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Electrochemical adsorption of neutral and ionic components in solutions of pyridine and derived ions
JOURSAL
OI:
ELECTROAS_-\L\-TICLXL
ELECTROCHEMICAL COMPONENTS IX
CHEMISTRY
XDSORPTION SOLUTIOX5 OF
OF NEUTRAL PYRIDINE _4ND
AND 1O;lrfIC DERIVED IONS
In the development of electrochemical kinetics, the relation between the adsorption of reactant species and the rate of the reaction, as a function of potential and solution concentration of reactants and other species, has been a matter of central importance The quantitative formulation of such effects was first given by FRUMKIN~ and has constituted one of the major contributions in a wide variety of papers from his school, c-g., with regard to the kinetics of cation discharge (e.g., ref. 2,3) and more recently in interesting \vays with regard to the special effects associated with anion reduction4 and the role of specificaIly adsorbed cations in such processes. In the present paper, it is therefore appropriate to present some further data on the electrochemical adsorption of a type of organic species, which can exist in either a neutral or ionic form, and which is the parent of a family of molecules and ions giving marked effects on the kinetics of, for example, hydrogen evolution from acid solutions at mercury5--7. Previous studies of the electrochemical adsorption of organic aminium ions at mercury have been reported by BLOMGRES AXD BOCKRIS~ and the relation between adsorption of neutral pyridine derivatives and their conjugated protonated ions has been investigated by CONW_~Y _=I) BARR--\DAS at mercury9 and at solid electrodesre~ri. The capacity behaviour of organic bases at mercury has been investigated by LOFGZZ AND &R%XEL= and the nature of the isotherm for adsorption of certain benzenesulphonate anions has been studied by PARSONS AXD PARR\-13. Little information is at present available on how neutral and ionic additives affect the distribution of other base electrol-yte ions in the double-layer and how these effects are related to the adsorption behaviour of the organic species. In the present work, we. therefore report (a) the effect of adsorption of pyridine in neutral form on the ionic components of adsorbed charge in KC1 solutions at mercury; (b) the nature of the adsorption of the protonated conjugate pyridinium ion and neutral pyridine; and (c) the role of the co-anion in determining the interaction effects associated with the adsorption of the organic cation_ In these studies, the classical elect-rocapillary method has been used. The thermodynamics of the adsorption of organic bases, B, and their conjugate ions, Bl%+, has been discussed that elsewhere 8.924 where it has been emphasised unique‘characterisation
of the adsorption
of the protonated J_ Electvoaozal.
species Chew_,
BH+ IO
or B is not (1~65)
485-502
486
B-
possible
owing BH+
However,
E.
COKWAl--,
to the involl-ement
t Hz0
in quite
R.
G.
B_%RRADAS,
P.
G.
HAZIILTOX,
J_ _\I. PARR\-
of the equilibrium
F+ I3 -i- HzO+ stron g acid
solutions,
PJH+ is the principal
organic
species
and
(I) in
neutral or slightly alkaline solutions, I3 is the predominant species. Also, quaternisation of B, in the case of pyridinc, produces an ion which cannot be in prototropic equilibrium with the solvent, yet is not too different in size from the protonated form PyH+. An elucidation is therefore possible of the role of PyH+ adsorption compared with that of Py adsorption from acid media, and also of the effects the PyH’ (and lMePy+) ions have on the HsO+ adsorption at mercury. ESPERIJIESTAL
Ap#aratws _4n apparatus for determining electrocapillary curves for mercuryby- the method of forcing a mercury- meniscus to a fiducial mark on a tapering fine capillary, described 9. Pressure control was by means of a copper bellows’” .was used as previo&sly operating with nitrogen gas cornmunic~tin g with the capillacand with a large bore (3 cm) manometer.. The position of the meniscus was viewed by means of a zoom binocular stereomicroscope. Other details were as described pm\-iouslys. The temperature wzis maintained at 30” by circulation of water through a glass spiral sealed in the cell and through a jacket around the reference electrode. I.
2.
So.zzctio~zs
1Vater was conductivitywater redistilled in a stream of nitrogen_ Pottisium chloride used for the electroly-te in some runs was the reagent-grade material recrystaJl.ised once and dried. HJ-drochloric acid used in other runs N-S prepared b>redistilling the constant boiling mixture followed b>- appropriate dilution in an allglass vessel with the redistilled water_ Pure p_\;ridine was prepared by distilling the reagent material under reduced pressure with a nitrogen leak in order to avoid atmospheric o_xidation. Solutions wex made up in water or the hydrochloric acid as required_ X-meth>-lp>Tidinium chloride was prepared bv quatemisation of pyridine with methyl iodide followed b\- conversion to the chloride solution by ion exchange until iodide \vas chemicallv undetectabii: with starch after treatment with an o_xidant_ Perchloric acid was the anal+ical-reagent material suitably diluted.
This was purified .&_
as described
previouslyl6.
~7ocedure
For the work in KC1 solutions, electrocapillary curves were determined for six concentzations of KCl: o-or, 0.03. 0-1, 0.3, T-O and 3.0 _V, in the absence of pyridinc. Sets of electrocapillar-y curves were also determined for seven concentrations of pyridine (0.01,.0_02, 0.05; 0.1, 0.2, 0.5 and I .M) in each of the’s& KC1 supporting electrolytes indicated~ above_ Calome~ reference electrodes were used and were immersed in the cell in each of the KC1 solutions used, so that .no liquid junction potentia.& or. dther Coi-rkc,tions were &cessary. ‘pi1t.k paper connections17 between the
referehce &&nai~
..cell corripartments were avoided.
J_ ‘Eledioangi~.
TO i,r965)
Chni..
4.85-50~
ADSORPTIOX
IN
SOLUTIOSS
OF
F’YRIDISE
ETC.
487
For the work in acid solutions, 0.33 _iV HCl solutions were used with X-methyl pyridine chloride at concentrations of 0.005, O.OL, 0.02, 0.05 and 0.x M. Similarly, 0.02 AI N-methylp_yridinium chloride was used with 0.01, 0-033, 0.1, 0.33, 1.0; 3-3 and Also, with pyridine, o. I 0.2, 1.0, 2.0 and 3.0 _X HCl and 1.0 _iV 5 IV aq. HCl solutions_ HClOq acid solutions were used at p>tidine concentrations of 0.01, 0.02,o.b5, O-I, 0.2 and 0.5 M_ For these systems, several different reference electrode arrangements were necessq. For the N-methylpyridinium chloride solutions in aq. HCl, a saturated calomel electrode was used in which contact with the bulk of the solution was made
through a closed, were plotted on
ungreased ground-glass sleeve. The resulting electrocapillary curves an “E-” scale as if the measurements had been made against an electrode reversible to the chloride ion at each of the chloride concentrations used. The corrections to the potential scale were hence composed of two contributions: (a) the liquid-junction potential which \vas measured experimentally, and (b) a Nernst correction expressed by E
snt.col=
E-
-
In “fzcl-”
F
+ constant
where the constant is the Eo of the calomel electrode_ The plots based on the internal E-‘scale were subsequently checked in t\\-o cases by using an internal silver/silver chloride electrode ; agreement \vas \-err good_ In the p>-ridine-HCI s_vstem, potentials were measured against both a silver/ silver chloride electrode and a calomel electrode at the bulk chloride concentration, but separated from the p_vridine-containing solution’ by an ungreased stopcock. The electrodes \\-ere checked against each other during the runs. The p>-ridine-HClOa svstem \\as studied using a saturated calomel electrode, since it 1va.s found that the hvdrogen electrode in this acid was prone to some random fluctuations of the order of 5- mV. The results were plotted against an E- scale defined in terms of a hjFthetica1 electrode reversible to the perchlorate ion. i-e.,
Esat.cal
= E-
where the constant calomel
-
RT 7
In “acioa -” + constant
is now the standard
1 sat-Kc1
11 HCIO,
potential
1 “electrcode
of the cell
re\-erjible
to ClO,
which cannot be determined. Thus. the potential scale L-not naI scale but is nevertheless con&Gent within the s_vstcm used.
” be related
to an ester-
RESULTS
of charge iJch-cl sol&iorrs in tk przsc~usz of p_vrdind The purpose of this part of the \\sork was to esamine how the adsorbed cation and anion charges at various electrode surface charges (or potentials) depended upon the pyridine concentration, or more particularl\T. on the surface excess of pytidine molecules at the interface_ In the neutral COzfree solutions studied. the p>tid.ine is PyH+ concentration is hence < ca. 10-d-J :lf for I M negligibly ionised (pK_4=5_1g18; pyricke) and the neutral molecule is the principal adsorbed speciesll_ The surface excess of potassium ‘ion was thus obtained from the derivative (@J/~~KcI)E,~ in the I.
~~o~nporrerrts
J_ Ekriroanal.
Chenr..
IO
(x965)
.+Sj-so=
‘@s usual
B.
way,
and
(8y/ZCd_)p~a
qs, the or
total
varying
(ay/fX
concentrations
relative
solubilitv
COXN’_4Y,
R.
charge
in the
cn~.)~~~~p~y_
from qs and FF~G+ _ In this derivation activity of pyricline. from the salting-out
E.
G.
The
of components
B_iRR4DAS,
solution total
P.
side
anion
G.
H_‘\~fILTOX,
of the
cliarge
of charge,
it is assumed
pyridinc
concentration,
of KC1 at a given
J_ >I.
PARR\
double-layer,
from
was
hence
that
tile presence
does
obtained
I;ot afiect
the
Tlie effect of KC1 on the activit!: of p)-riclin~ ma>- be estimated theory- g-i\-en by COS\VA\-, DESSO~-ER~ XSD SXIITH’ c-. l-he reduced
(the salting-out
constant)
is giver.
b>-
so-s -=-
.5
c>f
(1)
SO?ZL
in the absence of salt, S that in its where SO is. in the general case, the solubilitv h>-dration radius of the ion of presence at molar concentration 9~2. rh the primary charge ze, V2 the molar volume of the solute molecule, PZ its polarisation and R the
a.
o*
r
77”
.
e
e
\+ / /
b
0
~~,.~i~-~‘@+ipotient+ of chaige iti kg solns. .iti t+ absence ana, pr-ence _a); o’o~;~bj. 0.03; (c); 0-r: (d), 0_3;.(e)..z;‘(f). 3 N KCl. .. _. J_++cr&irliz;:&&n.. -
10-&6~).
485’502
C
/
df‘o.1
lki pyhdine:
ADSORPTIOS
IN
SOLCTIOXS
OF
PI-RIDISE
ETC.
489
0
Fig. z. Components pyridine_
of
charge
in
I Jf
KC1
soIn_
in the
prcscnce
of:
(a),
0.01;
(b).
o-5:
(c),
I Af
radius of the. spherical co-volume available to the ions (i.e., R is proportional to nt- *)_ since fs =so, where f is the activity coefficient of the non-electrolyte solute in the presence of salt (f=r as m+o); eqn. (I) gives f through the term (I-~/fi(~jm). Calculation from the previous data given for I(+ and Cl- ion@9 indicates I - I/’ is ca_ ‘o.zo-o.3~ for p_t-ridine at 7n = I M allowing for the concentration dependence arising in the term r/R;fis therefore 1-25-x.33_ Alternatively, RT lnfmay be calculated as t,he excess +r-ti.al molar free energy of pyridine arising from hydration associated with the KC1 ele&oJyte. Under. the conditions where changes of fpY ma.y be signi. frcant: at‘ constant, pyt-idine concentration, dy will be made up of the two terms rp; GP, at constant E-. The overall &.AP,-, as the KC1 concentration is -.-_Ftidp_Kci_changed from o to.1 M, is about 200 cal mole-l from t’Le change of RT ln fi The mean J_ E_Z~cfroantd_
Chem-.
IO (1965)
485-503
B.
490
E.
COSW_kY,
R.
G.
B_4RRADXS,
P.
G.
HXXILTOS,
-EC,!
(mV!
;-
i
--Ecot (mv) Fig- 3_ Detailed components 0-03; (b). r _% KCl.
and
sub-components
of charge
for
KC1 in 0.1
AI pyridine:
-300
L$o i-
i
i I 1 f, I 100 ‘It
/ i 3.
PzlXR?i
i
2+
cf
Id.
2
‘I
0
3
-I
(a),
ADSORPTIOX
IS SOLUTIOXS
OF P\-RIDISE
491
ETC.
tions of KCI. The effect of change of KC1 concentration on the activity of pyridine at is therefore negligible under most conditions and constant pJ-ridine concentrarion, small at the highest pyridine coverages. as a The components of charge ~I,L- and X’er- have been plotted in Figs. ra-If function of qs (= -qm) for various KC1 concentrations, at 10-1 M pqridine. The solid circles represent experimental pomts for the pure KC1 solutions and the open circles represent the corresponding data in the presence of pyridine (1.0 x 10-r AZ). The data for 1.0 x 10-1 M pyridine were chosen m order to minimise multiphcity of graphs and because this was a representative mterrnediate concentration within the range of pyridine concentrations studied (z&. 10-2-1.0 AZ). The electrocapillary data for the pure KC1 solutions were indistinguishable point for point (e-g_. in the case of 0.1 M ourresultsfor KCl) fromtheresults ofD~x-_ax~l~~=\~ ASD PERIES 17_ Correspondingly, NIX+ and r'cr as a function of qs agree \-ery well with those shown by DEVASATHXK AND PERIES~~ and also by DEVAS_XTH_-1~ XFD C_~s.w&\R_%Tx_+PO, for pure KClsolutions. In the derivation of values of (@/Zlu~;e~)E~l , all points of y VS. log ~*,RCI fell on a smooth curve with increasing slope, except the one at the highest (3 M) KC1 concentrationThis was a reproducible feature of all the results and may probably be accounted for by the effect discussed in the appendix of ref. 32_ The principal effect of co-adsorbed pyridine is to i~zcrease the specific adsorption of Cl- ions and, correspondingly, the equivalent cation charge in the diffuse-layerenough, greatest at potenThe extent of the increase of Cl- adsorption L surprisingly with tials cathodic to the e.c.m. (or at negative q,,.-values) ; the effect also increases KC1 concentration. For a given KC1 concentration, the effect depends on pyridine concentration as shown in Fig_ 2. The inner, Q-A and diffuse, q--,d, layer charge contributions to q- have been calculated and are shown in Fig. 3 for 0.03 and I hT KC1 solutions in the presence of 0.1 i1g pyridine. The plot for I N KC1 emphasises the marked effect of pyridine in enhancing the specific adsorption of Cl-. The Esin and Akrkov effect with regard to Cl- ion adsorption is shown in Fig. 4 in the absence of pyridine, and in the presence of IO- 1 _W pyridine. The corresponding shift of potential of zero charge (p.z.c_) with pJ-ridine concentration at two constant KC1 concentrations, is also shown. 2. 1sotAevms for adsorj%ion of rreutml j5yridi7ze from A-Cl solzttions Isotherms were obtained for pyridine adsorption from
0.03
and
I
M
KC1
solutions at the various indicated potentials. More points were obtained at lower surface coverages than in the previous workQ_ Data for r-4 expressed as “FT&” are given in Tables I and z for 0-03 N and I: N KC1 solutions. 3_ Components of charge in acidic solzrtions of $yridize and Iv-nret?EyZp~lridi~tizrnt chloride In solutions of p_yridine in hydrochloric acid, thesolute species are Py, PyHf, indiHaO+ and Cl-. The thermodynamic analysis Q-Q.14 based on the Gibbs equation cates that only the composite quantities r(= J’-‘-+ + Fry+), T-4 (= TP~H+ + I&) and in the usual way, &T -J-‘pY can be derived for the organic species and in addition, Per and +_ The conditions under which these quantities may be derived have been discussed previously_8SQV14. Typical results for adsorption of species from 0.1 M of electrode potenpyridine solut- ion in I i%Z aq_ HCl are shown in Fig. 5 as a function tial E- (see above) _ /_
Electroanal
Ch?WZ..
10
(1965)
48,--502
B. E.
492 T_?LBLE VALUES TR3TIOX
COXWAY,
r,
FOR
EXPRESSED
XS
(1s /fC
“ FTpy”
F-j
cm--r)
.\S .\
P. G.
H_MIILTOX,
4-5
3-2
6.3
300 400 500
::z 9.6 10.1
700 so0
99 I17
9m
IO__+
1000
6.6 5.9 4-4
rroo 1200
Ol-
-_-_ -
2.0 - 13-z
2.+
600
I-USCTIOS
--
cw -1.0 - 10-L
100
TRATION
BARRADAS,
O-ID3 h’ KC1
200
V_+LUES
G.
J. M.
P_+RR\-
1 OF
--Ece:_(m
TABLE
R.
5-0 - IO -” S.5 II_‘7
16.1 20-4 '4-7 2S.6 30-s
10-10-1
POTESTI
XL
-‘LSD
CONCES-
_--
- -__d 0 - IO-1
-j-u
10-L
I.0
-11
IL-5
10.0
II>.
I =j__{
19-3 24-i 31-g
7-5.6 33 S 37.s
31-r 39-9 -w-i
3i-S
-w-S
47-5
40-S _+G.z
48-7 51-G
51-5 545
20.1 26.9 31-z 34-S 3S-7
35-o
I
10.3 I+3 16.9 IS.5 17.6 19 1 10.3
31-3
422
49-O
50-Z
55-5
3-l-o
43-O
51-5
49-2
5S.I
14-9 13.0 10-r
3-l-2 30-s =7-7
47-5 45-O 4-k-O
54-3 56-3 56--l
54-5 56.$
57-9 65.2 61.8
61.1
2 OF
rw
FOR
ESPtiESSED I
iv
r-0
100
_lS
“FTm”
(in
,#.I
Cn-l-z)
AS
A
FUNCTIOX
OF
POTEXTI_+L
.aSD
COXCEX-
Kc1
-
no
ro-2
2.0
-
ro-1
5-O
-
ro-2
r-0
-
ro-1
2-o
-
IO-1
5.0
-
10-l
I.0
M
pIot
200
2.0
300 400
4-I 7-3
z.z zz
'3-3 S-8 12-I 14.9
-go0 IO00 1100
5-4
S-1 12-O
IO.7
15-Z 18.9
IS.2 14-7 '0.7 1g.s
25-5 33-o 32-s 35-8
6.7
rr.3
7-6 3-6
13-9 10.0
36-g 31-7 27-S
I-l-3 =9 0 2s.g 43 36-o3 46-4 466 52-r 45.2 46-r
1S.S
2=j_S
28_6
27-3
36-I
41-3
37-3
45-i 50-t 55 4 55-G 5S-4 58-7 59-2 GO.4
-+S.+ 55-r 58-d 5i_S 60-5 60.0 59-9 62.2
4-t-6 49-9 51-9 53-7 57-9 54-S 56-7
In the case of N-methylpyridinium chloride, no ambiguities arise with regard to the molecular significance of the thermodynamic surface excess quantities that can be &iv&l; the only difficulty is a more trivial one, that of estimating activities of the salts in the mixed electrolyte solution media_ The results for components of charge + as well as qS are shown in Fig_ 6 for 2 - IO+ M NJ?Grer+_; Frciand Fr,,, rnethylpyridinium chloride in 0.33 M aq. HCl_ Qualitativelyi the results are similar to those for the pyTidine/HCl system -(Fi& 5). except that the specific adsorption of Cl- is stronger in the more concentrated, ,I A& HCI solution,- as expected_ The hydrogen ion excess in the interphase remains constant in bothsysterr& throughout the range of electrode potentials studied f&m;_&& anodic to the cathodic Side through the-e.c.m. potential. Comparison of the _maiu features’of~-the reSdts $or the_ two systems studied, indicates that the principal $e&s ~adsorbG&-i .I$ridine solutibnS in >acid -is evidently the protonated pyrid+e -ion-andpeuti molecule adsorption‘ik not extensive.: The latter effect could only be 1 :isi@fi&nt ii the stand&d-free energy of adsorption of neutral -pyridine were some numerically greater than th+t _for the- pyri-dinium ion, since the -7+3+&lZrnole-~, +concen*kon -. of the ion & some-I95{ro!tirries l+rg&‘than that of the neutral molecule _ =J._-hXT+?&~~,_ -_
_-.-
Iti (X965)--485--509=
,:-~-
..
ADSORPTIOS
IS
SOLL-TIOSS
OF
PI-RIDISE
493
ETC.
2( IF
IEiII ,_
__+-+-+-+
--
-16
d -2c
I
I
.I
.2
-E_ Fig.
5
Components
of charge
I
I -6
I -5
I .4
I .3
I-
-7
-9
a
I.1
I.0
I2
(volts)
for 0.1 AI pyridine
in I Ar HCI
aq.
soln.
16 -
/ -20
0
.I
I .2
3
I -4
I .!i
I .6
I
I
.7
.8
I -9
I I.0
I I.1
I I.2
-
-E_(volts) Fig.
6:_-Combonen&of charge
for z
- x0-2 MN-methylpyridinium
chloride
J_ ElecLroamaZ.
in o-33 Chenz..
hi HCl
TO (1965)
aq.
soln.
485-502
B. 1. COI‘;\\'XI',R. G_ BARRADAS,
494
P. G. HAJIILTOX-,
J_ Jf. PARRY
in I _X strong acid solutions_ These effects confirm the behaviour found at solid metal electrodes in our prexious x\-orE;I’ on acricline and its ions. Electrocapillary data lvere also obtained in perchloric acicl solutions of pyridine in order to investigate the effect of a different co-anion. Substantial effects arc observed
(see Discussion)
_
DISCI_TSSIOS I. Effects
of pyridine on the components of charge in k-C2 The effects of neutral pyndine on r+ and r- for KC1 are insignificant for the most dilute solution of KCl; effects are first noticeable at 0.03 M KC1 but do not become appreciable until 0.3 M KCl; for I iW and 3 M KCl, large changes of P+ and qare evident at a pyridine concentration of 0.1 M (Figs. I, z and 3)_ The negative adsorption of the Cl- ion which is normally obsen-ed at potentials negative to the e-c-m. becomes positive and increases with increasing negative potential (Fig. 3) or electrode charge (Fig. I). Calculation of the diffuse-layer contribution to Q- from the values of i+ (assuming no specific adsorption of the cation, but see discussion below) enables the specifically a&orbed anion charge q-, I to be evaluated as shown in Fig. 3 _ The specifically adsorbed charge is then larger than q-, and in 3 M KC1 is almost equal and opposite to q+_ It must be remarked that at these high salt concentrations, use of the diffuselayer theory based on a continuous charge distribution in a continuous dielectric may be unsatisfactory for the-same kinds of reason that the ionic atmosphere model for bulk electrolyte solutions must break down in principle at moderate concentrations”“.“3. In fact, at concentrations of I _M and 3 M, there 1s virtually no diffuse-layer since the Debye-Hiickel reciprocal radius is 1/z--1/3.3 A-1. i.e.,a thickness comparable with the diameters of hydrated ions in the Helmholtz layer. The effect of p_yridine on q+ and q- may arise from one or more of the following possible effects: (i) cations can become specifically adsorbed in the presence of p_yridine at the surface of mercury, and the increase of q- snnply balances the increase of q+; (ii) anion specific adsorption is facilitated by the pyridine layer owing to displacement of oriented water at the interface which would tend to hold the Cl- ions more in the solution owing to hydration effects; (iii) ion-pairing between K+ and Cl- is ‘facilitated by the presence of the low dielectric constant pyridine layer, so that a greater surface excess of both ions in the interphase is possible_ It seems likely that (iii) is the most probable basis of the effect since the oriented pyridine layer may present a region of lower dielectric constant than that associated with the partly dielectrically saturated water layer normally present’+‘“. This view is also supported by-the far$ that the effect orily arises at high KC1 concentrations where most of the ca&n charge q+ is close to the electrode surface. Since only smaller effects occur at -pc$entials an;odic to the e-cm., where Cl- is normally specifically adsorbed, it is to be ‘con&ded_that enhanced spetific adsorption of Cl- originates in a different way from that ksociated with the normal specific adsorption, or the effect observed when the potenti& a& cat-hodic may only arise when the pyridine molec~ules are -oriented with %.& negative Nicentres outward to the solution ; this may allow inter&ionwith the Kt- ions +d penetr+i& df partly desolvated Cl- ions between the oriented pyridine ~01ecules~It is to be-noted that the isotherms for pyridine adsorption indicate a _c&t@uousl~jntg surface excess bf pyridinewith increasing potential ah&St up t&the
highest.cathodic_pbtentials
studied
(see Tables
I and z)_
ADSORPTIOX
IX SOLGTIOWS
Ol? PYRIDIXE
ETC.
495
The enhancement of ion adsorption, at a given KC1 concentration, depends on the pyridine concentration (or surface excess) as follows from the data in Figs. I and 3_ g Esin and Markov effects for the KC1 salt (at constant pyridine The correspondin concentration) and for the py-ridine (cf- FRCMKIN"~) (at constant KC1 concentration) are compared in Fig. 1. The results are consistent with the fact that the presence of p]ridine enhances the Cl- adsorption and the dEp.z.c. continues to increase with KC1 concentration instead of reaching the maximum exhibited in pure aqueous KC1 solution (Fig. 4). as also observedby DEVAK_ATHAX XXD PERIES~~. On the other hand, the shift of the e_c.m.in the presence of pyridineis almost independent of the KC1 concentration in 0.03 and I N KC1 solutions, so that the KC1 adsorption does not appreciably IsoihcTms
2.
affect
the pyridine
for pyridiue
orientation_
adsor$tion
The isotherms for pyridine adsorption exhibit more or less the same characteristics as those observed previously-y for hydroand halo-deril-atives of pyridine. The present data are, however, more reproducible and more points on the isotherms have been obtained than in the prexious work. Following the method employed in an earlier publicationg, the form of the isotherms has been tested by calculating the apparent standard free energies of adsorption, AGo (defined previously”), that are of isotherm, so that any interaction or related derived from a “Langmuir form” effects actually involved are reflected in a dependence of 4@ on 8. &values were caIculatedfromJ'r,-values usingestimatesofthe molecularareaofpy-ridinebasedon dimensions of space-filling models. The dependence of AGo on 83, which may arise from repulsion effects amongst oriented dipolesgm"s, is shown in Fig. 7_ The plots
5
T.. E
z I 1
‘. 0
Ic3
7 0
08
04 8 3/2
Fig. 7. -4pparentstandardelectrochemicalfreeenergiesofadsorptiondGOfor pyridineadsorption from I iV I
E.
496 tidicate
relatively
appreciable
sudden
coverages
E.
CONWAY,
orientation
as proposed
R.
G.
BARR_ADAS,
effects*
previouslv
at
P.
high
g. The effects
G.
HAMILTOX,
cathodic associated
3_ 31. PARR\-
potentials witl;
and
decreasing
--&o.-&ill-arise from (a) the change of population of water dipoles and (bj the increasing accommodation of pyridine dipoles in the inner region of the double-la_ver as the chemical potential of pyridine is increased in the solution_ A complementary approach may be made @, in terms of the parameter, AE, defined bJ.-
by calculating30
the surface
pressure,
A&y”-y+q,(E’J-E) &here ~0 and EO are the surface tensions and respective potentials for points on the electrocapillary curve for the-base solution at given values of qm, and “/ and E are _values for the same qm values in the presence of pyridinevalues of pm, for _ The curves of 6 vs. log C py are shown in Fig. S for various pyridiqe in I N KCl. The behaviour in 0.03 _M KC1 is similar. The surface pressure curves- differ significantly from those for thiourea adsorption at mercury, discussed p~viously by PARSOh’S 30. First, it is clear that they cannot be superimposed (except perhaps at low @lvalues) to give a composite curve simply by adding constant terms (proportional to the standard free energy of adsorption at zero coverage) to the log C-values for each value of q;I considered; the shapes of the curves themselves evidently that the interaction effects in the surface layer and/or depeud on +_ This indicates the effective area of the species depend on +_ Such an effect is consistent with the conclusions that may be drawn from the AGO-0 * plots (Fig. 7). _ Secondly, it will be seen that the surface pressure for pyridine adsorption does not-follow a single, -hvo-term virial equation of state (cf- ref. 30) and corresponding isotherm, as does the adsorption of thioureaao at all values of +_ This may arise since of Cl- will depend on (a) in the KC1 e 1ectioly-te used, the extent of specific adsorption qrn and is also a function of pyridine concentration (Figs. I and 2) ; and (b) a disorientation-orien@ion transition is evidently involved which would not allow represkrtation. of the surface pressure behaviour in terms of a single-valued second virial coefficient. Accordingly, no attempt will be made here to fit the results to a particular type of isotherm. :-i S6me~fukhkr insight into the equation(s) of state for the pyridine ad-layer may, however, beobtained by plotting @ as a function of Fry at various qm values, from the datain Fig_ -9 for & as a function of q,.,,. The derived plot is shoti in Fig. IO for 7 values of:&_- It is evident that‘ for almost all the values of qm, @ is a discozztinztoz~s function of r pp,.and at high coverages suddenly increases more with increasing Try than at lo&-doverage. This again supports the view that a relatively sudden onset of kientatioh occurs in region z.(Fig. -IO), giving rise to stronger repulsion effects and . -* It c&nld be~%rgued
that-the effects observed at high coverages I -_0_teMx in c&culatin~~ dG”-values. -Thus 29, if the isotherm -foe, 3~ concen*tio~$C; of pyridine, -:_ __e J __
e-KCedl;OlW-
,r_(ryi f3,y
-.
to
arise from be- used
the use of an incorrect were of the following
(3)
ADSORPTION
IX
SOLUTIOKS
OF
I’\-RIDINE
ETC.
497
3c T
E
”
a,
5
z
20
IO
0
Log Fig.
8.
(0).
+zO;
(ml>
(d),
-5-o;
-3-o;
Surface
pressure
+r5; ((-#).
(@)
( 0). -7;
-0.5
-1.0
-1.5 [Cpy]
curves
fI2.5; (D).
( for
C,, in g mole I-‘) pyridine
(v). fro; -ropCcm-r.
adsorption +7.5;
(A).
from I _V KC1 as a function of qm. (X), +5; (+I. +2-5; (e). O-0; t-b).
hence to a larger d@/drp,_ The points in Fig. IO have been drawn within two envelope regions, but for each series of points, the discontinuity in the G-r relation is clearly evident. At more negative qm-values, a larger coverage is reached before the transition occurs, but the value of d@/drin region z is then greater than that for more positive q,-values. The transition region occurs when the area available to the adsorbate is between 35-45 A-l per molecule of pyrkline. The projected molecular area of pyridine in the “flat” configuration is ca. 35 A *_ The maximum surface coverage attained is about 6.5 x IO-10mole cm-2 in agreement with our previous results9 ; such a coverage corresponds to an effective molecular area of ca. 25 As molecule-1 which could arise if the molecules stood on end in the oriented condition* with the N-atom directed towards or atiay fro-m the mercury surface, depending on qm_ 3_ BeIz+iozc7 of cations denved from pyridine _ By thesamemethod as that used previouslys, the AGo-values __ a -It is of interest to note that a disoriZntation-orientation transition also of py+ixie in montmorillonite with increasing uptake pf the s&batel3. -JJ-Ekctrob~rd:Ckenz.,
for
PyH+
occurs
IO
in
in HCl;
tlie
sorption
(Ig65)
485-502
B_ E.
49s
COXWAY,
R.
G.
BARRADAS,
,L~i.I,+-+~+\-F
cy
o-01
-2 O-
-I-
+20
+/+
+’
g_
T,
as a function
of qm for
I
0
G.
.
--A-
.
-10
(pLc cma2J
various
py-ridine
concns.
H_~MILTOX,
+ + +
I
I +I0 9,
Fi&
P.
izz I .X 1X1.
-: 0
J_ 31. PARRY
ADSORPTIOS
IN SOLUTIONS
I 01
I 0.2
OF PYRIDINE
I 0.4
I 03
I 0.5
ETC.
I 0.6
499
I 0.7
&2
Pig. II. -apparent standard free energies HCI aq. soln. as a function of 8 I_
of adsorption,
il?%,
of pyridinc
of adsorption;
dG 0, of
(h>-drochloride)
in o-33 N
-1.0
Fig. 12. Apparent sz&2_ as a fu?x&k
standard oi83.
free energies
0.1
N-methylpyx-idinium
chloride
B
500
PyH’
in HClOq
plotted
againsr
and
I3. COSXV..I>-,
R.
G.
S-methylpyridinium
04 cc/- refs- Sand
BARRADXS,
ions
9) in Figs_
II,
P.
G
HrlJiILTOS,
J.
31. P_%RRl-
in HCJ, ha%-e been obtained and are and 13_ In diLz[te HCl or perchloric
12
acid, the AGO-values are linear* in 8 8 indicating 8.9 that the leading effect giving rise to deviation from the Lan,oTnuir isotherm is a. coulombic nearest-neighbour repulsion energy reciprocal in the interionic distance in the ad-layer. The effects observed with N-methylp>ridinium cations and \-it11 the protonated pJ-ridine are very similar confirming that F;H+ is the principal organic species (c/- ref. 31) adsorbed from acidic solutions of yyridine. E_ potentials are indicated m these Figures in 1.olts.
3.5
2.5
2.2
I
1
01
02
I 0.3
I 0.4
I 05
I 06
I 0.7
I 0.0
8% Fig~3~ Apparent standard free energies of adsorption. perchloricacidsoln_asafunctionof8*. _
A% 0, of pyridine
(perchlorate) in I X
The effect arising from nearest-neighbour coulombic repulsions may appear oversimplified_ However, at moderate and high concentrations in free I :I electrolyte solutions, an analogous disordered lattice model22. 23 for nearest-neighbour ionic interactions; leads to a cz&-root law for log[activity coefficients] which is well supported by experiment23 and is to be preferred (except at high and infinite dilution) to the Debye-I&kel theory_ Nevertheless, the simple cube-root law is only obeyed after _hydratiion effects in Jog[actitity coefficient] have been removed23. The inter-ionic distances laterally across the double-layer are certainly comparable with, or smaller ‘than, th& in concentrated free electrolyte solutions (e.g., in 5 M KCl. the average inter-ionic_ &stance is only 4-5 &)_ Hence hydration effects may also be an important f&t&leading to ttio-dimensional non-ideality as they are in three-dimensional free *-electroIjrte solutions at high concentrations. __ I . rk-TI%f&t
tk~*Line.ar relations tie obtained for-the-ion ddsOrpti&s. without any inflections, made_ab&e_that theminflections in-the correspondin& plots for neutral tihg%roi6thedcid%&&_(see footnoteon p-.496_) _molecllles.~f(e*)arenotGrtEfacts
-$tip*or&-the-conte+on ‘&.~~~~W?+-_
@&=.;-rro
(x96$1
&-5Oi
ADSORPTION
XX SOLUTIOKS
OF
PYRIDXSE
ETC.
50=
Finally, co-adsorption of anions will be another effect to be considered; to diminish the repulsive interaction bettveen the adsorbed cations_
tend
it will This
is
probably the reason for the decreasing slopes of tire ~@?a--O* lines shown in Fig. 14 as the electrode potential is made less negative and anion adsorption can become
I
-o-7_,
401T_. -06
I I
‘a3 z E
0
+&
-zbQ
=Q$ :\
1 3.5 i-/
0
Ia
a I 30
I
.%-J*
i-
,‘\ Y
I
\ \ PI!9
\ x
I
\
x5cJ\
A+ ;7\
Ii
\
X
i .--__L--L_1__.___.I_1 /’ 01 02
0 I+ aq
l x\
-02
Fig. HCl
+
c
-03.0
i
z-5
,“’
\\
0
Y -
0
c3
_ “1 u
04
05
06
07
Apparent standard free energms of adsorption, ~16, of pyridine in V as indicated. soln. as a function of 13*_ E c;L~potentials
more significant (see Fig. 5). Such effects in the more dilute HCl solutions.
are absent
(hydrochloride)
in the perchloric
acid solution
in
z 117
and
ACIQiOWLEDGE3IEXTS
Grateful acknowledgement is made for support of this work carried out jointly One of us (J-M-P.)
acknowledges
the award
to the National Research Council, Canada, at the Universities of Ottawa and Toronto. of an N.R.C. Post-Doctoral Fellowship at (P.G.H.), the award of a Pro-
the University of Ottawa in 1962-63. and another vince of Ontario Government Fellowship for rg63-65.
A. comprehensive study of adsorption of pyridine in neutral and ionic forms at the mercury electrode is reported_ The effect of neutr4 pyridine on the components of charge for adsorption of KCI in the double-layer has been investigated and it is shown_fhat the presence of an ad-layer ieads to an apparent specific adsorption of the chloride ion on the cathodic as well as the anodic side of the electrocapillarymaximum. This effect may be due to ion-pair formation_ The components of charge for pyridine and N-meth.ipyridinium cation adsorption in acid solutions have been derived. Iso-
B. E. COS\VA\-,
502
R. G. B_~RRA13_~S,
I'. G. HAJIILTOS,
J_ AI. PARRY
for neutral py-ridine have been analysed in terms of surface pressures, CD, and two regions of the @-coverage relation are apparent: and ma>- be related to orientation principally by ion repuleffects. The isotherms for the ionic species are cha racteriscd
therms
sion effects.
REFERENCES I 2
A. N. A_ N_
FRUMKIN.
2.
FRUJKIN.
A&OX_
3 V. BAGOTSKII
AXD
Pkysik
C/MHZ., k11Gq (1933) 131; _-1&a Physicocirzm. l_J.R S.S., 6 (x937) 502. Electrocl.en~.El~c~roclre~~~_ E=g., 3 (1963) _-ST; zbzti., I (1961) 65. h-him.. 23 (1949) 413; DokZ. _qkad I\-auk. SSSII. J_ J_+BLOKOVA, Zh. Fi=
58 (1947) x387/L N_ FRUMKIK
AXD N. V_ XIKOLAJE\~'X-FEDOROXXCH. Il-estx. YlZosk. Ufaiu.. 12(+) (1957) zGg; (1956) x455_ _-~sD I. SLES5 J_ P~c~,CoZZec~ion Czech. Chem. Cotnmzrlr.. 6 (1934) 129. rgo; P. HERXSYJIF.XRO DYK.Z&-d., 6(1g34)zo+,zgo;R_ BRDIC54, ibid.,3 (1933) I@. Proc. C.l.T.C.E.(xg5~~, Buttcrworths, 6 l3. E. COXW_AY. J_ O'BI. BOCKFUS XND B. LOX-RECEK, London,Ig55.p_z07s DokLAkad. _Vazrk,SSSR. I 14 (1957) 1273; 120 (1958) 1294; 132 (IgGo) 1352; 7 S.MAimovs~~r. 133(rgbo) Iti. S E.BLOMGREK AND J. O'~i.Boc~~~~~.].P~ys.Clrs~tz.. 63 (1959) 1475. 319, 349; L. GIERST, PYOC. AND R. G. BARRADAS. EEectrockim. -3cta. 5 (1961) 9 B. E. CONWAY 1959, John \Vlley and Sons, Kew York, Symp_ Elecirode Processes, The Electrockem. Sot., rg6r.p. Iog_ IO l3_ E_Co?vwi~, R_ G.BXRR~DAS AND T. ZAWIDZKI,]. Phys.CAem.. 61 (1955) 676_ II B_ E. COSWAYAND R-G. BARRADAS, J_ ElecLroalznZ_Cher,l., G (1963) 314. 12 W_LORENZ.F_ MC%ZKEL AND N. nI~~~~~,Z.P~ysik.C~eln.,15 (rgGo) 145_ Sac-. 5g (1963) 141. 13 R. PARSOXS XP;D J- 31. P_~RRY. Tratrs. Faradq~ Theory and Prirrcifiles of Electrode Processes, Ronald Press Co , Xert* York. 1-f B. E. COKWAY. 4
r5 16 17 r8 =9
Zh- Fiz_K~imz..30
1965. chap- 3_ K. lli_ JOSHI -44~~R. PARSONS. EZecCrockim. -3&z, 4 (2961) r2g_ J. OX. BOCKRIS AXD R. PARSOXS, Trans. Faraday SOL. _15 (1919) gI6M. A. V_ DEVAXATHANAKD P.PEFUES. T~on.s_ ~amdaySoc.,~o (1954) 1~36
N-F. Htirnx~M.R. SPRINI~LE,].~~.C~~~~.SOC.,~~ (1931) 3469_ l3. E. CONXVAY, J_ E. DESKOYERS AND -4. C_ SMITH, Phi,?.;Tratzs.Roy_ SOG- (London). X256, (1964) 3S92o.M. A. V. DEVAXATHA~ _xxD S. G. CANAGXRATNA, bledrochim. -d&a, 8 (1963) 77_ 21 D_ C. GRAHAMLE, Chem. Rev.. 41 (1947) 441~ 22 H. S. FRAMK AND P. T. THOMPSOX;. J _ CIzenz. Phys., 3 I (1959) 1086; Stvzrclzrve of EZecZroZyte - Solzdiorrr, e&Ceil by XV. J_ H~JIER. John Wiley and Sons. New York. 1959, chap. S. 23 J_ E. DESNOYERSAKD B.E.CONWAY, J.Pkys.Ckena.. 68 (IgG4) 2305~ 24 B. E_ Cosza~.J_ O’Ai. BOCKRISAND I_ _4.AhI~rAR,Tra1zs. FaradaySoc..q7 (1951) 756_ 25 R. \V_ RAMP‘OLLA.R.C_B~ILLERA~~ C.P.S~~~~.j.C~etn.Plrys.. 30 (1959) 56626 27
A. N. FRu_vmK, Eugeb. Elrakt. Naturn-, 7 (1928) 25% l3_ E_ CorrivAy ASD E. GILEADI.C~~~.J_C~~?~_.~~ (1964) 90; Modem AspcclsofEZectrochenristry. Vol. 3,edited by J_ O’M. BOCKRISAND l3. E-CONWAY. Buttemvorth.London. 1964. chap.5_ 28 J_ S. MI-XHELL, Tmm-. Faru&y SOG.. 31 (1935) gSo_ 2g R. PARSONS. J_ II+troamaL C&m.. 8 (1964) 93_ 30. R1 PARSONS, Truns_ Far&uy Stic.. 51 (1955) 1518; ProcRoy: Sot. /Lorrdmr). A261 (1961) Tg_ 31 N. HACKERMANAXDA.C-AM~~~~~~. Ind.Eng-Chem.. 46(rg54)523_ 33 D. C. GRAHAME AND R. Pesows, J- Am. Chem. SOG.. 83 (1961) 1291. 33. R_ GREENE-KELLEY-,IT~~S. Faraday Soc-,5x (1955)412,425i_ EZectroana&.
Cicem.,
IO
(1965)
485-502
OI:
ELECTROAS_-\L\-TICLXL
ELECTROCHEMICAL COMPONENTS IX
CHEMISTRY
XDSORPTION SOLUTIOX5 OF
OF NEUTRAL PYRIDINE _4ND
AND 1O;lrfIC DERIVED IONS
In the development of electrochemical kinetics, the relation between the adsorption of reactant species and the rate of the reaction, as a function of potential and solution concentration of reactants and other species, has been a matter of central importance The quantitative formulation of such effects was first given by FRUMKIN~ and has constituted one of the major contributions in a wide variety of papers from his school, c-g., with regard to the kinetics of cation discharge (e.g., ref. 2,3) and more recently in interesting \vays with regard to the special effects associated with anion reduction4 and the role of specificaIly adsorbed cations in such processes. In the present paper, it is therefore appropriate to present some further data on the electrochemical adsorption of a type of organic species, which can exist in either a neutral or ionic form, and which is the parent of a family of molecules and ions giving marked effects on the kinetics of, for example, hydrogen evolution from acid solutions at mercury5--7. Previous studies of the electrochemical adsorption of organic aminium ions at mercury have been reported by BLOMGRES AXD BOCKRIS~ and the relation between adsorption of neutral pyridine derivatives and their conjugated protonated ions has been investigated by CONW_~Y _=I) BARR--\DAS at mercury9 and at solid electrodesre~ri. The capacity behaviour of organic bases at mercury has been investigated by LOFGZZ AND &R%XEL= and the nature of the isotherm for adsorption of certain benzenesulphonate anions has been studied by PARSONS AXD PARR\-13. Little information is at present available on how neutral and ionic additives affect the distribution of other base electrol-yte ions in the double-layer and how these effects are related to the adsorption behaviour of the organic species. In the present work, we. therefore report (a) the effect of adsorption of pyridine in neutral form on the ionic components of adsorbed charge in KC1 solutions at mercury; (b) the nature of the adsorption of the protonated conjugate pyridinium ion and neutral pyridine; and (c) the role of the co-anion in determining the interaction effects associated with the adsorption of the organic cation_ In these studies, the classical elect-rocapillary method has been used. The thermodynamics of the adsorption of organic bases, B, and their conjugate ions, Bl%+, has been discussed that elsewhere 8.924 where it has been emphasised unique‘characterisation
of the adsorption
of the protonated J_ Electvoaozal.
species Chew_,
BH+ IO
or B is not (1~65)
485-502
486
B-
possible
owing BH+
However,
E.
COKWAl--,
to the involl-ement
t Hz0
in quite
R.
G.
B_%RRADAS,
P.
G.
HAZIILTOX,
J_ _\I. PARR\-
of the equilibrium
F+ I3 -i- HzO+ stron g acid
solutions,
PJH+ is the principal
organic
species
and
(I) in
neutral or slightly alkaline solutions, I3 is the predominant species. Also, quaternisation of B, in the case of pyridinc, produces an ion which cannot be in prototropic equilibrium with the solvent, yet is not too different in size from the protonated form PyH+. An elucidation is therefore possible of the role of PyH+ adsorption compared with that of Py adsorption from acid media, and also of the effects the PyH’ (and lMePy+) ions have on the HsO+ adsorption at mercury. ESPERIJIESTAL
Ap#aratws _4n apparatus for determining electrocapillary curves for mercuryby- the method of forcing a mercury- meniscus to a fiducial mark on a tapering fine capillary, described 9. Pressure control was by means of a copper bellows’” .was used as previo&sly operating with nitrogen gas cornmunic~tin g with the capillacand with a large bore (3 cm) manometer.. The position of the meniscus was viewed by means of a zoom binocular stereomicroscope. Other details were as described pm\-iouslys. The temperature wzis maintained at 30” by circulation of water through a glass spiral sealed in the cell and through a jacket around the reference electrode. I.
2.
So.zzctio~zs
1Vater was conductivitywater redistilled in a stream of nitrogen_ Pottisium chloride used for the electroly-te in some runs was the reagent-grade material recrystaJl.ised once and dried. HJ-drochloric acid used in other runs N-S prepared b>redistilling the constant boiling mixture followed b>- appropriate dilution in an allglass vessel with the redistilled water_ Pure p_\;ridine was prepared by distilling the reagent material under reduced pressure with a nitrogen leak in order to avoid atmospheric o_xidation. Solutions wex made up in water or the hydrochloric acid as required_ X-meth>-lp>Tidinium chloride was prepared bv quatemisation of pyridine with methyl iodide followed b\- conversion to the chloride solution by ion exchange until iodide \vas chemicallv undetectabii: with starch after treatment with an o_xidant_ Perchloric acid was the anal+ical-reagent material suitably diluted.
This was purified .&_
as described
previouslyl6.
~7ocedure
For the work in KC1 solutions, electrocapillary curves were determined for six concentzations of KCl: o-or, 0.03. 0-1, 0.3, T-O and 3.0 _V, in the absence of pyridinc. Sets of electrocapillar-y curves were also determined for seven concentrations of pyridine (0.01,.0_02, 0.05; 0.1, 0.2, 0.5 and I .M) in each of the’s& KC1 supporting electrolytes indicated~ above_ Calome~ reference electrodes were used and were immersed in the cell in each of the KC1 solutions used, so that .no liquid junction potentia.& or. dther Coi-rkc,tions were &cessary. ‘pi1t.k paper connections17 between the
referehce &&nai~
..cell corripartments were avoided.
J_ ‘Eledioangi~.
TO i,r965)
Chni..
4.85-50~
ADSORPTIOX
IN
SOLUTIOSS
OF
F’YRIDISE
ETC.
487
For the work in acid solutions, 0.33 _iV HCl solutions were used with X-methyl pyridine chloride at concentrations of 0.005, O.OL, 0.02, 0.05 and 0.x M. Similarly, 0.02 AI N-methylp_yridinium chloride was used with 0.01, 0-033, 0.1, 0.33, 1.0; 3-3 and Also, with pyridine, o. I 0.2, 1.0, 2.0 and 3.0 _X HCl and 1.0 _iV 5 IV aq. HCl solutions_ HClOq acid solutions were used at p>tidine concentrations of 0.01, 0.02,o.b5, O-I, 0.2 and 0.5 M_ For these systems, several different reference electrode arrangements were necessq. For the N-methylpyridinium chloride solutions in aq. HCl, a saturated calomel electrode was used in which contact with the bulk of the solution was made
through a closed, were plotted on
ungreased ground-glass sleeve. The resulting electrocapillary curves an “E-” scale as if the measurements had been made against an electrode reversible to the chloride ion at each of the chloride concentrations used. The corrections to the potential scale were hence composed of two contributions: (a) the liquid-junction potential which \vas measured experimentally, and (b) a Nernst correction expressed by E
snt.col=
E-
-
In “fzcl-”
F
+ constant
where the constant is the Eo of the calomel electrode_ The plots based on the internal E-‘scale were subsequently checked in t\\-o cases by using an internal silver/silver chloride electrode ; agreement \vas \-err good_ In the p>-ridine-HCI s_vstem, potentials were measured against both a silver/ silver chloride electrode and a calomel electrode at the bulk chloride concentration, but separated from the p_vridine-containing solution’ by an ungreased stopcock. The electrodes \\-ere checked against each other during the runs. The p>-ridine-HClOa svstem \\as studied using a saturated calomel electrode, since it 1va.s found that the hvdrogen electrode in this acid was prone to some random fluctuations of the order of 5- mV. The results were plotted against an E- scale defined in terms of a hjFthetica1 electrode reversible to the perchlorate ion. i-e.,
Esat.cal
= E-
where the constant calomel
-
RT 7
In “acioa -” + constant
is now the standard
1 sat-Kc1
11 HCIO,
potential
1 “electrcode
of the cell
re\-erjible
to ClO,
which cannot be determined. Thus. the potential scale L-not naI scale but is nevertheless con&Gent within the s_vstcm used.
” be related
to an ester-
RESULTS
of charge iJch-cl sol&iorrs in tk przsc~usz of p_vrdind The purpose of this part of the \\sork was to esamine how the adsorbed cation and anion charges at various electrode surface charges (or potentials) depended upon the pyridine concentration, or more particularl\T. on the surface excess of pytidine molecules at the interface_ In the neutral COzfree solutions studied. the p>tid.ine is PyH+ concentration is hence < ca. 10-d-J :lf for I M negligibly ionised (pK_4=5_1g18; pyricke) and the neutral molecule is the principal adsorbed speciesll_ The surface excess of potassium ‘ion was thus obtained from the derivative (@J/~~KcI)E,~ in the I.
~~o~nporrerrts
J_ Ekriroanal.
Chenr..
IO
(x965)
.+Sj-so=
‘@s usual
B.
way,
and
(8y/ZCd_)p~a
qs, the or
total
varying
(ay/fX
concentrations
relative
solubilitv
COXN’_4Y,
R.
charge
in the
cn~.)~~~~p~y_
from qs and FF~G+ _ In this derivation activity of pyricline. from the salting-out
E.
G.
The
of components
B_iRR4DAS,
solution total
P.
side
anion
G.
H_‘\~fILTOX,
of the
cliarge
of charge,
it is assumed
pyridinc
concentration,
of KC1 at a given
J_ >I.
PARR\
double-layer,
from
was
hence
that
tile presence
does
obtained
I;ot afiect
the
Tlie effect of KC1 on the activit!: of p)-riclin~ ma>- be estimated theory- g-i\-en by COS\VA\-, DESSO~-ER~ XSD SXIITH’ c-. l-he reduced
(the salting-out
constant)
is giver.
b>-
so-s -=-
.5
c>f
(1)
SO?ZL
in the absence of salt, S that in its where SO is. in the general case, the solubilitv h>-dration radius of the ion of presence at molar concentration 9~2. rh the primary charge ze, V2 the molar volume of the solute molecule, PZ its polarisation and R the
a.
o*
r
77”
.
e
e
\+ / /
b
0
~~,.~i~-~‘@+ipotient+ of chaige iti kg solns. .iti t+ absence ana, pr-ence _a); o’o~;~bj. 0.03; (c); 0-r: (d), 0_3;.(e)..z;‘(f). 3 N KCl. .. _. J_++cr&irliz;:&&n.. -
10-&6~).
485’502
C
/
df‘o.1
lki pyhdine:
ADSORPTIOS
IN
SOLCTIOXS
OF
PI-RIDISE
ETC.
489
0
Fig. z. Components pyridine_
of
charge
in
I Jf
KC1
soIn_
in the
prcscnce
of:
(a),
0.01;
(b).
o-5:
(c),
I Af
radius of the. spherical co-volume available to the ions (i.e., R is proportional to nt- *)_ since fs =so, where f is the activity coefficient of the non-electrolyte solute in the presence of salt (f=r as m+o); eqn. (I) gives f through the term (I-~/fi(~jm). Calculation from the previous data given for I(+ and Cl- ion@9 indicates I - I/’ is ca_ ‘o.zo-o.3~ for p_t-ridine at 7n = I M allowing for the concentration dependence arising in the term r/R;fis therefore 1-25-x.33_ Alternatively, RT lnfmay be calculated as t,he excess +r-ti.al molar free energy of pyridine arising from hydration associated with the KC1 ele&oJyte. Under. the conditions where changes of fpY ma.y be signi. frcant: at‘ constant, pyt-idine concentration, dy will be made up of the two terms rp; GP, at constant E-. The overall &.AP,-, as the KC1 concentration is -.-_Ftidp_Kci_changed from o to.1 M, is about 200 cal mole-l from t’Le change of RT ln fi The mean J_ E_Z~cfroantd_
Chem-.
IO (1965)
485-503
B.
490
E.
COSW_kY,
R.
G.
B_4RRADXS,
P.
G.
HXXILTOS,
-EC,!
(mV!
;-
i
--Ecot (mv) Fig- 3_ Detailed components 0-03; (b). r _% KCl.
and
sub-components
of charge
for
KC1 in 0.1
AI pyridine:
-300
L$o i-
i
i I 1 f, I 100 ‘It
/ i 3.
PzlXR?i
i
2+
cf
Id.
2
‘I
0
3
-I
(a),
ADSORPTIOX
IS SOLUTIOXS
OF P\-RIDISE
491
ETC.
tions of KCI. The effect of change of KC1 concentration on the activity of pyridine at is therefore negligible under most conditions and constant pJ-ridine concentrarion, small at the highest pyridine coverages. as a The components of charge ~I,L- and X’er- have been plotted in Figs. ra-If function of qs (= -qm) for various KC1 concentrations, at 10-1 M pqridine. The solid circles represent experimental pomts for the pure KC1 solutions and the open circles represent the corresponding data in the presence of pyridine (1.0 x 10-r AZ). The data for 1.0 x 10-1 M pyridine were chosen m order to minimise multiphcity of graphs and because this was a representative mterrnediate concentration within the range of pyridine concentrations studied (z&. 10-2-1.0 AZ). The electrocapillary data for the pure KC1 solutions were indistinguishable point for point (e-g_. in the case of 0.1 M ourresultsfor KCl) fromtheresults ofD~x-_ax~l~~=\~ ASD PERIES 17_ Correspondingly, NIX+ and r'cr as a function of qs agree \-ery well with those shown by DEVASATHXK AND PERIES~~ and also by DEVAS_XTH_-1~ XFD C_~s.w&\R_%Tx_+PO, for pure KClsolutions. In the derivation of values of (@/Zlu~;e~)E~l , all points of y VS. log ~*,RCI fell on a smooth curve with increasing slope, except the one at the highest (3 M) KC1 concentrationThis was a reproducible feature of all the results and may probably be accounted for by the effect discussed in the appendix of ref. 32_ The principal effect of co-adsorbed pyridine is to i~zcrease the specific adsorption of Cl- ions and, correspondingly, the equivalent cation charge in the diffuse-layerenough, greatest at potenThe extent of the increase of Cl- adsorption L surprisingly with tials cathodic to the e.c.m. (or at negative q,,.-values) ; the effect also increases KC1 concentration. For a given KC1 concentration, the effect depends on pyridine concentration as shown in Fig_ 2. The inner, Q-A and diffuse, q--,d, layer charge contributions to q- have been calculated and are shown in Fig. 3 for 0.03 and I hT KC1 solutions in the presence of 0.1 i1g pyridine. The plot for I N KC1 emphasises the marked effect of pyridine in enhancing the specific adsorption of Cl-. The Esin and Akrkov effect with regard to Cl- ion adsorption is shown in Fig. 4 in the absence of pyridine, and in the presence of IO- 1 _W pyridine. The corresponding shift of potential of zero charge (p.z.c_) with pJ-ridine concentration at two constant KC1 concentrations, is also shown. 2. 1sotAevms for adsorj%ion of rreutml j5yridi7ze from A-Cl solzttions Isotherms were obtained for pyridine adsorption from
0.03
and
I
M
KC1
solutions at the various indicated potentials. More points were obtained at lower surface coverages than in the previous workQ_ Data for r-4 expressed as “FT&” are given in Tables I and z for 0-03 N and I: N KC1 solutions. 3_ Components of charge in acidic solzrtions of $yridize and Iv-nret?EyZp~lridi~tizrnt chloride In solutions of p_yridine in hydrochloric acid, thesolute species are Py, PyHf, indiHaO+ and Cl-. The thermodynamic analysis Q-Q.14 based on the Gibbs equation cates that only the composite quantities r(= J’-‘-+ + Fry+), T-4 (= TP~H+ + I&) and in the usual way, &T -J-‘pY can be derived for the organic species and in addition, Per and +_ The conditions under which these quantities may be derived have been discussed previously_8SQV14. Typical results for adsorption of species from 0.1 M of electrode potenpyridine solut- ion in I i%Z aq_ HCl are shown in Fig. 5 as a function tial E- (see above) _ /_
Electroanal
Ch?WZ..
10
(1965)
48,--502
B. E.
492 T_?LBLE VALUES TR3TIOX
COXWAY,
r,
FOR
EXPRESSED
XS
(1s /fC
“ FTpy”
F-j
cm--r)
.\S .\
P. G.
H_MIILTOX,
4-5
3-2
6.3
300 400 500
::z 9.6 10.1
700 so0
99 I17
9m
IO__+
1000
6.6 5.9 4-4
rroo 1200
Ol-
-_-_ -
2.0 - 13-z
2.+
600
I-USCTIOS
--
cw -1.0 - 10-L
100
TRATION
BARRADAS,
O-ID3 h’ KC1
200
V_+LUES
G.
J. M.
P_+RR\-
1 OF
--Ece:_(m
TABLE
R.
5-0 - IO -” S.5 II_‘7
16.1 20-4 '4-7 2S.6 30-s
10-10-1
POTESTI
XL
-‘LSD
CONCES-
_--
- -__d 0 - IO-1
-j-u
10-L
I.0
-11
IL-5
10.0
II>.
I =j__{
19-3 24-i 31-g
7-5.6 33 S 37.s
31-r 39-9 -w-i
3i-S
-w-S
47-5
40-S _+G.z
48-7 51-G
51-5 545
20.1 26.9 31-z 34-S 3S-7
35-o
I
10.3 I+3 16.9 IS.5 17.6 19 1 10.3
31-3
422
49-O
50-Z
55-5
3-l-o
43-O
51-5
49-2
5S.I
14-9 13.0 10-r
3-l-2 30-s =7-7
47-5 45-O 4-k-O
54-3 56-3 56--l
54-5 56.$
57-9 65.2 61.8
61.1
2 OF
rw
FOR
ESPtiESSED I
iv
r-0
100
_lS
“FTm”
(in
,#.I
Cn-l-z)
AS
A
FUNCTIOX
OF
POTEXTI_+L
.aSD
COXCEX-
Kc1
-
no
ro-2
2.0
-
ro-1
5-O
-
ro-2
r-0
-
ro-1
2-o
-
IO-1
5.0
-
10-l
I.0
M
pIot
200
2.0
300 400
4-I 7-3
z.z zz
'3-3 S-8 12-I 14.9
-go0 IO00 1100
5-4
S-1 12-O
IO.7
15-Z 18.9
IS.2 14-7 '0.7 1g.s
25-5 33-o 32-s 35-8
6.7
rr.3
7-6 3-6
13-9 10.0
36-g 31-7 27-S
I-l-3 =9 0 2s.g 43 36-o3 46-4 466 52-r 45.2 46-r
1S.S
2=j_S
28_6
27-3
36-I
41-3
37-3
45-i 50-t 55 4 55-G 5S-4 58-7 59-2 GO.4
-+S.+ 55-r 58-d 5i_S 60-5 60.0 59-9 62.2
4-t-6 49-9 51-9 53-7 57-9 54-S 56-7
In the case of N-methylpyridinium chloride, no ambiguities arise with regard to the molecular significance of the thermodynamic surface excess quantities that can be &iv&l; the only difficulty is a more trivial one, that of estimating activities of the salts in the mixed electrolyte solution media_ The results for components of charge + as well as qS are shown in Fig_ 6 for 2 - IO+ M NJ?Grer+_; Frciand Fr,,, rnethylpyridinium chloride in 0.33 M aq. HCl_ Qualitativelyi the results are similar to those for the pyTidine/HCl system -(Fi& 5). except that the specific adsorption of Cl- is stronger in the more concentrated, ,I A& HCI solution,- as expected_ The hydrogen ion excess in the interphase remains constant in bothsysterr& throughout the range of electrode potentials studied f&m;_&& anodic to the cathodic Side through the-e.c.m. potential. Comparison of the _maiu features’of~-the reSdts $or the_ two systems studied, indicates that the principal $e&s ~adsorbG&-i .I$ridine solutibnS in >acid -is evidently the protonated pyrid+e -ion-andpeuti molecule adsorption‘ik not extensive.: The latter effect could only be 1 :isi@fi&nt ii the stand&d-free energy of adsorption of neutral -pyridine were some numerically greater than th+t _for the- pyri-dinium ion, since the -7+3+&lZrnole-~, +concen*kon -. of the ion & some-I95{ro!tirries l+rg&‘than that of the neutral molecule _ =J._-hXT+?&~~,_ -_
_-.-
Iti (X965)--485--509=
,:-~-
..
ADSORPTIOS
IS
SOLL-TIOSS
OF
PI-RIDISE
493
ETC.
2( IF
IEiII ,_
__+-+-+-+
--
-16
d -2c
I
I
.I
.2
-E_ Fig.
5
Components
of charge
I
I -6
I -5
I .4
I .3
I-
-7
-9
a
I.1
I.0
I2
(volts)
for 0.1 AI pyridine
in I Ar HCI
aq.
soln.
16 -
/ -20
0
.I
I .2
3
I -4
I .!i
I .6
I
I
.7
.8
I -9
I I.0
I I.1
I I.2
-
-E_(volts) Fig.
6:_-Combonen&of charge
for z
- x0-2 MN-methylpyridinium
chloride
J_ ElecLroamaZ.
in o-33 Chenz..
hi HCl
TO (1965)
aq.
soln.
485-502
B. 1. COI‘;\\'XI',R. G_ BARRADAS,
494
P. G. HAJIILTOX-,
J_ Jf. PARRY
in I _X strong acid solutions_ These effects confirm the behaviour found at solid metal electrodes in our prexious x\-orE;I’ on acricline and its ions. Electrocapillary data lvere also obtained in perchloric acicl solutions of pyridine in order to investigate the effect of a different co-anion. Substantial effects arc observed
(see Discussion)
_
DISCI_TSSIOS I. Effects
of pyridine on the components of charge in k-C2 The effects of neutral pyndine on r+ and r- for KC1 are insignificant for the most dilute solution of KCl; effects are first noticeable at 0.03 M KC1 but do not become appreciable until 0.3 M KCl; for I iW and 3 M KCl, large changes of P+ and qare evident at a pyridine concentration of 0.1 M (Figs. I, z and 3)_ The negative adsorption of the Cl- ion which is normally obsen-ed at potentials negative to the e-c-m. becomes positive and increases with increasing negative potential (Fig. 3) or electrode charge (Fig. I). Calculation of the diffuse-layer contribution to Q- from the values of i+ (assuming no specific adsorption of the cation, but see discussion below) enables the specifically a&orbed anion charge q-, I to be evaluated as shown in Fig. 3 _ The specifically adsorbed charge is then larger than q-, and in 3 M KC1 is almost equal and opposite to q+_ It must be remarked that at these high salt concentrations, use of the diffuselayer theory based on a continuous charge distribution in a continuous dielectric may be unsatisfactory for the-same kinds of reason that the ionic atmosphere model for bulk electrolyte solutions must break down in principle at moderate concentrations”“.“3. In fact, at concentrations of I _M and 3 M, there 1s virtually no diffuse-layer since the Debye-Hiickel reciprocal radius is 1/z--1/3.3 A-1. i.e.,a thickness comparable with the diameters of hydrated ions in the Helmholtz layer. The effect of p_yridine on q+ and q- may arise from one or more of the following possible effects: (i) cations can become specifically adsorbed in the presence of p_yridine at the surface of mercury, and the increase of q- snnply balances the increase of q+; (ii) anion specific adsorption is facilitated by the pyridine layer owing to displacement of oriented water at the interface which would tend to hold the Cl- ions more in the solution owing to hydration effects; (iii) ion-pairing between K+ and Cl- is ‘facilitated by the presence of the low dielectric constant pyridine layer, so that a greater surface excess of both ions in the interphase is possible_ It seems likely that (iii) is the most probable basis of the effect since the oriented pyridine layer may present a region of lower dielectric constant than that associated with the partly dielectrically saturated water layer normally present’+‘“. This view is also supported by-the far$ that the effect orily arises at high KC1 concentrations where most of the ca&n charge q+ is close to the electrode surface. Since only smaller effects occur at -pc$entials an;odic to the e-cm., where Cl- is normally specifically adsorbed, it is to be ‘con&ded_that enhanced spetific adsorption of Cl- originates in a different way from that ksociated with the normal specific adsorption, or the effect observed when the potenti& a& cat-hodic may only arise when the pyridine molec~ules are -oriented with %.& negative Nicentres outward to the solution ; this may allow inter&ionwith the Kt- ions +d penetr+i& df partly desolvated Cl- ions between the oriented pyridine ~01ecules~It is to be-noted that the isotherms for pyridine adsorption indicate a _c&t@uousl~jntg surface excess bf pyridinewith increasing potential ah&St up t&the
highest.cathodic_pbtentials
studied
(see Tables
I and z)_
ADSORPTIOX
IX SOLGTIOWS
Ol? PYRIDIXE
ETC.
495
The enhancement of ion adsorption, at a given KC1 concentration, depends on the pyridine concentration (or surface excess) as follows from the data in Figs. I and 3_ g Esin and Markov effects for the KC1 salt (at constant pyridine The correspondin concentration) and for the py-ridine (cf- FRCMKIN"~) (at constant KC1 concentration) are compared in Fig. 1. The results are consistent with the fact that the presence of p]ridine enhances the Cl- adsorption and the dEp.z.c. continues to increase with KC1 concentration instead of reaching the maximum exhibited in pure aqueous KC1 solution (Fig. 4). as also observedby DEVAK_ATHAX XXD PERIES~~. On the other hand, the shift of the e_c.m.in the presence of pyridineis almost independent of the KC1 concentration in 0.03 and I N KC1 solutions, so that the KC1 adsorption does not appreciably IsoihcTms
2.
affect
the pyridine
for pyridiue
orientation_
adsor$tion
The isotherms for pyridine adsorption exhibit more or less the same characteristics as those observed previously-y for hydroand halo-deril-atives of pyridine. The present data are, however, more reproducible and more points on the isotherms have been obtained than in the prexious work. Following the method employed in an earlier publicationg, the form of the isotherms has been tested by calculating the apparent standard free energies of adsorption, AGo (defined previously”), that are of isotherm, so that any interaction or related derived from a “Langmuir form” effects actually involved are reflected in a dependence of 4@ on 8. &values were caIculatedfromJ'r,-values usingestimatesofthe molecularareaofpy-ridinebasedon dimensions of space-filling models. The dependence of AGo on 83, which may arise from repulsion effects amongst oriented dipolesgm"s, is shown in Fig. 7_ The plots
5
T.. E
z I 1
‘. 0
Ic3
7 0
08
04 8 3/2
Fig. 7. -4pparentstandardelectrochemicalfreeenergiesofadsorptiondGOfor pyridineadsorption from I iV I
E.
496 tidicate
relatively
appreciable
sudden
coverages
E.
CONWAY,
orientation
as proposed
R.
G.
BARR_ADAS,
effects*
previouslv
at
P.
high
g. The effects
G.
HAMILTOX,
cathodic associated
3_ 31. PARR\-
potentials witl;
and
decreasing
--&o.-&ill-arise from (a) the change of population of water dipoles and (bj the increasing accommodation of pyridine dipoles in the inner region of the double-la_ver as the chemical potential of pyridine is increased in the solution_ A complementary approach may be made @, in terms of the parameter, AE, defined bJ.-
by calculating30
the surface
pressure,
A&y”-y+q,(E’J-E) &here ~0 and EO are the surface tensions and respective potentials for points on the electrocapillary curve for the-base solution at given values of qm, and “/ and E are _values for the same qm values in the presence of pyridinevalues of pm, for _ The curves of 6 vs. log C py are shown in Fig. S for various pyridiqe in I N KCl. The behaviour in 0.03 _M KC1 is similar. The surface pressure curves- differ significantly from those for thiourea adsorption at mercury, discussed p~viously by PARSOh’S 30. First, it is clear that they cannot be superimposed (except perhaps at low @lvalues) to give a composite curve simply by adding constant terms (proportional to the standard free energy of adsorption at zero coverage) to the log C-values for each value of q;I considered; the shapes of the curves themselves evidently that the interaction effects in the surface layer and/or depeud on +_ This indicates the effective area of the species depend on +_ Such an effect is consistent with the conclusions that may be drawn from the AGO-0 * plots (Fig. 7). _ Secondly, it will be seen that the surface pressure for pyridine adsorption does not-follow a single, -hvo-term virial equation of state (cf- ref. 30) and corresponding isotherm, as does the adsorption of thioureaao at all values of +_ This may arise since of Cl- will depend on (a) in the KC1 e 1ectioly-te used, the extent of specific adsorption qrn and is also a function of pyridine concentration (Figs. I and 2) ; and (b) a disorientation-orien@ion transition is evidently involved which would not allow represkrtation. of the surface pressure behaviour in terms of a single-valued second virial coefficient. Accordingly, no attempt will be made here to fit the results to a particular type of isotherm. :-i S6me~fukhkr insight into the equation(s) of state for the pyridine ad-layer may, however, beobtained by plotting @ as a function of Fry at various qm values, from the datain Fig_ -9 for & as a function of q,.,,. The derived plot is shoti in Fig. IO for 7 values of:&_- It is evident that‘ for almost all the values of qm, @ is a discozztinztoz~s function of r pp,.and at high coverages suddenly increases more with increasing Try than at lo&-doverage. This again supports the view that a relatively sudden onset of kientatioh occurs in region z.(Fig. -IO), giving rise to stronger repulsion effects and . -* It c&nld be~%rgued
that-the effects observed at high coverages I -_0_teMx in c&culatin~~ dG”-values. -Thus 29, if the isotherm -foe, 3~ concen*tio~$C; of pyridine, -:_ __e J __
e-KCedl;OlW-
,r_(ryi f3,y
-.
to
arise from be- used
the use of an incorrect were of the following
(3)
ADSORPTION
IX
SOLUTIOKS
OF
I’\-RIDINE
ETC.
497
3c T
E
”
a,
5
z
20
IO
0
Log Fig.
8.
(0).
+zO;
(ml>
(d),
-5-o;
-3-o;
Surface
pressure
+r5; ((-#).
(@)
( 0). -7;
-0.5
-1.0
-1.5 [Cpy]
curves
fI2.5; (D).
( for
C,, in g mole I-‘) pyridine
(v). fro; -ropCcm-r.
adsorption +7.5;
(A).
from I _V KC1 as a function of qm. (X), +5; (+I. +2-5; (e). O-0; t-b).
hence to a larger d@/drp,_ The points in Fig. IO have been drawn within two envelope regions, but for each series of points, the discontinuity in the G-r relation is clearly evident. At more negative qm-values, a larger coverage is reached before the transition occurs, but the value of d@/drin region z is then greater than that for more positive q,-values. The transition region occurs when the area available to the adsorbate is between 35-45 A-l per molecule of pyrkline. The projected molecular area of pyridine in the “flat” configuration is ca. 35 A *_ The maximum surface coverage attained is about 6.5 x IO-10mole cm-2 in agreement with our previous results9 ; such a coverage corresponds to an effective molecular area of ca. 25 As molecule-1 which could arise if the molecules stood on end in the oriented condition* with the N-atom directed towards or atiay fro-m the mercury surface, depending on qm_ 3_ BeIz+iozc7 of cations denved from pyridine _ By thesamemethod as that used previouslys, the AGo-values __ a -It is of interest to note that a disoriZntation-orientation transition also of py+ixie in montmorillonite with increasing uptake pf the s&batel3. -JJ-Ekctrob~rd:Ckenz.,
for
PyH+
occurs
IO
in
in HCl;
tlie
sorption
(Ig65)
485-502
B_ E.
49s
COXWAY,
R.
G.
BARRADAS,
,L~i.I,+-+~+\-F
cy
o-01
-2 O-
-I-
+20
+/+
+’
g_
T,
as a function
of qm for
I
0
G.
.
--A-
.
-10
(pLc cma2J
various
py-ridine
concns.
H_~MILTOX,
+ + +
I
I +I0 9,
Fi&
P.
izz I .X 1X1.
-: 0
J_ 31. PARRY
ADSORPTIOS
IN SOLUTIONS
I 01
I 0.2
OF PYRIDINE
I 0.4
I 03
I 0.5
ETC.
I 0.6
499
I 0.7
&2
Pig. II. -apparent standard free energies HCI aq. soln. as a function of 8 I_
of adsorption,
il?%,
of pyridinc
of adsorption;
dG 0, of
(h>-drochloride)
in o-33 N
-1.0
Fig. 12. Apparent sz&2_ as a fu?x&k
standard oi83.
free energies
0.1
N-methylpyx-idinium
chloride
B
500
PyH’
in HClOq
plotted
againsr
and
I3. COSXV..I>-,
R.
G.
S-methylpyridinium
04 cc/- refs- Sand
BARRADXS,
ions
9) in Figs_
II,
P.
G
HrlJiILTOS,
J.
31. P_%RRl-
in HCJ, ha%-e been obtained and are and 13_ In diLz[te HCl or perchloric
12
acid, the AGO-values are linear* in 8 8 indicating 8.9 that the leading effect giving rise to deviation from the Lan,oTnuir isotherm is a. coulombic nearest-neighbour repulsion energy reciprocal in the interionic distance in the ad-layer. The effects observed with N-methylp>ridinium cations and \-it11 the protonated pJ-ridine are very similar confirming that F;H+ is the principal organic species (c/- ref. 31) adsorbed from acidic solutions of yyridine. E_ potentials are indicated m these Figures in 1.olts.
3.5
2.5
2.2
I
1
01
02
I 0.3
I 0.4
I 05
I 06
I 0.7
I 0.0
8% Fig~3~ Apparent standard free energies of adsorption. perchloricacidsoln_asafunctionof8*. _
A% 0, of pyridine
(perchlorate) in I X
The effect arising from nearest-neighbour coulombic repulsions may appear oversimplified_ However, at moderate and high concentrations in free I :I electrolyte solutions, an analogous disordered lattice model22. 23 for nearest-neighbour ionic interactions; leads to a cz&-root law for log[activity coefficients] which is well supported by experiment23 and is to be preferred (except at high and infinite dilution) to the Debye-I&kel theory_ Nevertheless, the simple cube-root law is only obeyed after _hydratiion effects in Jog[actitity coefficient] have been removed23. The inter-ionic distances laterally across the double-layer are certainly comparable with, or smaller ‘than, th& in concentrated free electrolyte solutions (e.g., in 5 M KCl. the average inter-ionic_ &stance is only 4-5 &)_ Hence hydration effects may also be an important f&t&leading to ttio-dimensional non-ideality as they are in three-dimensional free *-electroIjrte solutions at high concentrations. __ I . rk-TI%f&t
tk~*Line.ar relations tie obtained for-the-ion ddsOrpti&s. without any inflections, made_ab&e_that theminflections in-the correspondin& plots for neutral tihg%roi6thedcid%&&_(see footnoteon p-.496_) _molecllles.~f(e*)arenotGrtEfacts
-$tip*or&-the-conte+on ‘&.~~~~W?+-_
@&=.;-rro
(x96$1
&-5Oi
ADSORPTION
XX SOLUTIOKS
OF
PYRIDXSE
ETC.
50=
Finally, co-adsorption of anions will be another effect to be considered; to diminish the repulsive interaction bettveen the adsorbed cations_
tend
it will This
is
probably the reason for the decreasing slopes of tire ~@?a--O* lines shown in Fig. 14 as the electrode potential is made less negative and anion adsorption can become
I
-o-7_,
401T_. -06
I I
‘a3 z E
0
+&
-zbQ
=Q$ :\
1 3.5 i-/
0
Ia
a I 30
I
.%-J*
i-
,‘\ Y
I
\ \ PI!9
\ x
I
\
x5cJ\
A+ ;7\
Ii
\
X
i .--__L--L_1__.___.I_1 /’ 01 02
0 I+ aq
l x\
-02
Fig. HCl
+
c
-03.0
i
z-5
,“’
\\
0
Y -
0
c3
_ “1 u
04
05
06
07
Apparent standard free energms of adsorption, ~16, of pyridine in V as indicated. soln. as a function of 13*_ E c;L~potentials
more significant (see Fig. 5). Such effects in the more dilute HCl solutions.
are absent
(hydrochloride)
in the perchloric
acid solution
in
z 117
and
ACIQiOWLEDGE3IEXTS
Grateful acknowledgement is made for support of this work carried out jointly One of us (J-M-P.)
acknowledges
the award
to the National Research Council, Canada, at the Universities of Ottawa and Toronto. of an N.R.C. Post-Doctoral Fellowship at (P.G.H.), the award of a Pro-
the University of Ottawa in 1962-63. and another vince of Ontario Government Fellowship for rg63-65.
A. comprehensive study of adsorption of pyridine in neutral and ionic forms at the mercury electrode is reported_ The effect of neutr4 pyridine on the components of charge for adsorption of KCI in the double-layer has been investigated and it is shown_fhat the presence of an ad-layer ieads to an apparent specific adsorption of the chloride ion on the cathodic as well as the anodic side of the electrocapillarymaximum. This effect may be due to ion-pair formation_ The components of charge for pyridine and N-meth.ipyridinium cation adsorption in acid solutions have been derived. Iso-
B. E. COS\VA\-,
502
R. G. B_~RRA13_~S,
I'. G. HAJIILTOS,
J_ AI. PARRY
for neutral py-ridine have been analysed in terms of surface pressures, CD, and two regions of the @-coverage relation are apparent: and ma>- be related to orientation principally by ion repuleffects. The isotherms for the ionic species are cha racteriscd
therms
sion effects.
REFERENCES I 2
A. N. A_ N_
FRUMKIN.
2.
FRUJKIN.
A&OX_
3 V. BAGOTSKII
AXD
Pkysik
C/MHZ., k11Gq (1933) 131; _-1&a Physicocirzm. l_J.R S.S., 6 (x937) 502. Electrocl.en~.El~c~roclre~~~_ E=g., 3 (1963) _-ST; zbzti., I (1961) 65. h-him.. 23 (1949) 413; DokZ. _qkad I\-auk. SSSII. J_ J_+BLOKOVA, Zh. Fi=
58 (1947) x387/L N_ FRUMKIK
AXD N. V_ XIKOLAJE\~'X-FEDOROXXCH. Il-estx. YlZosk. Ufaiu.. 12(+) (1957) zGg; (1956) x455_ _-~sD I. SLES5 J_ P~c~,CoZZec~ion Czech. Chem. Cotnmzrlr.. 6 (1934) 129. rgo; P. HERXSYJIF.XRO DYK.Z&-d., 6(1g34)zo+,zgo;R_ BRDIC54, ibid.,3 (1933) I@. Proc. C.l.T.C.E.(xg5~~, Buttcrworths, 6 l3. E. COXW_AY. J_ O'BI. BOCKFUS XND B. LOX-RECEK, London,Ig55.p_z07s DokLAkad. _Vazrk,SSSR. I 14 (1957) 1273; 120 (1958) 1294; 132 (IgGo) 1352; 7 S.MAimovs~~r. 133(rgbo) Iti. S E.BLOMGREK AND J. O'~i.Boc~~~~~.].P~ys.Clrs~tz.. 63 (1959) 1475. 319, 349; L. GIERST, PYOC. AND R. G. BARRADAS. EEectrockim. -3cta. 5 (1961) 9 B. E. CONWAY 1959, John \Vlley and Sons, Kew York, Symp_ Elecirode Processes, The Electrockem. Sot., rg6r.p. Iog_ IO l3_ E_Co?vwi~, R_ G.BXRR~DAS AND T. ZAWIDZKI,]. Phys.CAem.. 61 (1955) 676_ II B_ E. COSWAYAND R-G. BARRADAS, J_ ElecLroalznZ_Cher,l., G (1963) 314. 12 W_LORENZ.F_ MC%ZKEL AND N. nI~~~~~,Z.P~ysik.C~eln.,15 (rgGo) 145_ Sac-. 5g (1963) 141. 13 R. PARSOXS XP;D J- 31. P_~RRY. Tratrs. Faradq~ Theory and Prirrcifiles of Electrode Processes, Ronald Press Co , Xert* York. 1-f B. E. COKWAY. 4
r5 16 17 r8 =9
Zh- Fiz_K~imz..30
1965. chap- 3_ K. lli_ JOSHI -44~~R. PARSONS. EZecCrockim. -3&z, 4 (2961) r2g_ J. OX. BOCKRIS AXD R. PARSOXS, Trans. Faraday SOL. _15 (1919) gI6M. A. V_ DEVAXATHANAKD P.PEFUES. T~on.s_ ~amdaySoc.,~o (1954) 1~36
N-F. Htirnx~M.R. SPRINI~LE,].~~.C~~~~.SOC.,~~ (1931) 3469_ l3. E. CONXVAY, J_ E. DESKOYERS AND -4. C_ SMITH, Phi,?.;Tratzs.Roy_ SOG- (London). X256, (1964) 3S92o.M. A. V. DEVAXATHA~ _xxD S. G. CANAGXRATNA, bledrochim. -d&a, 8 (1963) 77_ 21 D_ C. GRAHAMLE, Chem. Rev.. 41 (1947) 441~ 22 H. S. FRAMK AND P. T. THOMPSOX;. J _ CIzenz. Phys., 3 I (1959) 1086; Stvzrclzrve of EZecZroZyte - Solzdiorrr, e&Ceil by XV. J_ H~JIER. John Wiley and Sons. New York. 1959, chap. S. 23 J_ E. DESNOYERSAKD B.E.CONWAY, J.Pkys.Ckena.. 68 (IgG4) 2305~ 24 B. E_ Cosza~.J_ O’Ai. BOCKRISAND I_ _4.AhI~rAR,Tra1zs. FaradaySoc..q7 (1951) 756_ 25 R. \V_ RAMP‘OLLA.R.C_B~ILLERA~~ C.P.S~~~~.j.C~etn.Plrys.. 30 (1959) 56626 27
A. N. FRu_vmK, Eugeb. Elrakt. Naturn-, 7 (1928) 25% l3_ E_ CorrivAy ASD E. GILEADI.C~~~.J_C~~?~_.~~ (1964) 90; Modem AspcclsofEZectrochenristry. Vol. 3,edited by J_ O’M. BOCKRISAND l3. E-CONWAY. Buttemvorth.London. 1964. chap.5_ 28 J_ S. MI-XHELL, Tmm-. Faru&y SOG.. 31 (1935) gSo_ 2g R. PARSONS. J_ II+troamaL C&m.. 8 (1964) 93_ 30. R1 PARSONS, Truns_ Far&uy Stic.. 51 (1955) 1518; ProcRoy: Sot. /Lorrdmr). A261 (1961) Tg_ 31 N. HACKERMANAXDA.C-AM~~~~~~. Ind.Eng-Chem.. 46(rg54)523_ 33 D. C. GRAHAME AND R. Pesows, J- Am. Chem. SOG.. 83 (1961) 1291. 33. R_ GREENE-KELLEY-,IT~~S. Faraday Soc-,5x (1955)412,425i_ EZectroana&.
Cicem.,
IO
(1965)
485-502