from the solution to the electrode.
when one thallium
B. J. BOWLES
716
If the adsorption
takes place on a uniform
surface we can write
where E is the potential, E,, some defined potential, B a constant depending on the number of electrons involved, Mthe fractional surface coverage and c the concentration of adsorbent relative to some standard concentration c,,. If c,, is defined for a solution of unit activity, E, is the potential of a half-covered electrode in such a solution. In this case as E is varied the fractional coverage a varies in a sigmoid manner for constant c due to the nature of equation 2. The value of u will approach unity as E is made very negative and zero as E is made very positive, and will vary most rapidly with potential when tc has a value of 0.5.
FIG. 9. Scheme for non-uniform
absorber.
A non-uniform adsorber may be considered as a number of strips of uniform adsorber arranged adjacent to each other in order of ascending E,,, Fig. 9. In this case the sigmoid distribution of IX will appear as the strip is traversed due to the variation of the left hand side of equation (2) by altering the value of E,,, but keeping If E is now E constant (the cross-hatched portion represents the adsorbed material). altered, the sigmoid distribution will take up a new position as shown by the dotted line, and the amount of adsorbent shown by diagonal shading will be adsorbed from the solution. This simplified picture is complicated by the variation in concentration of a solution of finite extent due to the adsorption/desorption phenomenon, which imposes a skewness on the curves of Fig. 9. In addition, Fig. 9 shows a linear distribution of sites with E,, which is not possible for a surface of finite area. The variation of apparent saturation coverage with applied potential may be it corresponds to the unshaded portions readily understood from the above discussion; above the sigmoid curves of Fig. 9. Since the surface has a finite area, the total amount of material that can be adsorbed is limited, and hence there must be upper and lower limits of E,, beyond which sites are virtually non-existent. The existence of the positive limit can be seen in Fig. 8, where the apparent saturation coverage approaches zero. The frequency
Specific adsorption
of cations on electrodes-I.
Tl+ on Pt
727
distribution of sites with E,, is probably of the form of one or more Gaussian curves superimposed. However, the straight slope of the portion of Fig. 8 that is uncomplicated by oxygen adsorption seems to indicate that the distribution of adsorption sites with different values of E, is approximately constant over the range available for study with the potentials and concentrations used here. Nevertheless, the idea of a constant distribution of adsorption sites should not be taken too seriously, as a large number of types of distribution lead to the same sort of behaviour within experimental error over much of their range.
FIG. 10. Thallium concentration
vs potential, at constant coverage.
The relationship between the values of E, and E may be derived from equation (2) but the value of the constant B is unknown. Preliminary experiments on the charge excess during adsorption indicate that the charge transfer involved is of the order of one electron per thallium atom. However, Fig. 10, log (thallium concentration) US electrode potential at constant coverage, shows slopes equivalent to charge transference of between O-25 and 0.6 electrons per atom adsorbed depending on the surface coverage, but there are doubts as to whether their values are a true representation of the actual average number of electrons transferred. This will be discussed more fully in a later paper. If the graph of saturation coverage us potential, Fig. 8, is extrapolated linearly to zero coverage it is found that the adsorption phenomenon ceases at about I.02 V. This is in rough agreement with the value obtained by extrapolation of the adsorption coefficient to zero in Fig. 7. This value of I.02 V where adsorption just ceases is about 1.3 V removed from the standard potential for the thallium/thallous reaction, and is still further removed by another 0.35 V from the deposition potential of thallium from solutions of micromolar concentrations such as used here. This difference is connected with the difference in binding energy of thallium to the most active centres
B. J. BOWLES
728
on the platinum and the binding energy of subsequent atoms of thallium to thallium atoms already deposited. It is expected that the adsorption behaviour of this system will be of some use in the rigorous purification of solutions from contaminants present in very small concentrations. In fact, the properties of the system given above have been used in this laboratory to remove traces of thallous and other metallic ions from solutions in HCI prior to polarographic determinations sensitive to thallium of about IOU7 M.
FIG. 11. Integral adsorption
coefficient as function of potential for various coverages.
As a guide to the capabilities of the adsorption, Fig. 11 gives the integral adsorption coefficient as a function of potential for various coverages of the geometric surface of smooth platinum. This investigation has revealed that the adsorption of cations on an electrode takes place to a greater extent than has been realized before. The strength of the adsorption at low electrode potentials and the gradual decrease of the adsorption coefficient to zero as the potential is increased encourages the belief that the phenomeSuch applications could be (a) the purificanon has practical analytical applications. tion of solutions from certain contaminants to concentrations of less than 10PIO M (b) the concentration of certain cations from solutions of very low concentration and (c) the separation of cations. REFERENCES 1. G. GOUY, Ann. Chim. Phys. 29, 145 (1903); 9, 75 (1906). 2. G. GOUY, Ann. Phys. 6, 5 (1916); 7, 129 (1917). 3. A. FRUMKINand A. D. OBRUCHEVA, Z. anor
Specific absorption
of cations on electrodes-I.
TI+ on Pt
5. N. A. BALASCHOVA, 2. phys. Chem. 207,340 (1957). 6. A. D. OBRUCHEVA,Dokl. Akad. Nauk SSSR 120,431 (1958). 7. G. C. BARKER,private communication. 8. A. D. OBRUCHEVA,Zh. Fiz. Khim. 26,1148 (1952). 9. E. GLUECKAUF,Nature, Lond. 156, 748 (1945). 10. M. W. BREITER,J. Electrochem. Sot. 109, 425 (1962). 11. A. FRUMKIN,Symposium on Electrode Processes (ed. E. Yeager), Philadelphia (1959).
729