- Email: [email protected]
Evidence for competitive adsorption of fluoride ions in studies carried out at constant ionic strength
&EO&&
%ETTE
_
~htt~t@~.d’Eh&&tie &ved:;l&d
-&tet$u&du
C.N.kS..
1, Place A. Briam&-92190 Medon-Bellevue
(France)
J&x 1981; in revise&form StbA~gust :I!XI)
: ..
ABtiCr
:
.-
1
.T& adsorptian of the bromideion oil a (1 silver &&r&e from mix&d solutions of s&urn bromide and [email protected]$de was investigated by measuring the doudfe-layer capacity as a function of solution composition_ _n?e a&y& of the total inner-layer (diffkential) capacity as a function of the elect&e charge bhogs clear‘kdence for com&titive &cific adsor$t&m of the ‘fluoride ion when the bromi& cokxntration~is sm@kwith respkt to the fluoride c¢&ion_ J%e total inner-lay& cap&y is ksolvcd
16,
into i&. comp&znt parts with the-aid of diffuse layer ihe& The (integral) capacity measured at cotistant ekctrod~ charge is a.constant -when the fluoride adsorption appears not to_interfere. The (differential) capacity measured at. constant amqmt adsorbed as .a function of the_ electrode charge presents a maximum at Zerocharge, when thi: fluoride influence is negligible; which may bc attributed to the solvent propertks. With c&re@ed values of the eIcctr&e ciiarge vs. potential plots for a base elecklyte without aniottic adsorpti& the standard free energy of br&ide adsorpt&x~ is shown to be linearty dependent on
the electrode charge, as for the bromide aqueous solutions/mercury
system.
INTRODUCTION
The numerous studies of ionic adsorption on mercury have been made so far with two main types of measureme&:. oqe using the.single.salt in aqueous solutions tid one using solutiofis the i&c strength of which is~m~+i~$ainedconstant by addition’of fluoride as b.&&.el~tr~lyt& For & &en anion, signif&a~~‘diffe&ike.s are observed when re.sults’obt+ied from the two techr+c&s .&r6 &mpared:_‘Thk- tiost striking dkcrepaxicies con&n the c&j&onent of the i+ef layer. at tionsigt ekctrode charge, wti_ch is-a tin+ant.& most Casey for the~s&gle tilt soIu&& and which is dependent .on. tIie elect&de dhtige’foi the soIutions~at_~ns&ion& strength,-an$on the other han&the stFdz@d- fr+ .&er&y -&icJ~& iinearl~- dependent on .the’eIFtrdfle charge foilthe si&k sali stiIutio&ra@ no~~line&~+petidedent for the other solutions. : ,Hills .&d- &eVes [ 1) have suggest&i ihat -.thes$ .diScrepan&s between the two kinds of iGti$ a& 8~t$&ibl~:;.iii a &&~eiiii& ad&ptibn:~6f ;the fluoride ions ovi&g ;_tOi th_e__ f&i : t@ ;e_~ i ;dis~~~~~ies 1ti$i .~~i@e;-ved io decrease .:e :_th$ +!c$fic -$&f$$&.~.of_, -i&ji&
.
io&
.;:.
&&‘&&b: f&k
~~~~~‘~~~c~~~~~,~u~~.~.~~~:~,~~~p~~,,
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131.:&&y’@~~bI&’
&
_,&&pGqp
.of
(Q&~.‘$j&jQ&i : of hi&@? [io@ .._._ ..
..’ _ _-, : _j .* -j -.,_ ’ ~~~7~8/~2,Y~~~looo/so27s:@Z~ 1482 &&&~&u~ia~!L&~ : : .m:.‘_), :: “._ :_ I _ _... -_ ... .,~:,~-l..: ._.c 1.. :_ : .:I.:_. . . ..y -‘_ ,... :_’ ‘. ;_ . ..I-.. . ..._._.,: -.,I - :’ .,_
.-. I .I
:-
312 ‘:
from ICI + KF solutions [3], while the adsorption of nitrate ions from -KN@ solutions [4] differs notably from the adsorPtion of nitrate ions in NHiNO, + NH,F solutions [5]. The results presented here support this hypothesis. RESULTS AND DISCUSSION
The double-layer capacity at a (I 10) silver- eIe&de was measured in ~aqueous solutions of xM N&r + (0.04 - x)M NaF at 25”C, using the same experimental conditions as given in ref. 6. The potential range of investigation was extended from - 1.5 to -0.2 V, with respect to a saturated calomel electrode, just before the formation of silver bromide. Solutions were prepared from “Suprapure Merck” NaF and NaBr and “Miilipore” water of bigb purity. The capacity of
C/GF.crn-’
Fig. I. Differential
solutions O.oOS(--.---
of xM )
(110)
Ag/
NaBr
+
N aF
capacity of the electrical double layer of a (I IO) silver .el&trode in. c&t&t
NaFk+(O.W:x)M
Ne
x=0
(
(4): 0.01 (- -.a-)(5):0.02(---)(6);0_04(--m-)(7).
) (I):
0.00125
(- - - - ;,-) (2): &JO25 (-I-,
_
: -:
-with (3);
-_
.~
. .
: $3
Fig. 2. Apparent density of charge due to specifically adsorbed bromide ions (a’) apparent density of ekctkde charge (u) and bulk concentraticn of bromide ions.
as a function of the
by assuming equality of the electrode charge u and the reversible work req*uiredto form the unit area of new surface y for all solutiotis at sufficiently negative vajues of Q (- 20 pC cmm2). These calculations were carried ‘out with the aid of a Tektronix TEK 31 progra&mable computeti. The curves of Ay vs. tog 6 at constant potential -were djfferentiated grtiphically. The deta@d mode of calculation is siinilar to that given in ref. 6. It should- be mentioned that -the migimug~ .potentid-is .&ken as the
pzc for the pure NaF solution; thi+ is only true iri thk absknce of spec$ic ads&ion. As fluoride.hzqbk& ihown-to present a_slight specific adsbrption [7], a smallkrror is mad& in the determinatigi of the elect&de charge. From the high yahxs of -capacity for the @ied so~utiqns, thfi system&c. error one the ekctrode charge-potent@ variations &tained by, back i&egratiori is &en sr&&r~~d may be considered as negligible_ ‘:=L .-- : _‘;_.. ~_ .: y _ _
-The- ?rariations df .oi ~aS,,afuUn@& of .-tG eleft&l~ .ch$rge’ u and the N&r +xxxitr@jy~ z&-ep’o;hi+iz Fig, 5. From n&&ive eIe&& char&$ td & A::+ 60 @C cti?.?, &e ..s+&) :curVes ~r&e+3h$:T pyl w&$&&n .*tEh, iorgc con&&$i~~ ISOGr: . .: +. .Gh&‘& &nl;;c-dontitiatioi _& fc%md;$amely: t&4 adso&&i $@ge: a’ id :. :.-::.
314
decreases. When u > 60 pC cmw2 the inverse order is observed; owing to this striking. and anomalous variation of ui with ionic concentration, the thermodynatnic analysk T was only carried out for - 20 c o c + 20 pC cm-‘, i.e. the electrode charges range
concerned with the first peak on C(E) curves. THE INNER-LAYER
CAPACITY
AND
I-i-S COMPONENTS
The (differential) total inner-layer capacity C’ was then calculated from diffuse layer theory by means of the equation [8] modified for solid surfaces: R(C)-’
=
(Ci)_’ +
(Cd)_‘(l
Tt_&7’/&t)
(1)
where R is the roughness factor determined from the 0.04 M NaF solution according to the method given in ref. 9. Here, R is taken as 1.15 * 0.05; this value is somewhat higher than that, approaching the exact value, determined in hexafluorophosphate solution [7]; Cd is the capacity of the diffuse layer. The coefficient G/au is the slope of the plots of Fig. 2. The C’(u) curves are shown in Fig. 3. For the greater part of the electrode charge range, namely from - 10 to -2 PC cm-* and from + 10 to +20 PC cm-l, the total
0
!g)bC.cm-’ , -10
0
10
20
w
Fig. 3. Total inner-layer (differential) capacity as a function of electrode charge, exphsed p’i -&t real area. and bromide concentration: x=0.(- . . -)(I): 0.00125(-_I -_) (2); o.wI25 (- - - - ---) (3); 0.005 (---) (4); 0.01 (- -_) (5); 0.02 ( ) (6); 0.04 (- - - -) (7).
. . .. ._
..
3ii
-.._
.-.I.--
-. -mner-l~~~~capac;~~“~.~~~~~-
.w&
,:.
,, @s
bromide :c&entr&on;Th&
-~~~~~~‘p~~~;j~~~~~~~~~~~~
.+~~<+~+I~0
.&&ion
,pG ~c&~?,-~C~f-&i~
,agrees in-&e
$@I~. 5-F-g &i&t; and .in ‘the jger~‘@kction: for. the:two s&&k b~~~~~~.lconcentrations;.For_ th&t$ : two ’ .s&~ons, the”-total inner&j& capacity-
,~$di@@ftjr
coxrgs ne@!$ ihe:mukh bigher CT!values of. thelpure .NaF ~solutionresulting from- an intqkre& &ppIi+ion:of the double-layer-m&h%in theabsence of. spe@fic adsorfkion;
as demo_nstrated$nref._.7;The increase of C,!,Gth dkease,of [KBr] (or of.& ican be.. exp@is@ .in terms :of:-.th& in&qe ~f,Spec$icallyadso~bgd F T-.which is neglected in
&applic@n of.:the &odel.:S&b & o‘bservation is rendered.possibb through& high value._ of C,i-for &ilk& ti_th &s&k to those- for me&&; for this reason the experimental double-layer capacity t&i beconk? vkti s&&tive to the effect of the diffuse layer [7}. The tiegkct of :the &@ic-adsorptioti of fluoride .may also explain the anomalous.v&&on .of ai,‘&nst Q, -asa function of the bromide~concentration, when o >-60 pC cm-.? (Fig. 2). It Imust.be. noted that a similar variation has been observed on gold [ 101in halide solutions at constant ionic strength-with NaF as base electrolyte. -. -The.. total. @ner:layer- (differential) capacity was then resolved into differential components measured at Constant electrode charge and at constant amount adsorbed through the identity [I I]. (c’)Y,’
= (,ci)-l
f
(,cy’
ad/a;
(2)
where J?, the differential capacity of the inner layer measured at constant amount adsorbed, and ,iCiS the differential capacity measured at constant electrode charge, are defined by ,c’
(3)
= afr/qP2
and ,ici =
ad/a&2
(4)
where q5M-2jthe potential drop across the inner layer, wascalculated by subtracting the potent@ of zero -charge‘@xc) and the potential difference in the diffuse layer (+‘) from the ,me&ured cell emf (E): _
*“-‘-=E-p~-&
..
-6)
The d@iren~ kmponent of .the .inner-layer capticity ,,C’ was derkd from Fig. 4 where the potenti;nt~dropa&ss the inner layer is plotted ti a function of ei at con.&& ektrode chtiges.- In Fig.4 the data obtained for the chloride ion in .xM NaCl+ (C&O4 i X)M-N& solu~onS are also ‘reported (dotted lines) [6]. For the bromide_:.solutions-.whenl O_fl@ 5 X.G 0.04,. as the plots are ~hne& the differential capacity may.bereplaced by the integrz&capaci$ &!; _f+he+more, ,the parallelism observe~-impZiesthaf.~~~~~;tK’ is c&tant,‘.~then~.~~~‘,~, 85 k !v.CsF cm:?: These &:o:p&&&& obsen$jns:~ &&&de’&& ror&~adsor@&n Gas studied m -‘. single~_salt~ ~~u~_o@$s~$h~&- &hde; [2&~~briji$ide~@~, ,_+o$e: @I (@,s$ite-.,of-.me @nail scatter._of the pbin~forT:‘@s’:salt) _yd nitrate-141, in: @&oti a~~.+.&nt .;“. .. . or : ..~~. .:-.
.-.
.-
316 ionic strength with fluoride when the ionic adsorption is sufficiently strong; such as iodide [3] and bromide for u > 6 pC [14{. On the other hand, for the bromide solutions when 0.00125 GX G 0.005, the plots +“-2 (u’) are neither line& nor I parallel. The striking feature is that-the bromide plots tend to join the chloride plots [6]. In this latter case, if the plots may be considered as linear, they are no longer increases with the electrode charge. This variation may be parallel and ,X’ compared to that observed on mercury in solutions at constant ionic strength for an ion, the specific adsorption of which is recognized as weak, such .as nitrate [5], perchlorate [ 151and perhaps also chloride [ 1q (the actual situation of this latter ion appearing poorly defined on mercury)_ Consequent to the evidence of. specific adsorption of fluoride in the range of the small bromide concentrations shown on C’ (a) curves, the different variation of qP-* with ui may also be attributed to the same phenomenon. By analogy, it may be advanced that in the previous study of the chloride ion [6], the high values of ,iK’ should also bc due to the competitive adsorption of fluoride. The extrapolated values at ui = 0 of the linear parts of the r&M_2(a’) plots (Fig. 4) do not coincide with the $“-2 values calculated according to eqn. (5) for the 0.04 M
Fig. 4. Potential drop across the inner layer as a function of charge of the adsorbed bromide. at cons&t
electrode charges. Dotted tines are relative to the NaCl+NaF
data [a].
-317. :
.
-_
?&F;-&htitq@:The .ionic ~adkuption of fluoride:m_ayalso:-be responsible’for this d&ergence-.asshoti below. ... :; : : : ._. .. .. : -. -.(1)3t has beerkshownm ref.cl:thatthe C(E)-or. C’(a) curves &~symmetriLl~m the absence of specifk+k+ption, therefore-the+%! (al F 0) values must ako be symmetricalwith respectto a - 0. Consequently,the potent@ drop .#!.2(e.i z 0) in the- inn&r lay&r-:at.~positive:charges is expected to be more :positivc than .tbose calculatedfor-NaF. -.~ ...: :-I-. .. (2) T+:pzc in th& absence of spe&ic. adsorption is about 15-mV more positive .than the capacity minimum potential in the 0.04% NaF solution [A. The.electrode charge -is therefore evaluated with a positive error in NaF, the #‘-* values in bromide solutions sh0ti.d.be decreased.These preceding. observations converge to minimize the difference between extrapolated and calculated +“-’ vahres, when Q’ = 0. :The differemial Componentof the inner-layer. capacity measured at a cmstant amount of adsorbed ,Ci was further calculatedfrom-eqn.(2). Here, ,$’ as a.function -of the ekctrode charge and bromideconcentrationis given in Fig. 5: The inner-layer capacity from ICPF, sohrtionsis also representedin this figure; this curve has been shown to depend on the behaviourof water molecules[7]. The variation of ,,C’ is
(c3p-‘tn-2 -16
.-0
io
:
._
2G
Fig. 5. Diffe~&tial co&onent of the h&r-layer &xity me&u&$ at c&&& amount .adso&d &a function of ekckqle charge. and bromide concentra&: x:0 (-- - - - - - - -) (I); O.tJO125(& - - - -) (2); ‘. ) (7). l+‘dotted 0.0025 (- 1-x (3); 0.005 ( --) <4); 0.01)~(----) (5);.O-02 ( --: - -) (6); 0.04 ( _’ lineisrelativetochebexafluoropb~spha~~dalajl]. :C..-. ‘:i -.. -. ..
31s
notable in that the maximum is a function of the electrode charge for mixed solutions. When 0.01 sz x =Z0.04, the maximum is situated at the zero charge and ,C’ is slightly dependent on the bromide concentration. When 0.005 ax~=O, the position of the maximum varies from +2 to +5 pC cm-* and ,C’ increases markedly as the bromide concentration decreases. These observations are still consistent with the competitive adsorption of the fluoride ion. In fact when 0.01
,Ki/,iKi
= (x, -x,)/x*
where xl and x2 are respectively the distances from the electrode plane lep) to the inner Helmholtz plane (iHp), and from the ep to the outer Helmholtz plane (oHp). For the (110) silver/NaBr system, when the more concentrated solutions are considered, the distance ratio obtained from the values of ,K’ and ,iKi deduced from Fig. 4 is equal to 0.6 -C0.1. This value is comparable with those determined on mercury: 0.5 [ 141and 0.3 [20] in KBr + KF mixed solutions or 0.4 [ 121in a single salt solution. If 0.6 is-a reliable value for the present interface, the small increase with respect to mercury could be interpreted with the same hypothesis made in ref- 7, namely a decrease of the inner-layer thickness due to the definition of the electrode plane. On addition, it may be reasonably envisaged that a bromide ion is as well fitted into the rails of the (110) face as a water molecule. Under these conditions x,) could be maintained as a constant, while x2 should decrease from mercury (x2 to (I 10) silver. With the more dilute solutions in NaBr and with the NaCl+ NaF solutions 161, the distance ratio decreases and tends towards to 0.15; this datum is comparable
319 with that obtained on .mercury in KCl + KF solutions [16]. The cause of this small value may once more be attributd to the specific adsorption of fluoride. DEPENDENCE OF THE STANDARD TRODE CHARGE
FREE ENERGY
OF IONIC ADSORPTION
ON ELEC-
The variation of another adsorption characteristic is notably different when results from single salt solutions are compared with those from solutions at constant ionic strength. This characteristic is the electrode charge dependence of the standard free energy of adsorption, which is derived from the composite curves of the surface pressure of adsorbed ions measured at constant electrode charges as a function of the logarithm of concentration. When the ionic adsorption’ is strong, a linear electrode charge dependence of the standard free energy would be observed for the two systems, for example with ICI [2] and KI + KF [3]. In contrast, when the ionic adsorption is less strong, this electrode charge dependence would be linear with single salt solutions and non-linear with solutions at constant ionic strength, and this would be the case of the nitrate ion [4,5]. In the present study the surface pressures have not been calculated to evaluate this electrode charge dependence; the potential shift approach has been used. According to the thermodynamic analysis of the ionic adsorption in solutions at constant ionic strength [3]:
Wb @T/F) [
-a lnx
1 (z) *=
(7)
x
where Eb is the measured cell emf in the base solution in the absence of bromide ions. If the isotherm for bromide adsorption is congruent with respect of the electrode charge, i.e. the shape is independent of the electrode charge, eqn. (7) can be integrated [3]:
where ln fl is related to the standard free energy of adsorption by RTh@=
-AG,,
(9)
Equation (8) implies that the plot of AE against the logarithm of bromide concentration is the adsorption isotherm with the qi scale multiplied by a ln B/au. values for the NaBr + NaF solutions/(1 10) silver system. are The (EL -E) plottcdagainst the logarithm of bromide concentration in Fig. 6, for various electrode charges. The plots are parallel straight lines’which are therefore segnients of a common adsorption isotherm. Thus, a composite curve, not represented here, can be obtained by shifting parallel to the abscissa. It is evident in Fig.6 that these displacements +xrenot at all proportional to the electrode charge (see Fig. 8 where they are plotted against Q). Ccnsequently, a non-linear electrode charge dependence of the standard free energy may be advanced for the bromide adsorption. However,
320
[Eb-E)/”
6~
l
0.:
10
l
8
l
6
l4 +2
0
0.2
0.
-2 -4
0:
0.’
X
-3
-2
-1
b
Fig. 6. (E, - E) against logarithm of bromide concentration taken from the 0.04 AI NaF solution.
-3
-2
-1
e
at constant electrode charges. when Eb is
Fig. 7. (E,--E) against logarithm of bromide concentration at constant electrode charges. when E,, is taken from a symmetrical E:o) curve. corresponding to an abs~ncz of specific adsorption for the base electrolyte (71.
the E, values in the NaF base solution are not those corresponding to an absence of specific adsorption. From the symmetrical shape of the C(E) ewe of the (110) electrode in KPF, solutions 171,the E(a) curve without specific adsorption is also symmetrical with respect to u = 0. With the corrected Eb values, the dE(l& x) plots are represented in Fig. 7. The individual segments are still parallel st+ght lines, but the striking feature is that the displacement of the individual segments, b obtain the composite curve, is then proportional to the electrode charge..These shifting .values are given in Fig. 8. When the infhxence of the specific adsorption of flu&de is eliminated, a linear electrode charge dependence of the skiard free energy is found __._:: . for the bromide adosprtion. This conclusion may be confirmed when ui is plotted vs. E, at]+st&k&xk&k charges, allawing the Eb contribution in eqn. (8) to be eliminated. ‘HI+!(E) +ms are represented in Fig. 9. These parallel straight lines justify the applika@ity c$,q.
-
a/pC_cm-2 0
5
10
15
*
o-
-1.1
0
-1.0
‘E/V
(Sk.3 c
-0.9
Fig. 8. Dependen% of the standard free ene& of adsorption of bromide on electrode charge: (-0-) is-taken from the E(o) curve relative to NaF, (-.X-) E,., is taken from a symmetrical E(o) curve. Fig. 9. Charge due to the specifically adsorbed bromide as a function of the measured All emeat electrode charges.
E,
constant
tid the standard free energy of bromide adsqpeon on’ (1 i0) &&r & cle&ly linearly dependent on the electrode charge, since (a In /3/i30)is a con++ value in the whble electr+le charge rang% equal -to-that of Fig. 8 when E,, is taken from a symmetrical E(a) curve, for-W base electrol~e. The value of (&E/au’), is 0.016 * 0.002 PF-*- crk2; for .the present system, (a+2/api); ‘isfound to be essentially con+ant and equal to 0.005 pF_ t Icin2. The value of. (&J#‘-~/&T~.), is therefore 0.01 1 + 0.002 PF :‘, &, which agrkes with that-deduced from the straight lines in : Fig. 4 (0.012 -C0.002). (S),
322
on single-crystal siIver ekxtrodes, in particular that of the xc&d capacitive peak, is to be carried out, either in mixed solutions with KPF, as base electrolyte, if, the anionic adsorption of this electrolyte is negligible in the region of high positive electrode charges, or in single salt solutionsACKNOWLEDGEMENT
The author is indebted to Dr. Roger Parsons for useful discussions and for the improvement of the presentation of this work. REFERENCES 1 G.J. Hills and R.M. Reeves. J. Electroanai. Chem.. 3 1 (1971 i 269.
2 3 4 5 6 7 8 9 10
DC. Grahame. J. Am. Chem. Sec. 80 (1958) 4201. E Dutkiewicz and R. Parsons. J. hectroanal Chem.. ii (1966) 100. R. Payne. J. Ekctrochem. Sot.. 113 (1966) 999. R. Payne. J. Phys. Chem.. 69 (1965) 4113. G. Valette. A. Hameiin and R. Parsons, Z. Phys. Chem.. 113 f 1978) 71. G. Valette. J. Ekctroanal. Chem.. 122 (1981) 285. M.A.V. Devanathan. Trans. Faraday !Soc.. 50 (1954) 373. G. Valette and A. Hamelin. J. Electroanal. Chem.. 45 (1973) 301. J.P. Beilier. Thesis. Paris. 1980. 11 R. Parsons, Trans. Faraday Sot.. 55 (1959) 99. 12 J. Lawrence. R. Parsons and R. Payne. J. Ekctroanal. Chem.. 16 (1968) 193. 13 D.C.
Grahameand
R. Parsons,
J. Am. Chem.
14 G.J. Hills and R.M. Reeves. J. Ekctroanal. 15 R. Payne. J. Phys. Chem.. 70 (1966) 204.
SIC.. 83 (1961)
1291.
Chem.. 42 (1973) 355.
16 R. Payne. Trans. Faraday Sot_. 64 (1968) 1638. 17 R.J. Watts-Tohin, Philos. Msg.. 6 (l%l) 133. 18 J.O’M. Bockris. M.A.V. Devanathan and K. Muiler. Proc. R. Sot. (Land.). 19 J.M. Parry and R. Parsons. Trans. Faraday Sot.. 59 ( 1963) 24 1. 20 A.R. Scars and P.A. Lyons, J. Elcctroanal. Chem.. 42 (1973) 69.
A274
(1%3)
55.
%ETTE
_
~htt~t@~.d’Eh&&tie &ved:;l&d
-&tet$u&du
C.N.kS..
1, Place A. Briam&-92190 Medon-Bellevue
(France)
J&x 1981; in revise&form StbA~gust :I!XI)
: ..
ABtiCr
:
.-
1
.T& adsorptian of the bromideion oil a (1 silver &&r&e from mix&d solutions of s&urn bromide and [email protected]$de was investigated by measuring the doudfe-layer capacity as a function of solution composition_ _n?e a&y& of the total inner-layer (diffkential) capacity as a function of the elect&e charge bhogs clear‘kdence for com&titive &cific adsor$t&m of the ‘fluoride ion when the bromi& cokxntration~is sm@kwith respkt to the fluoride c¢&ion_ J%e total inner-lay& cap&y is ksolvcd
16,
into i&. comp&znt parts with the-aid of diffuse layer ihe& The (integral) capacity measured at cotistant ekctrod~ charge is a.constant -when the fluoride adsorption appears not to_interfere. The (differential) capacity measured at. constant amqmt adsorbed as .a function of the_ electrode charge presents a maximum at Zerocharge, when thi: fluoride influence is negligible; which may bc attributed to the solvent propertks. With c&re@ed values of the eIcctr&e ciiarge vs. potential plots for a base elecklyte without aniottic adsorpti& the standard free energy of br&ide adsorpt&x~ is shown to be linearty dependent on
the electrode charge, as for the bromide aqueous solutions/mercury
system.
INTRODUCTION
The numerous studies of ionic adsorption on mercury have been made so far with two main types of measureme&:. oqe using the.single.salt in aqueous solutions tid one using solutiofis the i&c strength of which is~m~+i~$ainedconstant by addition’of fluoride as b.&&.el~tr~lyt& For & &en anion, signif&a~~‘diffe&ike.s are observed when re.sults’obt+ied from the two techr+c&s .&r6 &mpared:_‘Thk- tiost striking dkcrepaxicies con&n the c&j&onent of the i+ef layer. at tionsigt ekctrode charge, wti_ch is-a tin+ant.& most Casey for the~s&gle tilt soIu&& and which is dependent .on. tIie elect&de dhtige’foi the soIutions~at_~ns&ion& strength,-an$on the other han&the stFdz@d- fr+ .&er&y -&icJ~& iinearl~- dependent on .the’eIFtrdfle charge foilthe si&k sali stiIutio&ra@ no~~line&~+petidedent for the other solutions. : ,Hills .&d- &eVes [ 1) have suggest&i ihat -.thes$ .diScrepan&s between the two kinds of iGti$ a& 8~t$&ibl~:;.iii a &&~eiiii& ad&ptibn:~6f ;the fluoride ions ovi&g ;_tOi th_e__ f&i : t@ ;e_~ i ;dis~~~~~ies 1ti$i .~~i@e;-ved io decrease .:e :_th$ +!c$fic -$&f$$&.~.of_, -i&ji&
.
io&
.;:.
&&‘&&b: f&k
~~~~~‘~~~c~~~~~,~u~~.~.~~~:~,~~~p~~,,
61’: &eon$
131.:&&y’@~~bI&’
&
_,&&pGqp
.of
(Q&~.‘$j&jQ&i : of hi&@? [io@ .._._ ..
..’ _ _-, : _j .* -j -.,_ ’ ~~~7~8/~2,Y~~~looo/so27s:@Z~ 1482 &&&~&u~ia~!L&~ : : .m:.‘_), :: “._ :_ I _ _... -_ ... .,~:,~-l..: ._.c 1.. :_ : .:I.:_. . . ..y -‘_ ,... :_’ ‘. ;_ . ..I-.. . ..._._.,: -.,I - :’ .,_
.-. I .I
:-
312 ‘:
from ICI + KF solutions [3], while the adsorption of nitrate ions from -KN@ solutions [4] differs notably from the adsorPtion of nitrate ions in NHiNO, + NH,F solutions [5]. The results presented here support this hypothesis. RESULTS AND DISCUSSION
The double-layer capacity at a (I 10) silver- eIe&de was measured in ~aqueous solutions of xM N&r + (0.04 - x)M NaF at 25”C, using the same experimental conditions as given in ref. 6. The potential range of investigation was extended from - 1.5 to -0.2 V, with respect to a saturated calomel electrode, just before the formation of silver bromide. Solutions were prepared from “Suprapure Merck” NaF and NaBr and “Miilipore” water of bigb purity. The capacity of
C/GF.crn-’
Fig. I. Differential
solutions O.oOS(--.---
of xM )
(110)
Ag/
NaBr
+
N aF
capacity of the electrical double layer of a (I IO) silver .el&trode in. c&t&t
NaFk+(O.W:x)M
Ne
x=0
(
(4): 0.01 (- -.a-)(5):0.02(---)(6);0_04(--m-)(7).
) (I):
0.00125
(- - - - ;,-) (2): &JO25 (-I-,
_
: -:
-with (3);
-_
.~
. .
: $3
Fig. 2. Apparent density of charge due to specifically adsorbed bromide ions (a’) apparent density of ekctkde charge (u) and bulk concentraticn of bromide ions.
as a function of the
by assuming equality of the electrode charge u and the reversible work req*uiredto form the unit area of new surface y for all solutiotis at sufficiently negative vajues of Q (- 20 pC cmm2). These calculations were carried ‘out with the aid of a Tektronix TEK 31 progra&mable computeti. The curves of Ay vs. tog 6 at constant potential -were djfferentiated grtiphically. The deta@d mode of calculation is siinilar to that given in ref. 6. It should- be mentioned that -the migimug~ .potentid-is .&ken as the
pzc for the pure NaF solution; thi+ is only true iri thk absknce of spec$ic ads&ion. As fluoride.hzqbk& ihown-to present a_slight specific adsbrption [7], a smallkrror is mad& in the determinatigi of the elect&de charge. From the high yahxs of -capacity for the @ied so~utiqns, thfi system&c. error one the ekctrode charge-potent@ variations &tained by, back i&egratiori is &en sr&&r~~d may be considered as negligible_ ‘:=L .-- : _‘;_.. ~_ .: y _ _
-The- ?rariations df .oi ~aS,,afuUn@& of .-tG eleft&l~ .ch$rge’ u and the N&r +xxxitr@jy~ z&-ep’o;hi+iz Fig, 5. From n&&ive eIe&& char&$ td & A::+ 60 @C cti?.?, &e ..s+&) :curVes ~r&e+3h$:T pyl w&$&&n .*tEh, iorgc con&&$i~~ ISOGr: . .: +. .Gh&‘& &nl;;c-dontitiatioi _& fc%md;$amely: t&4 adso&&i $@ge: a’ id :. :.-::.
314
decreases. When u > 60 pC cmw2 the inverse order is observed; owing to this striking. and anomalous variation of ui with ionic concentration, the thermodynatnic analysk T was only carried out for - 20 c o c + 20 pC cm-‘, i.e. the electrode charges range
concerned with the first peak on C(E) curves. THE INNER-LAYER
CAPACITY
AND
I-i-S COMPONENTS
The (differential) total inner-layer capacity C’ was then calculated from diffuse layer theory by means of the equation [8] modified for solid surfaces: R(C)-’
=
(Ci)_’ +
(Cd)_‘(l
Tt_&7’/&t)
(1)
where R is the roughness factor determined from the 0.04 M NaF solution according to the method given in ref. 9. Here, R is taken as 1.15 * 0.05; this value is somewhat higher than that, approaching the exact value, determined in hexafluorophosphate solution [7]; Cd is the capacity of the diffuse layer. The coefficient G/au is the slope of the plots of Fig. 2. The C’(u) curves are shown in Fig. 3. For the greater part of the electrode charge range, namely from - 10 to -2 PC cm-* and from + 10 to +20 PC cm-l, the total
0
!g)bC.cm-’ , -10
0
10
20
w
Fig. 3. Total inner-layer (differential) capacity as a function of electrode charge, exphsed p’i -&t real area. and bromide concentration: x=0.(- . . -)(I): 0.00125(-_I -_) (2); o.wI25 (- - - - ---) (3); 0.005 (---) (4); 0.01 (- -_) (5); 0.02 ( ) (6); 0.04 (- - - -) (7).
. . .. ._
..
3ii
-.._
.-.I.--
-. -mner-l~~~~capac;~~“~.~~~~~-
.w&
,:.
,, @s
bromide :c&entr&on;Th&
-~~~~~~‘p~~~;j~~~~~~~~~~~~
.+~~<+~+I~0
.&&ion
,pG ~c&~?,-~C~f-&i~
,agrees in-&e
$@I~. 5-F-g &i&t; and .in ‘the jger~‘@kction: for. the:two s&&k b~~~~~~.lconcentrations;.For_ th&t$ : two ’ .s&~ons, the”-total inner&j& capacity-
,~$di@@ftjr
coxrgs ne@!$ ihe:mukh bigher CT!values of. thelpure .NaF ~solutionresulting from- an intqkre& &ppIi+ion:of the double-layer-m&h%in theabsence of. spe@fic adsorfkion;
as demo_nstrated$nref._.7;The increase of C,!,Gth dkease,of [KBr] (or of.& ican be.. exp@is@ .in terms :of:-.th& in&qe ~f,Spec$icallyadso~bgd F T-.which is neglected in
&applic@n of.:the &odel.:S&b & o‘bservation is rendered.possibb through& high value._ of C,i-for &ilk& ti_th &s&k to those- for me&&; for this reason the experimental double-layer capacity t&i beconk? vkti s&&tive to the effect of the diffuse layer [7}. The tiegkct of :the &@ic-adsorptioti of fluoride .may also explain the anomalous.v&&on .of ai,‘&nst Q, -asa function of the bromide~concentration, when o >-60 pC cm-.? (Fig. 2). It Imust.be. noted that a similar variation has been observed on gold [ 101in halide solutions at constant ionic strength-with NaF as base electrolyte. -. -The.. total. @ner:layer- (differential) capacity was then resolved into differential components measured at Constant electrode charge and at constant amount adsorbed through the identity [I I]. (c’)Y,’
= (,ci)-l
f
(,cy’
ad/a;
(2)
where J?, the differential capacity of the inner layer measured at constant amount adsorbed, and ,iCiS the differential capacity measured at constant electrode charge, are defined by ,c’
(3)
= afr/qP2
and ,ici =
ad/a&2
(4)
where q5M-2jthe potential drop across the inner layer, wascalculated by subtracting the potent@ of zero -charge‘@xc) and the potential difference in the diffuse layer (+‘) from the ,me&ured cell emf (E): _
*“-‘-=E-p~-&
..
-6)
The d@iren~ kmponent of .the .inner-layer capticity ,,C’ was derkd from Fig. 4 where the potenti;nt~dropa&ss the inner layer is plotted ti a function of ei at con.&& ektrode chtiges.- In Fig.4 the data obtained for the chloride ion in .xM NaCl+ (C&O4 i X)M-N& solu~onS are also ‘reported (dotted lines) [6]. For the bromide_:.solutions-.whenl O_fl@ 5 X.G 0.04,. as the plots are ~hne& the differential capacity may.bereplaced by the integrz&capaci$ &!; _f+he+more, ,the parallelism observe~-impZiesthaf.~~~~~;tK’ is c&tant,‘.~then~.~~~‘,~, 85 k !v.CsF cm:?: These &:o:p&&&& obsen$jns:~ &&&de’&& ror&~adsor@&n Gas studied m -‘. single~_salt~ ~~u~_o@$s~$h~&- &hde; [2&~~briji$ide~@~, ,_+o$e: @I (@,s$ite-.,of-.me @nail scatter._of the pbin~forT:‘@s’:salt) _yd nitrate-141, in: @&oti a~~.+.&nt .;“. .. . or : ..~~. .:-.
.-.
.-
316 ionic strength with fluoride when the ionic adsorption is sufficiently strong; such as iodide [3] and bromide for u > 6 pC [14{. On the other hand, for the bromide solutions when 0.00125 GX G 0.005, the plots +“-2 (u’) are neither line& nor I parallel. The striking feature is that-the bromide plots tend to join the chloride plots [6]. In this latter case, if the plots may be considered as linear, they are no longer increases with the electrode charge. This variation may be parallel and ,X’ compared to that observed on mercury in solutions at constant ionic strength for an ion, the specific adsorption of which is recognized as weak, such .as nitrate [5], perchlorate [ 151and perhaps also chloride [ 1q (the actual situation of this latter ion appearing poorly defined on mercury)_ Consequent to the evidence of. specific adsorption of fluoride in the range of the small bromide concentrations shown on C’ (a) curves, the different variation of qP-* with ui may also be attributed to the same phenomenon. By analogy, it may be advanced that in the previous study of the chloride ion [6], the high values of ,iK’ should also bc due to the competitive adsorption of fluoride. The extrapolated values at ui = 0 of the linear parts of the r&M_2(a’) plots (Fig. 4) do not coincide with the $“-2 values calculated according to eqn. (5) for the 0.04 M
Fig. 4. Potential drop across the inner layer as a function of charge of the adsorbed bromide. at cons&t
electrode charges. Dotted tines are relative to the NaCl+NaF
data [a].
-317. :
.
-_
?&F;-&htitq@:The .ionic ~adkuption of fluoride:m_ayalso:-be responsible’for this d&ergence-.asshoti below. ... :; : : : ._. .. .. : -. -.(1)3t has beerkshownm ref.cl:thatthe C(E)-or. C’(a) curves &~symmetriLl~m the absence of specifk+k+ption, therefore-the+%! (al F 0) values must ako be symmetricalwith respectto a - 0. Consequently,the potent@ drop .#!.2(e.i z 0) in the- inn&r lay&r-:at.~positive:charges is expected to be more :positivc than .tbose calculatedfor-NaF. -.~ ...: :-I-. .. (2) T+:pzc in th& absence of spe&ic. adsorption is about 15-mV more positive .than the capacity minimum potential in the 0.04% NaF solution [A. The.electrode charge -is therefore evaluated with a positive error in NaF, the #‘-* values in bromide solutions sh0ti.d.be decreased.These preceding. observations converge to minimize the difference between extrapolated and calculated +“-’ vahres, when Q’ = 0. :The differemial Componentof the inner-layer. capacity measured at a cmstant amount of adsorbed ,Ci was further calculatedfrom-eqn.(2). Here, ,$’ as a.function -of the ekctrode charge and bromideconcentrationis given in Fig. 5: The inner-layer capacity from ICPF, sohrtionsis also representedin this figure; this curve has been shown to depend on the behaviourof water molecules[7]. The variation of ,,C’ is
(c3p-‘tn-2 -16
.-0
io
:
._
2G
Fig. 5. Diffe~&tial co&onent of the h&r-layer &xity me&u&$ at c&&& amount .adso&d &a function of ekckqle charge. and bromide concentra&: x:0 (-- - - - - - - -) (I); O.tJO125(& - - - -) (2); ‘. ) (7). l+‘dotted 0.0025 (- 1-x (3); 0.005 ( --) <4); 0.01)~(----) (5);.O-02 ( --: - -) (6); 0.04 ( _’ lineisrelativetochebexafluoropb~spha~~dalajl]. :C..-. ‘:i -.. -. ..
31s
notable in that the maximum is a function of the electrode charge for mixed solutions. When 0.01 sz x =Z0.04, the maximum is situated at the zero charge and ,C’ is slightly dependent on the bromide concentration. When 0.005 ax~=O, the position of the maximum varies from +2 to +5 pC cm-* and ,C’ increases markedly as the bromide concentration decreases. These observations are still consistent with the competitive adsorption of the fluoride ion. In fact when 0.01
,Ki/,iKi
= (x, -x,)/x*
where xl and x2 are respectively the distances from the electrode plane lep) to the inner Helmholtz plane (iHp), and from the ep to the outer Helmholtz plane (oHp). For the (110) silver/NaBr system, when the more concentrated solutions are considered, the distance ratio obtained from the values of ,K’ and ,iKi deduced from Fig. 4 is equal to 0.6 -C0.1. This value is comparable with those determined on mercury: 0.5 [ 141and 0.3 [20] in KBr + KF mixed solutions or 0.4 [ 121in a single salt solution. If 0.6 is-a reliable value for the present interface, the small increase with respect to mercury could be interpreted with the same hypothesis made in ref- 7, namely a decrease of the inner-layer thickness due to the definition of the electrode plane. On addition, it may be reasonably envisaged that a bromide ion is as well fitted into the rails of the (110) face as a water molecule. Under these conditions x,) could be maintained as a constant, while x2 should decrease from mercury (x2 to (I 10) silver. With the more dilute solutions in NaBr and with the NaCl+ NaF solutions 161, the distance ratio decreases and tends towards to 0.15; this datum is comparable
319 with that obtained on .mercury in KCl + KF solutions [16]. The cause of this small value may once more be attributd to the specific adsorption of fluoride. DEPENDENCE OF THE STANDARD TRODE CHARGE
FREE ENERGY
OF IONIC ADSORPTION
ON ELEC-
The variation of another adsorption characteristic is notably different when results from single salt solutions are compared with those from solutions at constant ionic strength. This characteristic is the electrode charge dependence of the standard free energy of adsorption, which is derived from the composite curves of the surface pressure of adsorbed ions measured at constant electrode charges as a function of the logarithm of concentration. When the ionic adsorption’ is strong, a linear electrode charge dependence of the standard free energy would be observed for the two systems, for example with ICI [2] and KI + KF [3]. In contrast, when the ionic adsorption is less strong, this electrode charge dependence would be linear with single salt solutions and non-linear with solutions at constant ionic strength, and this would be the case of the nitrate ion [4,5]. In the present study the surface pressures have not been calculated to evaluate this electrode charge dependence; the potential shift approach has been used. According to the thermodynamic analysis of the ionic adsorption in solutions at constant ionic strength [3]:
Wb @T/F) [
-a lnx
1 (z) *=
(7)
x
where Eb is the measured cell emf in the base solution in the absence of bromide ions. If the isotherm for bromide adsorption is congruent with respect of the electrode charge, i.e. the shape is independent of the electrode charge, eqn. (7) can be integrated [3]:
where ln fl is related to the standard free energy of adsorption by RTh@=
-AG,,
(9)
Equation (8) implies that the plot of AE against the logarithm of bromide concentration is the adsorption isotherm with the qi scale multiplied by a ln B/au. values for the NaBr + NaF solutions/(1 10) silver system. are The (EL -E) plottcdagainst the logarithm of bromide concentration in Fig. 6, for various electrode charges. The plots are parallel straight lines’which are therefore segnients of a common adsorption isotherm. Thus, a composite curve, not represented here, can be obtained by shifting parallel to the abscissa. It is evident in Fig.6 that these displacements +xrenot at all proportional to the electrode charge (see Fig. 8 where they are plotted against Q). Ccnsequently, a non-linear electrode charge dependence of the standard free energy may be advanced for the bromide adsorption. However,
320
[Eb-E)/”
6~
l
0.:
10
l
8
l
6
l4 +2
0
0.2
0.
-2 -4
0:
0.’
X
-3
-2
-1
b
Fig. 6. (E, - E) against logarithm of bromide concentration taken from the 0.04 AI NaF solution.
-3
-2
-1
e
at constant electrode charges. when Eb is
Fig. 7. (E,--E) against logarithm of bromide concentration at constant electrode charges. when E,, is taken from a symmetrical E:o) curve. corresponding to an abs~ncz of specific adsorption for the base electrolyte (71.
the E, values in the NaF base solution are not those corresponding to an absence of specific adsorption. From the symmetrical shape of the C(E) ewe of the (110) electrode in KPF, solutions 171,the E(a) curve without specific adsorption is also symmetrical with respect to u = 0. With the corrected Eb values, the dE(l& x) plots are represented in Fig. 7. The individual segments are still parallel st+ght lines, but the striking feature is that the displacement of the individual segments, b obtain the composite curve, is then proportional to the electrode charge..These shifting .values are given in Fig. 8. When the infhxence of the specific adsorption of flu&de is eliminated, a linear electrode charge dependence of the skiard free energy is found __._:: . for the bromide adosprtion. This conclusion may be confirmed when ui is plotted vs. E, at]+st&k&xk&k charges, allawing the Eb contribution in eqn. (8) to be eliminated. ‘HI+!(E) +ms are represented in Fig. 9. These parallel straight lines justify the applika@ity c$,q.
-
a/pC_cm-2 0
5
10
15
*
o-
-1.1
0
-1.0
‘E/V
(Sk.3 c
-0.9
Fig. 8. Dependen% of the standard free ene& of adsorption of bromide on electrode charge: (-0-) is-taken from the E(o) curve relative to NaF, (-.X-) E,., is taken from a symmetrical E(o) curve. Fig. 9. Charge due to the specifically adsorbed bromide as a function of the measured All emeat electrode charges.
E,
constant
tid the standard free energy of bromide adsqpeon on’ (1 i0) &&r & cle&ly linearly dependent on the electrode charge, since (a In /3/i30)is a con++ value in the whble electr+le charge rang% equal -to-that of Fig. 8 when E,, is taken from a symmetrical E(a) curve, for-W base electrol~e. The value of (&E/au’), is 0.016 * 0.002 PF-*- crk2; for .the present system, (a+2/api); ‘isfound to be essentially con+ant and equal to 0.005 pF_ t Icin2. The value of. (&J#‘-~/&T~.), is therefore 0.01 1 + 0.002 PF :‘, &, which agrkes with that-deduced from the straight lines in : Fig. 4 (0.012 -C0.002). (S),
322
on single-crystal siIver ekxtrodes, in particular that of the xc&d capacitive peak, is to be carried out, either in mixed solutions with KPF, as base electrolyte, if, the anionic adsorption of this electrolyte is negligible in the region of high positive electrode charges, or in single salt solutionsACKNOWLEDGEMENT
The author is indebted to Dr. Roger Parsons for useful discussions and for the improvement of the presentation of this work. REFERENCES 1 G.J. Hills and R.M. Reeves. J. Electroanai. Chem.. 3 1 (1971 i 269.
2 3 4 5 6 7 8 9 10
DC. Grahame. J. Am. Chem. Sec. 80 (1958) 4201. E Dutkiewicz and R. Parsons. J. hectroanal Chem.. ii (1966) 100. R. Payne. J. Ekctrochem. Sot.. 113 (1966) 999. R. Payne. J. Phys. Chem.. 69 (1965) 4113. G. Valette. A. Hameiin and R. Parsons, Z. Phys. Chem.. 113 f 1978) 71. G. Valette. J. Ekctroanal. Chem.. 122 (1981) 285. M.A.V. Devanathan. Trans. Faraday !Soc.. 50 (1954) 373. G. Valette and A. Hamelin. J. Electroanal. Chem.. 45 (1973) 301. J.P. Beilier. Thesis. Paris. 1980. 11 R. Parsons, Trans. Faraday Sot.. 55 (1959) 99. 12 J. Lawrence. R. Parsons and R. Payne. J. Ekctroanal. Chem.. 16 (1968) 193. 13 D.C.
Grahameand
R. Parsons,
J. Am. Chem.
14 G.J. Hills and R.M. Reeves. J. Ekctroanal. 15 R. Payne. J. Phys. Chem.. 70 (1966) 204.
SIC.. 83 (1961)
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16 R. Payne. Trans. Faraday Sot_. 64 (1968) 1638. 17 R.J. Watts-Tohin, Philos. Msg.. 6 (l%l) 133. 18 J.O’M. Bockris. M.A.V. Devanathan and K. Muiler. Proc. R. Sot. (Land.). 19 J.M. Parry and R. Parsons. Trans. Faraday Sot.. 59 ( 1963) 24 1. 20 A.R. Scars and P.A. Lyons, J. Elcctroanal. Chem.. 42 (1973) 69.
A274
(1%3)
55.