Electroanalytical Chemistry and lnterJacial Electrochemistry, 41 (1973) 145-157 145 © Elsevier Sequoia S.A,, Lausanne - Printed in The Netherlands

A D S O R P T I O N OF INORGANIC ANIONS ON A MERCURY ELECTRODE FROM SOLUTIONS IN FORMAMIDE

II. A D S O R P T I O N OF CHLORIDE AND NITRATE IONS*

RICHARD PAYNE Air Force Cambridoe Research Laboratories, L. G. Hanscom Field, Bedford, Mass. 01730 (U.S.A.)

(Received 17th June 1972)

INTRODUCTION

The adsorption of anions on mercury from aqueous solutions has been studied extensively but is still imperfectly understood. The behavior of strongly adsorbed ions such as iodide for example appears to be qualitatively different from that of less strongly adsorbed ions such as chloride and nitrate for reasons that appear to be related in part at least to the well-known structural properties of water. It is therefore of considerable interest to study the behavior of the same ions under comparable conditions in a relatively unstructured solvent such as formamide. In the previous paper 1 the adsorption behavior of iodide was described. The behavior of chloride and nitrate ions in solutions of the respective potassium salts is described here.

EXPERIMENTAL

The experimental procedure was similar to that described for the KI system 1. The capacity was measured as a function of potential and concentration for seven concentrations of the salt in the range 0.01-1.0 M using the a,c. bridge described previously 2 at a frequency of l kHz. The interracial tension at the electrocapillary maximum was measured over the same concentration range using the capillary electrometer described previously 3. A 0.1 M aqueous KC1 calomel reference electrode was used in a cell of the type

Hg x M K X 0.1 M K X

in formamide in formamide II

I 0.1 M KC1 [ Hg2C12 Hg

in water I I[ I I

The liquid junction between the aqueous and formamide solutions was left undisturb- ed during a series of measurements. The liquid-junction potential between the two formamide solutions was calculated from the Henderson equation using a value for t +, the cation transference number, of 0.38_+0.01 for KNO3 and 0.40_+0.01 for KC1 calculated from the conductance data of Dawson and Berger 4 and published tables

* Presented at the meeting on The Electrical Double Layer held by the Society for Electrochemistry in Bristol 17th-19th April, 1972.

146 R. PAYNE

of the limiting ionic conductances 5. The transference number for KC1 solutions in formamide according to recent accurate measurements of Notley and Spiro 6is 0.42. However, the use of this more accurate value did not affect the calculated liquid junction potentials by more than 2 mV.

Solutions were prepared by weight using Analytical Reagent grade salts purified by recrystallization from permanganate-distilled water. Technical grade formamide was purified by triple distillation in vacuo using a 20 cm Vigreux column. The temperature of the distilling solvent was held below 60 ° C to avoid decomposition The product appeared to be free from ammonia which is formed by decomposition when the solvent is dried over alkali before distillation. Mercury was purified by triple distillation and capillaries were prepared and treated with dimethyldichloro- silane as described previously 7.

Potentials of zero charge were measured for each solution by the method of the streaming mercury electrode 8. All solutions were de-aerated by passing oxygen-free nitrogen through the solution after filtering through a cotton-wool plug and presaturating with purified formamide. The electrocapillary measurements were conducted in an air thermostat controlled to _+ 0.5 ° C. All other measurements were made in a water thermostat controlled to _+ 0.05 ° C.

RESULTS

1. The K N O 3 system Comparison of the electrocapillary and capacity curves for KNO 3, KC1 and

the previously reported KI system shows that the order of specific adsorption of anions in formamide is NO3 < C1- < I- as in aqueous solutions. However, as noted elsewhere 9, ~o the adsorption from formamide is comparatively weak probably due to strong solvation of the anion in solution by a hydrogen-bonding type of mechanism. Specific adsorption of the NO3 ion is particularly weak compared with its behavior in water as suggested by the capacity curves in Fig. 1 which show that the dependence on the concentration is quite low in the anodic region. The deep minimum close to the potential of zero charge (p.z.c.) in the more dilute solutions is due primarily to the effect of the diffuse layer.

The capacity curves were integrated as described previously to give values of the capacity (C), potential measured with respect to a reference electrode revers- ible to the anion (E-), interfacial tension (7) and the function 3 - = 7 + q E - , all interpolated to integral values of the charge on the metal (q). The integration constants were taken from the measured potential of zero charge and the interfacial tension at the p.z.c. The measured potentials were converted to an E- scale using values of the transference number of the cation calculated as described above and the published activity coefficients of Vasenko ~ ~ which however were obtained from freezing point measurements and refer to a temperature of 2.5 ° C. No serious error seems to be introduced by the use of these values however.

The occurrence of some specific adsorption of anions was confirmed by comparing the results in various ways with the predictions of the diffuse layer theory. The capacity of the diffuse layer C d was calculated from the expression :

C a = ( 2 R T / F ) ( q 2 + 4A2)4 (1)

assuming the absence of specifically adsorbed ions. The charge in the diffuse layer qd is

A D S O R P T I O N OF CI- AND NO~ AT Hg F R O M F O R M A M I D E 147

35

3C

:=.,

6

'~ 20

2

Q C3

IO

KNO 3 in formamide, 25°C

potent ia l (V) vs. oq.o.tN CE

Fig. 1. Differential capacity curves for solns, of KNO 3 in formamide at 25 ° C. Concn. (molality): (1) 0.916, (2) 0.444, (3) 0.184, (4) 0.0903, (5) 0.0450, (6) 0.0163, (7) 0.00918. Vertical arrows indicate potential of zero charge.

then equal to the electrode charge with the sign reversed. The diffuse layer parameter A is given by:

A = (eRTc/2000 lr) ~ (2)

where c is the concentration of the electrolyte in tool l - 1 and e is the dielectric constant of the solvent. For formamide at 25 °, e = 109.5 and A = 6.94 ~/c I~C cm- 2 If specific adsorption is absent the capacity should be given by Grahame's series condenser formulal 2.

1 1 1 = + (3)

in which C i the capacity of the inner region is independent of the concentration. A plot of I/C against 1/C d at constant electrode charge therefore should be linear with a slope of unity. The results in Fig. 2 confirm that this is the case for q = 0 but at more positive charges the slope decreases indicating the occurrence of some specific adsorption. This is confirmed in Fig. 3 where the function 4 - = 7 + qE- computed from the experimental data is plotted as a function of the chemical potential of the electrolyte and compared with values calculated from the diffuse layer theory accord- ing to the expression"

4- = - 2 R T ~ {A exp [arc sinh(q/2"A)- 1]} d In a+ + 4o (4)

The constant of integration 4o was taken as the experimental point for the lowest concentration. The results clearly indicate diffuse layer behavior at q = 0 but deviations occur at more positive values of q at the higher concentrations consistent with the

148 R. PAYNE

0.09

'7 .0 .08

% ~ 0 . 0 7

L

0.06

0,05

' ' ' ' 2 "50C OllO KNO 3 in formomide,

• C em -2

~ 6

0.04 °'hi I 8 i (~ I P i

0 0.01 O. 0.03 0.04 0,05 ( Ca)-I/crn z # r -1

39C

:580

3 7 0

360

i

KNO 3 in formomide, 25"C

~ 4

~ 6

8

l I I I I i - 0 2 -0,1 0"0

(2 RT/F) In o ± / V

Fig. 2. Plot of eqn, (3) for KNO3 data. Charge on electrode is indicated for each line.

Fig. 3. Comparison of function ~- computed from exptl, data for KNO3 solns. (O) with values calcd, from diffuse layer theory assuming absence of specific adsorption (--). Electrode charge is indicated for each line.

I0

% u

u

+ g.., 5 kL

I I

KNO 5 in formomlde, 25"C O.090~M

I() 0 -I0 q / # C cm ~

~,c o

8 ~_=

KNO3 in formamide, 25"C / B

• . / . . . . / /

•

• • / e

" - - . . . . . _ j .

• J ° ------.~. ~.. /

-0. 5 -0.2 0.1 ( 2RT/F )lna_. / V

i , r ,

/ q = l O ~ C ¢~z

Fig. 4. Surface excess of cations as function of electrode charge for two concns, of KNO3.

Fig. 5. Charge due to specifically adsorbed nitrate ions as function of chemical potential of electrolyte at indicated constant values of electrode charge.

occurrence of specific adsorption of anions. The slope of the plots in Fig. 3 is equal to the surface excess of cations (F+) with the sign reversed. The experimental plots were differentiated graphically to give F + as a function of q and the concentration. The results shown for two concentrations in Fig. 4 show the characteristic minimum in the curve of F+ against q. Adsorption of cations in the diffuse layer increases with the positive charge on the electrode due to the effects of specific adsorption of anions.

The results were further analyzed to give the total surface excess of anions (F_) and the charge due to specifically adsorbed anions (ql) using standard methods. However, because the adsorption is weak the calculation ofq 1 from the surface excess is subject to the well-known uncertainties of the diffuse layer calculation 3 especially for the lower concentrations. However, it is worth noting that the resulting constant charge isotherms flatten off at low concentration to a value of ql numerically close to the value of the electrode charge (Fig. 5). This behaviour is found commonly in systems of this kind and is discussed below.

2. The K C I system The strong concentration dependence of the capacity and the negative shift

of the p.z.c, for the KC1 solutions shown in Fig. 6 are indicative of relatively strong specific adsorption of C1- ions. The results were analyzed as before using the activity coefficients given by Vasenkol 1 for solutions at 2.5°C. Somewhat different activity coefficients were estimated for KCI solutions at 25°C from the data of Povarov et al. 13 for NaC1 and CsC1 solutions at 25°C but the use of these values had little effect on

4C

3~

lOt

t )

u. 30 = k

o ~25

a

P5

i E

KCl in formamide, 25"C

- ; -,b -,'s Potentiel/V vs. O.1N CE

/ KCI in formamide , 25"C 1

.00813 M

.75~

ADSORPTION OF CI- AND NO~ AT Hg FROM FORMAMIDE 149

6 J6 6 -,3 q/pC ¢m -2

Fig. 6. Differential capacity curves for solns, of KCI in formamide at 25 ° C. Concns. (molality): (1) 0.733, (2) 0.360, (3) 0.175, (4) 0.0898, (5)0.0391, (6) 0.0165. (7) 0.00813. Vertical arrows indicate potential of zero charge.

Fig. 7. Surface excess of cations as function of electrode charge for indicated concns, of KCI.

-0.6

>

a l

-0--0.4

_=

-0.2 I

Ltj

150 ~t. PAYNE

the analysis. The analysis was also repeated for both sets of activity coefficients using a computer differentiation of the ~- vs. In a± curves again with little effect on the results. The surface excess of cations in the KC1 system is positive at all concentrations and all potentials (Fig. 7) confirming the presence of specifically adsorbed anionic charge in excess of the charge on the electrode.

The charge due to specifically adsorbed C1- ions was calculated in the usual way from the surface excess of cations and the diffuse layer theory assuming that all cations are in the diffuse layer. The potential difference generated by the adsorbed anions is plotted in Fig. 8 as a function of the amount adsorbed at constant electrode charge. The plots appear to be linear and parallel for q ~< 6 ~C cm- z as found in the KI system 1 and in many of the aqueous systems studied 14. However, the slope, 0.070 cmZ//iF, is surprisingly large compared with the value of 0.022 cmZ/~F found pre- viously for the KI system. At more positive values of q the linearity deteriorates and the slope apparently decreases in contrast to the behavior of the KI systems where excellent linear and almost parallel plots were observed over the entire range of q. The experimental isotherms at constant charge are shown in Fig. 9. The dependence of the amount adsorbed on the concentration of the electrolyte is surprisingly small. This effect, evidently due in part to a residual electrical dependence of the amount adsorbed present even when the electrode charge is kept constant, is discussed below.

The Esin and Markov coefficients in these systems have been discussed pre- viously 1'9'1°. They provide independent confirmation for the conclusions drawn from the electrocapillary measurements that (i) anions are specifically adsorbed in the order NO3 < C1- < I- and (ii) the ionic surface excesses for a negatively charged

q=O pC cm "2 KCI in formamide, 25"C

IO

/ 12

1614

-z'0 Specifically adsorbed charge /,tiC cm -2

-2C

% 0

8

U)

q/pC cnf 2 KCl in formarnide, 25"C I ~ • /

Fig. 8. Potential difference generated by specifically adsorbed chloride ions as function of amount adsorbed and charge on the metal indicated for each line.

Fig. 9. Experimental adsorption isotherms for chloride ions at constant charge on the metal.

-o'2 -o.', 6 (2RT/ F) Ina, /V

ADSORPTION OF CI- AND NO~ AT Hg FROM FORMAMIDE 151

electrode in all three systems are consistent with the predictions of the diffuse layer theory assuming no specific adsorption in this region of polarization.

DISCUSSION

1. Specific adsorption of nitrate The measurements for KNO 3 solutions show that specific adsorption of

nitrate is quite weak in formamide as compared with water t5 but is definitely present. The difference is probably due to differences in the solvation energy. As is well known, large anions such as NO~, C102 and PF 6 tend to be rejected by the water "structure" as shown by the lowering of the surface tension of water that they pro- duce 16. The so-called "'squeezing out" effect seems to be the principal contribution to the adsorption energy in water and, as would be expected, is unimportant in the relatively unstructured formamide. Specific adsorption of nitrate and similar large anions evident in normal polar solvents like dimethylsulfoxide iv suggests the occur- rence of some kind of specific interaction with mercury. This possibility however, is apparently discounted, for water at least, by Damaskin and co-workers 18'19 who concluded on the basis of measurements in concentrated solutions that NO~ and C102 ions are adsorbed solely by the "squeezing-out" forces and are in fact separated from the electrode by a layer of water molecules. The implications of this model remain unclear but it obviously involves a radical departure from the Stern model. If the ions are excluded from the inner layer, the distinction between specifically adsorbed ions and diffuse layer ions disappears. The ions in this case are excluded from the region of high field strength. Consequently it seems difficult to explain the profound effect of the adsorbed ions on the potential and capacity observed 15,2o in more dilute solutions on the basis of this model.

2. Specific adsorption of chloride a. The potential generated by specifically adsorbed ions. As noted earlier the

adsorption of chloride ions generates an unusually large potential difference across the inner layer (Fig. 8). Consequently the effect of the specific adsorption o n the capacity curves in Fig. 6 is larger than would be expected in view of the relative weakness of the adsorption. According to Grahame 2~'22 the potential difference across the inner layer ~b"-2 can be expressed as the sum of separate contributions from the charge on the electrode and the specifically adsorbed charge :

ql q~m-2= q. + _ _ (5)

qK' qlK i

The component integral capacities in eqn. (5) are defined by:

qK i = ei/aTrX2 (6)

and :

q,K! = d/4~(x2 - x l) (7)

where e 1 is the mean dielectric constant within the inner region and Xl and x2 are respectively the distances of the inner and outer Helmholtz planes from the interface. In a previous paper 23 it was suggested that an additional term should be included in

152 R. PAYNE

eqn. (5) to account for the potential drop resulting from displacement of oriented solvent dipoles by the adsorbing ions. According to Levine 24 this effect is already accounted for in the dielectric constant. However, it should be pointed out that the replacement of a permanently oriented solvent dipole by an ion is unlikely to produce much effect on the average polarizability of the layer but it will produce a potential shift of 4rt/~/e i (/~ is the normal component of the dipole moment) which should be taken into account. As noted by Frumkin and Damaskin zS, unless this effect is con- sidered it is not possible to explain the shift of the potential of zero charge produced by the adsorption of organic compounds from aqueous solutions. In the formamide system there is some experimental evidence to suggest that the surface potential due to orientation of the solvent dipole is small 9, close to the p.z.c, at least, and the replace- ment effect therefore may be relatively unimportant in this case. This will be assumed in the following discussion.

The component of the capacity qlK i is given by the slope of the plots in Fig. 8, while qK i is obtained by interpolating ~b "-2 atconstant qX. The value of qK i is then given by the ratio Aq/Ac~ m-2 where A represents the difference from the value at the p.z.c. The steep slope of the plots of qS"- 2 against ql close to the p.z.c, corresponds to the unusually low value of 14.3/~F cm-2 for qlK i compared with 45 pF cm-2 found for the iodide systemL However, qK i is also low, 8.3 #F cm-2 compared with ~ 20 #F cm-2 for the iodide system. Consequently the ratio of the two capacities, which according to eqns. (6) and (7) is equal to the distance ratio (x2-xO/x2, isnot much different in the two systems. This is not so surprising since qK i and q,K i depend on the same parameters.

The experimental values for qg i give for the ratio e~/x2 in eqn. (6), 1.0 × 10- 8 cm- 1 for the chloride system (for 0 ~< q ~< 6) and 2.3 x 10- 8 cm- ~ for the iodide system. If the thickness of the layer is determined by the size of the solvent molecule assumed adsorbed in a flat orientation, the value of x2 indicated by molecular models is about 3 A. For the chloride system this gives a value of 3 for eJ which is reasonably close to the optical dielectric constant of formamide. However, if this value of gi is now applied to the iodide results, x2 has the impossibly low value of 1.3 A. It must be concluded therefore that the dielectric constant is higher in the iodide system. For x2 = 3/It the value of ci for the iodide system is 7. The results therefore seem to indicate that specifically adsorbed chloride ions polarize the surrounding solvent molecules more completely than iodide ions. This seems qualitatively consistent with the known properties of the ions.

b. The adsorption isotherm. The constant charge isotherms for both the nitrate and the chloride systems shown in Fig. 5 and 9 flatten off at low concentrations to values of the specifically adsorbed charge numerically close to the charge on the electrode. Similar behavior was noted previously in the corresponding aqueous solutions ~ 5,23 and now appears to be a general feature in all adsorption systems of this type, i.e., solutions containing only a salt of the active anion 26. The effect does not occur for solutions maintained at constant ionic strength by the addition of an inert electrolyte 2°'23. This suggests that the effect is related to the diffuse layer the composi- tion of which is generally quite different in the two types of system 23. The important factor appears to be the potential difference (4)2) across the diffuse layer. In a recent review 27 de Levie has shown that isothermswith flat segments resembling the experimental isotherms can be generated from Frumkin's isotherm with an explicit

ADSORPTION OF C1- AND NO3- AT Hg FROM FORMAMIDE 153

term in 4)2 in the adsorption energy:

0 (1 - 0) - t exp [AO + (zF/R T)q~2] = fia+ (8)

in which 0 is the fractional coverage of the electrode, fl the adsorption equilibrium constant and A the lateral interaction parameter. The form of the isotherm seems to be relatively unimportant. The approach of q 1 to - q at low concentrations depends only on the presence of the 4) 2 term in the exponent. Since ~b2 cannot be kept constant at constant q while the concentration varies it will be obvious that isotherms like eqn. (8) cannot be congruent in the charge.

The low coverage approximation of Frumkin's isotherm was used with the ~b2 term included in a recent paper by d'Alkaine et al. z6, to interpret the adsorption of azide ions in aqueous solutions. Their isotherm which can be written in the form :

l n ( - q l) + A'q I + (zF/RT)c~2 = fl' a+ (9)

is of the same form as the virial isotherm. The chloride adsorption results were com- pared with eqn. (9) by plotting -q~ against In ( -q l /a+) + zFc~z/RT at constant charge on the metal which should give linear plots. The results are shown in Fig. 10. A satisfactory straight line is obtained for q = 4 but for larger values of q the plots are curved. The same plots with the ~b 2 term excluded are also curved. The experimental results therefore are inconsistent with eqn. (9). The variation in the slope of the plots in Fig. 10 is formally equivalent to an increase in the composite isotherm parameter A' with increasing amount adsorbed.

~-15 u

L)

° - I0

.D

no o

> , - 5

U

ca. 03

KCI in formamide, ,15 0 2

-20

0 , , I ]

= 1~2 ~ - I 5~

0 ©

-tool-

8

; -

O - - I KCI in formamide,i 25°C -010,~ -0.2 -0.11 ()

0.05 O. i 0.1'5 0.2 0.25 03 (RT/F')In(-q'/a±)-,~z /V (2RT/F]Ina+_ /V

Fig. 10. Test of eqn. (9) for the chloride data. Charge on the metal is indicated for each curve. { - O ) including q~2 term, (©) omitting 4) 2 term.

Fig. 11. Experimental adsorption isotherms for chloride ions at constant values of electrode potential (E- -(RT/F) In a±) indicated for each curve.

154 R. PAYNE

The physical significance of the term in ~b2 in eqns. (8) and (9) as pointed out by d'Alkaine et al. is that it accounts for the energy of transfer of the anion across the diffuse layer. Since this is automatically taken into account if the potential of the electrode rather than its charge is kept constant, it is of interest to see whether the results can be better described by a constant potential form of isotherm. The amounts adsorbed were therefore interpolated for various constant values of the potential measured with respect to a fixed reference point. The resulting constant potential isotherms shown in Fig. 11 are more "normal" than the corresponding constant charge isotherms of Fig. 9 in the sense that they are not so flat and properly approach zero amount adsorbed at low concentrations. An attempt was made to fit these isotherms to eqn. (9) (with the q52 term omitted)by plotting _ q l against In ( - q 1/a + ). The resulting plots shown in Fig. 12 are again markedly curved. The adsorption of chloride ions from formamide therefore cannot be described by this type of isotherm either at constant charge (with or without allowance for the variation of 4)2) or at constant potential.

In view of the foregoing discussion it is of interest to re-examine the iodide results. In the previous paper 1 it was shown that the isotherms at constant charge in this system are congruent within the experimental error and can be represented by a virial form of isotherm with no allowance for the variation of the diffuse layer potential. However, the results can be represented equally well when the q~2 term is included as shown by the linear plots in Fig. 13. As in the case of the chloride system the constant charge isotherms for iodide are quite flat (Fig. 14)whereas the corresponding constant 2s

-20

E u o

g-I

~-I(

,~-5

0 0

i i i i

-0.15~ E=-O.IV

KCI in formamide, 25eC~

\

-0.25

- 0 . L" 2 4 6

In ( - q l l a t )

zc

I

mamide , 25"C ~ I 6 , u C cn

o.io o.~'s o~o o.~s o ~ (RT/F) ln( -c l ' /a+) - ¢ka / V

Fig. 12. Test of eqn. (9) with the term in Sz omitted for the adsorption of chloride at constant potential. Value of potential (E- - (R T/F) In a _+ ) is indicated for each curve.

Fig. 13. Test of eqn. (9) for adsorption of iodide at constant charge on the metal.

ADSORPTION OF C1 AND NO3 AT Hg FROM FORMAMIDE 155

~ t i i i i K[ in formamide , 25"C KI in forrnQmide , 25"C

-40 . / ~ , ~ q= 2 4 ~ C crrT t - 40 E=_0,5 v

/ zo . . . . . . ~ . . , 6

~501- ~ / . .-*-- ~ m . -30 o.s

"~ -0.7

t : .0-,

-0.5 -o.i ; °o.~ -o.z -d b (2 RT/F) In a+_/V (2RT/F) In o+_/V

Fig. 14. Experimental adsorption isotherms for iodide ions at constant charge on the metal indicated for each curve.

Fig. 15. Experimental adsorption isotherms for iodide ions at constant values of the electrode potential (E- - (R T/F) In a ± ) indicated for each curve.

-4C

E u O -3C ::L

g

.~ -20

=

.3

i

KI in formamide, 25"C

E= -O,hV

\o - 0 . 6 \ \o

-0.7

0

-0.8 0 %0

\ x

0 , 2 0.05 0.1 0.15 0.2

(2RT/F)In (-q'la+) / V Fig. 16. Test of eqn. (9) with the term in q~2 omitted for adsorption of iodide at constant potential (E- - RT/F) In a±) indicated for each curve.

156 R. PAYNE

potential isotherms are qualitatively different (Fig. 15). The iodide results at constant potential were plotted in the form of _ql against In (-ql/a±) as before and give satisfactory linear plots (Fig. 16). It appears therefore that the adsorption of iodide can be described within the experimental error by isotherms at constant charge (with or without allowance for the variation of 4)2) or at constant potential, whereas the adsorption of chloride can be described by neither. No complete explanation of these observations is possible at the present time. However the considerable qualitative differences in shape between the isotherms at constant charge and at constant potential suggests the presence of a large concentration dependent term in the adsorp- tion energy under constant charge conditions which is absent at constant potential. This can only be partially explained by the neglect of the energy of transfer across the diffuse layer in the constant charge isotherms.

ACKNOWLEDGEMENT

The experimental part of this work was performed at the University of Bristol during the tenure of a Senior Research Fellowship of the Science Research Council.

SUMMARY

Specific adsorption of chloride and nitrate ions on a mercury electrode from solutions of the respective potassium salts in formamide has been studied by measure- ments of the double layer capacity and the interfacia! tension as a function of con- centration. Specific adsorption of nitrate although weak is confirmed by comparison of the results with the diffuse layer theory. Chloride is more strongly adsorbed but its behavior, as in aqueous solutions, is more complex than that of the corresponding iodide system. The results are examined in terms of a low-coverage approximation to Frumkin's isotherm at constant charge on the metal, with allowance for the varia- tion of the diffuse layer potential, and also at constant potential across the double layer. The adsorption of chloride obeys neither form of isotherm whereas the adsorp- tion of iodide can be described by both. Adsorbed chloride ions produce an unusually large potential difference across the inner region of the double layer which is consis- tent with a mean dielectric constant of 3 compared with a value of 7 calculated for the iodide system.

REFERENCES

1 R. Payne, J. Chem. Phys., 42 (1965) 3371. 2 G. J. Hills and R. Payne, Trans. Faraday Soc., 61 (1965) 316. 3 K. M. Joshi and R. Parsons, Electroehim. Aeta, 4 (1961) 129. 4 L. R. Dawson and C. Berger, J. Amer. Chem. Soe., 79 (1957) 4269. 5 R. Parsons, Handbook of Electrochemical Constants, Butterworths, London, 1959. 6 J. M. Notley and M. Spiro, J. Chem. Soc., B (1966) 362. 7 R. Payne, J. Electroanal. Chem., 7 (1964) 343. 8 D.C. Grahame, E. M. Coffin, J. I. Cummings and M. A. Poth, J. Amer. Chem. Soc., 74 (1952) 1207. 9 R. Payne, J. Phys. Chem., 73 (1969) 3598.

10 R. Payne in P. Delahay and C. W. Tobias (Eds.) Advances in Electrochemistry and Electrochemical Engineering, Vol. 7, Interscience, New York, 1970, p. 1.

11 E. N. Vasenko, Zh. Fiz. Khim., 23 (1949} 959.

ADSORPTION OF CI- AND NO 3 AT Hg FROM FORMAM1DE 157

12 D. C. Grahame, Chem. Rev., 41 (1947) 441. 13 Yu. M. Povarov, Yu. M. Kessler and A. I. Gorbanev, Elektrokhim(va, 1 (1965) 1174. 14 R. Payne, J. Electroanal. Chem., in press. 15 R. Payne, J. Electrochem. Sot., 113 (1966) 999. 16 J.E.B. Randles, in P. Delahay and C. W. Tobias (Eds.) Advances in Electrochemistry and Electrochemical

Engineering, Vol. 3, Interscience, New York, 1963, p. 1. 17 R. Payne, .J. Amer. Chem. Soc., 89 (1967) 489. 18 B. B. Damaskin, A. N. Frumkin, V. F. Ivanov, N. I. Melekhova and V. F. Khonina, Elektrokhimiya, 4

(1968) 1336. 19 B. B. Damaskin, V. F. Ivanov, N. I. Melekhova and L. F. Maiorova, Elektrokhimiya, 4 (1968) 1342. 20 R. Payne, J. Phys. Chem., 69 (1965) 4113 ; 70 (1966) 204. 21 D. C. Grahame, Z. Elektrochem., 62 (1958) 264. 22 D. C. Grahame, J. Amer. Chem. Soc., 80 (1958) 4201. 23 R. Payne, Trans. Faraday Soc., 64 (1968) 1638. 24 S. Levine, J. Colloid Interfacial Sci., 37 (1971) 619, 25 A. N. Frumkin and B. B. Darnaskin in J. O'M. Bockris and B. E. Conway, (Eds.), Modern Aspects of

Electrochemistry, Vol. 3, Butterworths, London, 1964, p. 149. 26 C. V. d'Alkaine, E. R. Gonzalez and R. Parsons, J. Electroanal. Chem., 32 (1971) 57. 27 R. de Levie, J. Electrochem. Soc., 118 (1971) 185C.