Problems in the polarography of chromate

ELECTRQANALYTICr

Elsevier Publishing

PROBLEMS

CHEMISTRY

Company.

IN THE

AND

Amsterdam

INTERFACLAL

POLAROGRAPHY

PoZynrcr L)epavtmnZ.

~Veizmcmn InstiluLe

(Received November

14th.

ELECTROCREhlISTRY

- Printed in The Netherlands

of Science.

49

OF CHROMATE

Rehovot

(Israel)

1966)

INTRODUCTION

The first systematic studies of the polarography of chromate as a function of pH were made by LIKGANE AND KOLTHOFF I_ They found four distinct waves with half-waves in the potential regions : I--o,35 V ; II -1.0 V; III-x.5 V; IV-x-7 V relative to the normd( calomel electrode (NCE), at pH = 7- In region IV, the chromate is reduced by six electrons to metallic chromium, whereas in regions II and III, three and four electrons, respectively, take part in reducing the chromate to the trivalent or bivalent chromium ion. As regards region I, where the diffusion current is not proportional to the chromate concentration, these authors assumed the formation of insoluble layers of chromium hydroxide which impede the current, and discarded previous assumptions of the possibility of intermediate degrees of reduction. Their results were in accordance with the theoretical linear increase in diffusion current as a function of concentration in the presence of 0.1 and 1.0 N -0.7 V relative to NCE, respectively. They NaOH at potentials above -0.8 and assumed that the reduction product under these conditions is the more soluble anionic form of trivalent chromium, C~OZ-. The conclusions of KOLTHOFF _~ND LINGAXE were criticized by GIERST d aJ.2-9 on three main grounds. I. These authors claimed that the theoretical diffusion current cannot be achieved even at extremely low chromate concentration. If an insoluble product were responsible for the reduction in the diffusion current, such a result would however be expected. 2. The reduction in diffusion current is even more pronounced at all&line pH than it is in neutral solution, if the ionic strength is sufficiently low. The increase in NaOH concentration is equivalent to an increase in the ionic strength; both shift the pri+zcipuJ wane. the term coined by GTERST et al. for wave II, towar more positive potentials. 3_ The existence of an insulating layer on the mercury surface is not indicated by typicalirregularitiesincurrentosciilat;i~ns,~is usuallyobservedinsimila.rcases. The first claim is substantiated only in the case of strongly alkaline solution. At pH 7, as will be shown later (f?i.g. z), the current in region I becomes equal to that in region 11 at a chromate concentration of 2.10 -5 M or lower- In the presence of 0.01 N Na&ZOs_to.~ N NaX03, the current in region I does not reach the theoretical value even when the chromate concentration is as low as 0.2 x IO-~ M; the ratio of L current to concentration, however, incre&e-s ~4th decreasing chromate concentration, J. ElectroanaZ. Chum.; 15 (1967) +g--6o

I.

50 The second any

possible

argument adsorption

is essentially

correct.

of chromium

k = ko e?:p(ar%F/RT) i - 4 - (Z&z

The

in any

estimate

form,

of the potential

in the

F

’ -

R_ MILLER

Wd neglecting

relation4*5

(1)

- r)?ppd]

is, however, questionable. 1n this relation: k is the rate constant of the electrode process; ko is the rate constant when the value of the exponent is zero ; is the transfer coefficient; a is the number of electrons participating in the rate-controlling step; z _ is the P o 1arization potential of the electrode (referred to, for convenience, ZA ye

potential difference from the electrocapillary maximum) ; is the charge of the depolarizer; is the potential prevailing at a distance, 6, from the surface at which reduction occurs. The third argument which refers to the shape of the current-time

as the

the electrocurves,

is

very u-eak, as it neglects the consideration that the dependence of the current on the g-row&g thickness of the insoluble layer and on its charge may vary in different cases. The object of the present study is to investigate the existence of an insoluble layer at the electrode surface by solubilization experiments. If the current is impeded by an insoluble layer, it should be possible to restore the theoretical diffusion current by solubilizing this layer. In principle, a subsequent reaction is being considered. A preceding reaction, Cr042- + HCrOd-, postulated by GREENS AND WALKLEY~ may exist, but should contribute only negligibly to the amplitude of the “prewave”_ The significance of the part played by any preceding reaction, Cr042- +HCrOa-, in reducing the current in the pre-wave is considered doubtful for three reasons: I. Addition of EDT-4 increases the current amplitude of the prewave even at alkaline pH. where its only action is solubihzation of the insoluble chromium layer by complex formation_ 2. Polyelectrolytes at minute concentrations that could not possibly affect this reaction have, when adsorbed on the surface, a very marked effect on the measured currents. 3_ The peak on the prewave, which aever exceeds the value of the theoretical diffusion current, cannot be explained by postulating a preceding reaction without assuming increased reaction rates when the surface active chromate is adsorbed_ It is, however, iu keeping with the concept of an insoluble layer being displaced from the surface by the chromate ion.

The polarographic measurements were carried out using a Shimadzu RPz polarograph. Large drop-times ( - 15 set) were employed in order to better record the events occurring on the mercury surface. Sodium chromate, the supporting electrolytes, and EDTA employed in these experiments were of analytical grade. In part of the experiments, the following deoxyribonucleic acid (DNA), was polyelectrolytes were employed : Calf thymus purchased from Worthington Co. and degraded by sonication The preparation of copolymer of +vinylpyridine with methacrylic acid (VPMA) at a monomolar ratio

POLAROGRAPHY

OF

51

CHROMATE

of 4 : 6 and of poly-4-.vinylethylpyridonium bromide (PVEPB), is described elsewhere7. The copolymer of lysine with glutamic acid (LGA), at a monomolar ratio of - 1-5 : I (L to GA) determined

was

obtained

from

in a Spinco

the Biophysics

Model

cmssec-1; were DDxA = 9-2 x 10-8 4-g x 10-7 cm?sec-1. cm%ec-1; DLCX = carried out electrolytes-

in neutral

medium

Department.

The

E ultracentrifuge

for

diffusion

using a synthetic cm%ec-1: DVpJrA = 3.25 x 10-7 The

DNA,

diffusion and

coefficient

in alkaline

coefficients

boundary

Dp-pB

determinations

media

for the

other

cella.

=

10-7

were poly-

RESULTS

PoZarogvaplzy of chromate at ~~eutraZ PH i~t ambuffered solution The half-wave potential of the prewave was found to be independent of the concentration of NaN03 which semed as supporting electrolyte (Fig. I). The peak decreased as a function of ionic strength, until at o 5 N NaN03 it vanished. It seems that even the weakly surface-active N03at high concentraticn interferes with the

Fig. I. Polarograms electrolyte. (-4j 0.6.

of chromate in unbuffered (B) 0.3. (C) 0.1, (D) 0.03

soln_ N

at pH

6.5

for different

concns.

of supporting

Na.NOs.

Fig. 2. The ratio of the current tothe theoretid diffusion current (iI&) non-bufferedsoLn.Conm.of ch.romate:(~),z -IO-~;(Z) ~-xo--C;(3),z-ro-~;

against potentnialinneutr,~

J. Etecfroand.

(+),s-IO-~;(J),zo-"

.W.

Cirevn., rg (1967) &N%I

I. R. _MLCLER

52

specific adsorption of chromate_ The salt concentration has, at most, only a negligible to the behavior at effect on the amplitude of the prewave, in- contradistinction alkaline pH. If there is an insoluble or adsorbed layer of the products of the electrode process, it does not can-y sufficient charge at neutral pH to affect appreciably the trvlsport of ions. There is, however, a shift with salt concentration in the half-wave potential of the principal wave. It is about 30% smaller than that which occurs at layer at neutral pH. It is pH 12. indicatm g a lower charge density on the surface difficult to decide conclusively whether the half-wave potential shift is ascribable only to the effect of salt on the potential of the diffused double layer or also to screening of the charges of the chromate ion. The marked specific effect of the cations suggests, however, that the second mechanism may be of importance. The ratio, i/i&, (Fig. 2) decreases with increasing chromate concentration, the effect being most pronounced in the minimum current region of the “prewave”. At chromate concentrations lower than 2.10-s M, the “prewave” merges with the prin-

Fig_ 3_ Effect of EDT-4 COIXXI_ of added EDTA

on the polarograms of 5 -10-a &I chromate in non-buffered (at pH 64 : (a), o; (b), 5 -x0-4 M; (c), 3 -IO-J M.

0.x N Na.NOa.

cipal wave, the peak disappears, and the polarogram levels off _ The addition of EDTA, which forms compIcxes with Cr s+ and has its maximal buffering capacity at pH 6.~,~ may &se solubilization of the insoluble film of chromium -hydroxide if any has formed. Solnbilization may be achieved by lowering the pH-in the surface as

POLAROGWHY

OF

53

CHROMATE

well as by direct binding of Cra+ by EDTA. The rise of current in the “prewave” until the diffusion current values of the principal wave are reached (as shown in Fig. 3) can be brought about by this solubilization process. At a chromate concentration of 5 - IO-* M or higher, Cr-EDTA, which is the product of the surface reaction, tends to become adsorbed at the surface and hence affect the shape of the current-time curves. Cr-EDTA is not adsorbed on a pure mercury surface at these polarization and concentrations either at neutral or alkaline pH. At alkaline pH, no adsorption phenomena are observed even when the electroreduction of chromate is carried out in the presence of much higher chromate concentrations. The conclusion is inevitable, therefore, that Cr-EDTA is adsorbed on a surface which differs in its properties from the pure mercury surface and that the [OH-] ions, which are a product of the electrode process:

HCrOd- = 3 H~0+3

e -_) Cr(OH)3+4

OH-

(2)

are not responsible for the enhancement of adsorption of Cr-EDTA at higher concentrations of chromate _ Another effect of addition of EDTA on the polarographic behavior of chromate is observed in the potential region between o and -0.3 V relative to NCE. Anew wave appears in this region, evidently caused by a preceding reaction.

Fig.

4. Effect EDTA;

(r), no

Fig.

5_ i/id

for

of

EDTA

an

(2). ~-IO-~;

5

the (3).

polaropm 1-5 x ro-=;

- 10-4 M chromate

Supporting electrolytes: ( 0),

o-r

at N

-0.85

NaNOs;

of 5 - IO-~ lkf (5). 3.3 X IO-*

chromate at pH M

r r a.rd o-r N NaNOa :

EDTA.

V relativeto NCE (A). I N NaNOs.

against

COIUXI-

of

added

EDTA.

At alkalirre pH, EDTA does not have a buffering effect and can influence the polarographic current only via its effect on the adsorbed layer on the mercury surface. Figure 4 shows the augmentation of the polarographic current on addition of EDTA at pH x1-5. Also, EDTA c&.rses a shift in the half-wave potential of the prewave‘ toward more positive values, presum ably by lowering the equilibrium concentration J. ElectroanJ.

Chew..

15 (1967)

49-60

I.

54

R.

MILLER

of EDTA required to bring about of free Cra+ near the surface. The concentration the same increase in current is considerably larger in alkaline than in neutral solutions. EDTA is also more effective at higher ionic strength (Fig_ 5) which is in accordance with the concept of interaction between a negatively charged molecule and a negatively charged surface. This interaction results in the removal of negatively charged substance from the surface. In any other situation, e.g., adsorption of EDTA, further reduction rather than an augmentation of the polarographic current should be expected. It can be seen in Fig. 3 that the plots of (i/id) US_ concentration of added EDTA are of sigmoidal shape. The augmentation of the current due to the addition of EDTA is weak below a certain concentration and increases more rapidly with EDTA from EDTA concentrations of about 1-2 x 10-p and 1-5 x x0-2&f for I and 0.x .V NaNOa, respectively. Probably the EDTA d oes not cause any appreciable increase in the current until it reduces the surface charge by solubilization to a significant extent. It should be noted that at alkaline pH values the addition of EDTA does not V as might be expected (Fig. 4)_ cause the appearance of the wzve at -0.05 Effect 0faddedpoZ_yelectro2ytes

on the ccpezzave"

The polyelectrolytes used in these experiments are surface active to a varied degree. They affect the polarographic currents only if adsorbed_ Native DNA is very weakly surface active, but its adsorption on the surface is considerably enhanced by denaturation. At neutral pH, native DNA affects the amplitude of the prewave as well as the current-time relation only at chromate concentrations above IO-” M. At high chromate concentrations, it affects the current even in the presence of excess EDTA, which has a buffering activity at neutral pH. This excludes the possibility that the (C)H)ions evolved in the electrode process cause denaturation of DNA prior to its adsorption_ The adsorbability of all the negatively charged polyelectrolytes employed in these experiments increased with ionic strength; at ionic strength lower than I= 0.01, no effect of the negatively charged polyelectrolytes on the polarographic current could be detected. In Fig. 6, the relation between the adsorption of denatured DNA at neutral pH and its effect on the instantaneous current, is demonstrated in the current-time curves. At brief times corresponding to amall surface concentrations, the adsorbed DN_4 augments the instantaneous current, which reaches its maximum value when about two-thirds of the surface is covered by DNA, as estimated from the amount adsorbed, r, calculated by the relation10 r = 0.743 D’/2W”CnrL*

(3)

After the maximum value is attained, the impeding effect of the adsorbed DNA becomes noticeable. Only after full saturation, when polymoleculti adsorption may be reachedrr, does the instantaneous current in the presence of DNA assume values that are lower than in the presence of the same concentration of chromate without at any pH, or any additives_ However, if the current is raised by adding EDTA buffer at pH values below pH 7, the subsequent addition of DNA always reduced the current. We suggest, a.s the most plausible interpretation of these results, that the adsorbed DNA molecules bind the chromium ions in their domain, leaving the depleted holes of low resistance in the. chromium hydroxide layer. It should be borne in tid that DNA has a very low buffering capacity below pH 8.5 and cannot

POLAROGRAPHY

OF

CHROMATE

55

cause an increase in the diffusion current by neutralization of the (OH)- evolved in the electrode process. Moreover, the evolved (OH)- is about fifty times the equivalent of the adsorbed DNA. Thus unlike EDTA, DNA does not affect the current by a precedent reaction at potentials positive relative to the prewave. The effect of the adsorbed polyelectrolytes is even more pronounced at alkaline pH. It seems surprising that adsorption of negatively charged polyelectrolytes greatly enhances transport of the depolarizer across the surface layer (Fig. 7)_ It is difficult JA

iD t

Fig.

6. Current-time

soln. and various mequiv.!ml.

for 5 -10-a M chromate in the presence of noa-buffered 0.1 N KNO3 of added DN_4: (I), no DNA; (2) x-53 x 10-0 mequiv./ml; (3). 0.76~ 10-4

curves

concns.

Fig. 7_ Current-time cures for 5. x0-4 M 0.03 N Na&Ox and different negatively 0.3 mg/ml LGA; (3). 0.25 mg/ml VPMA.

chromate at -0.9 V relative to NCE in the presence of charged polyelectrolytes: (I), no polyelectrolyte; (2).

to determin e whether the adsorbed polyanion acts here by binding the chromium or by locally increasing the ionic strength near the surface. From the full polarograrns it seems, however, that the first mechanism is the more likely, as the adsorbed polyanions do not shift the principal wave as would be expected from the variation in ionic stiength3. Polycations increase the amplitude of the prewave at even lower surface concentrations than do polyanions. In the case of PVEPB. the full diffusion current at drop-times greater than IO set is only recovered at about half-saturation of the surface, while at shorter drop-times, the smaller surface concentrations of PVPEB attained10 suffice for frill current recovery (Fig. 8)_ In this case, inversion by the adsorbed cation of the surface charge on the surface layer, which is composed to a large extent of CrO2-, seems to play a predominant role in the enhancement of the chromate transport to the electrode However, the possibility of bindi& of CrOzby the polycation and hence hole formation in the surface layer cannot be excluded. J- Eleztroond.

Chem:,

15 (1967)

qg-60

T.

56

R.

SfILLER

>A 25-

ZP -

Is-

1.0 -

_Fig.

8_ Cm-rent-time curvesat -0-g V relativeto?SCE for5*Io-41%2 chromateinthepresence of 0.x iV I-iazCOa andva.riousconcns.ofquatemizedpoly-q-vinylpyridine (PVEPB). (x),noPVEPB; (a), 2.5 X 10-J; (3), 5*10-s: (4). 7-5 x 10-5; (5). 10-4; (6). x.75 x IO-~; (7), 3 -10-a eq/l PVEPB. DISCUSSION

I_

of fh.e am~Zitzd&

Defimdew

on the pemave

in

neutraZ

soZutions

effect on the diffkion current of a thin layer of constant thickness near the surface has been discussed previouslyl”. The concentration ratio of the depolarizer on each side of the boundary between the thin layer and the bulk was given by the distribution coefficient. K. and the difhrsion coefficients in the thin layer and iu the bulk were & and Dr, respectively. In the present case, we introduce the additional boundary condition that the thickness of the layer is not constant, but increases with the amount of reduction product deposited on =be electrode, namely: The

da -=&

v3’t

5d.N At

dt

=

-

(41

AtnF

where 6 is the thickness of the dyer at time t, _N is the number of moles of depolarizer crossing the surface and Y’ is the partial molar volume of the product in the layer_ At is the surface area at time t. Equation (13) of ref, 12 assumes. with this boundary condition, the following form: . sc =_7_o8 x zo%m

s/3tr/=Drl/zBr

= TLF,T&

-where e ti +$e rate of flow of mercury in g/set, tt is-the number ating i_n the reduction process, F is a Faraday unit. xx = G&r J. E~~rc%nwZ.

-h {(6-S0)/K&]

~km.,i-I~

(~67)

49-60

(&/2x$ “‘]

6)

of elections

particip(6)

OF

POLAROGlUPHiT

CHROMATE

57

where Cc0 is the concentration of the depolarizer in the bulk of the solution and 80 is the thickness of a part of the surface layer which is fully permeable to the depolarizer_ Introdu_cing the value of d in eqn_ (6) from eqn. (4) and insefiing into eqn. (5). we obtain for the instantaneous current:

However, the numerator in eqn. impending barrier. Hence: (id/it)

= I f

(7) is the ideal d_iffuSon current,

,)I%

(B/nFA)

ito, without

any

cSo/fil

(8)

where j3 = ( DI~zi!)l'~(~/KDn) Fig. g. (id/&) is plotted above

(id/it)

t.

against

r-5. The value of

I

0

A straight

idt/nFA.

t I

i&fnFA

line is obtained

the intercept

at

at values

of the straight

of

line with

the line (&/it) = I, corresponds :o (&/a). From here (So/c) = z - 10-9, which means that for crystalline chrome oxide where 6 per chromium atom is 15 cm3, 80=3 ft, whereas for a highly hydrated chromium hydroxide when 5 = IOO, 60 would be 20 A.

I

0

1

,

I

2

1.

6

I

6

10

,'d@..=d d

Fig. g- The (is/i)again&

ratio of the theoretical instantaneous yO idf/nFA.

diffusion current

to the

measured

current

The value of /3 obtained from the slope of the straight line is 8 - 108 from which the value obtained for the product (fl/~Dn) is about 2 -IO la_ It is impossible to separate the parameters of their product. -In this treatment, the effect of the surface charge of +he thin layer- on the current has not been considered. However, the values of K, and therefore also of #?, depend on the electrical poteutial, which is likely to increase with the thickness of the lay& formed due to increased alkalinity neartpe surface brought about by-the electrode process -(equ. -2). As K is expeCted to decrease with increasing negative i J-‘EZeboysaLCiicm.,

x5.(1967)

4g-60

I.

58 potential of the surface layer, the lower values of bations of chromate are qualitatively reasonable. 2.

SolubiEization

of the &soluble

film

j3

by a compZexing

obtained

R.

MILLER

at the higher concen-

agent buffering

Consider first the solubilizing effect due to buffering action alone. In t-his model, we assume that the equilibria (eqns. g and IO) near the surface are instantaneously established and that the diffusion of the reactants to the surface and away from it are rate-determining in the dissolution process. [Cr3*] [OH-]3

= K1

[OH-I CBuf El =

(9)

K

(10)

2

L’BUf-1

An insoluble film is formed if the rate of film formation is larger than its dissolution. If the rates are diffusion-controlled, assuming equal diffusion coefficients for chromate and chromium hydra_xide, an insoluble film is formed when (Cm -Ca) 7 [Cr3’] dFor Ki = 0.5 x x0-30 13, insoluble films should thus be formed in well-buffered x0-3 M chromate solutions above pH 5_ In fact, formation of insoluble films is not observed up to pH 7_ In carbonate6 or phosphate buffer, and at a chromate concentration of 5 - Io-4 M, (i/id) in the “prewave” is less than I at pH-values above 7_ It is possible, however, that even below this pH an insoluble film is formed which is swollen or leaky enough to be permeable to ions. The higher negative charge on the insoluble film at the higher pH values may also contribute to its lower permeability to chromate. Under these circumstances, no simple model is likely to explain quantitatively the solubilizmg action of buffering substances_ Solubilization by complexation alone will occur if the complexing agent has no buffering activity, or a buffering activity only in the pH region in which the film is in any event insoluble. The rate of formation of the film uuder these conditions is

(1x1 where k is the complexation rate near the surface and the index, y, indicates the complexing agent. The completing agent diffuses to the surface, forms a complex at a rate KCy,a and the complex formed then diffuses back into the solution. This is equivalent to a totally irreversible polarographic process which has been dealt with by many authors_ According to DEL-~=Y~~ C,,& = Cy” exp (kit/D,)

erfc (kP/2fD,1/2)

(12)

where D, is the diffusion coefficient of the complexing agent and Cyo is its concentration in bulk. For kTlf2/Dyl/2d I, eqn. (14) gives Cy,b +C,O. For addidonal boundary conditions, the value of BI in eqn. (5) becomes kCyoIt

BI =Co/[r+fl

(13)

A

-where It = J_- Ekctroand.

I

jxp Checrn.,

(k’tfDy)

erfc (kT1’~fD.“S)

15 (x967) 49-60

dt

(14)

POLAROGRAPHY OF CHROMATE The for

third the

term

59

in the denominator

dissolution

of the

insoluble

on the right-hand

side of eqn.

layer,

exceed

may

never

the

(13),

which

second

stands

term

which

stands for its formation. However, EDTA in non-buffered solution solubilizes only by its complexing action but also as a result of its buffering capacity, and calculated Cva is not proportional to Cyc as is required by eqn. (12) _ 3_ Evolzction The

of thf3 reduction

most

HCrOaThe rate obtained

likely +

H+

T

%

reaction

in tAe presence

of excess

batffeeuiq

agent

is:

H&r04

2

(15)

constants of the precedent reaction in the presence from the reduction in the instantaneous current:

i/id

of excess

buffer

can be

FC;C)

=

(16)

_4CCOrding to KOUTECK+l5.16, (K/[IILl

of N2C704

-wave

precedent

not the

for

a large

ratio

of dissociation

constant

K,

to

[H+],

B I)

k2 = (K”-%“-/[H]z)

(7/1at)

(=7)

The values of x are tabulated by KOUTECK+ as function of F(X). At pH-values between 7 and 8.2, in the presence of 0.04 N phosphate buffer and 0.1 N KN03 and with t = IO set, we obtained for x/m+] the value (7 23) .1010 cm3mol-I. For reaction (15) (K = 3. IO-~ mole cm-3)13, the rate constant of dissociation calculated from eqn. (17) is about 3 - 1013 cm set-i and kl e 1010 cmssec-i_ If the buffer concentration is of the order of that of the depolarrzer, as in the case illustrated in Fig. z, the rate constant of the precedent reaction can be obtained from the equations derived by &?EK et aZ.l7_ The reaction in this case is: HCrOa-

+

XH22-

&

kk:!

XHs-

+

H2Cr04

(a)

3e+ zH,O

i Cr(OH)3+3

2 XHz2-

+ 3 (OH)-

.+

and as can be seen, it includes Significamzc

of the principal

According

3 XHs-

+

a precedent

(ISI

OH-

3 Hz0

(b)

reactIon

(a) and a parallel reaction

(b).

wave

to our conclusions,

Cr014-

may

be reduced

in the

whole

potential

region starting at about -0-3 V relative to NCE. The principal wave (according to the term coined by GIERST et d-2.3) which is very sensitive to salt concentration, cannot be simply the reduction of CrOsz- as suggested by these authors. An alternative possibility is that the difference between the “prewave” and the “principle wave” is not related to the form of the depolarizer but to the location of the reduction process_ While in the “prewave”. CrO4”- and HCrOaare reduced at the mercury surface after penetration through the adsorbed layer, the reduction process of the principle wave takes place on the boundary between the adsorbed layer and the bulk solution_ The reduction rate at this boundary is a function of the potential on the- site of the J_ EZectroanaC.

Chem.,

15 (1967)

4g-60

I.

60

R.

MLLLER

depolarizer. This potential is a sum of the contributions of the surface layer and the charges of the depolarizer itself_ The ionic strength has an‘kffect on both components of the potential. The potential of the boundary is a faction of pH. The conductance of the surface layer as well as its porosity and surface potential can be modified by incorporation of organic surfactants and polyelectrol_ytes_ It is possible that the penetration of HCr04- through the layer to the surface is faster than that of CrOd*-_ In &is case the preceding reaction, CrOrz-+HCr04may be of importance. ACKNOVTLEDGEMEXTS

The help of Miss D. g-ratefizlly acknowledged.

BACH

and Mr. H.

GRAJZT in carrying

out this work is

The mechanism of electroreduction of chromate was investigated by applying complex@ agents and polyelectrolytes that bind trivalent chromium (the electroreduction product) thus affecting the current in the polaro_~aphic prewave. It was found that amounts of ad-sorbed polyelectrolytes so minu-c. as to be insufficient to affect any preceding reactions, caused an increase in the pokzagraphic current, until, under suitable conditions, the theoretical diffusion current could be recovered. Similar augmentation of the current was observed when agents that could solub&e the barely soluble layer possibly formed by the products of the electrode process, were added. It was concluded that the thin layer formed near the surface is responsible for reducing the polarographic current in the prewave. Its impeding effect is a function of its structure, thickness and charge_ REFERENCES I J_ J_ LZNGANE

AND I_ M. KoLTHoFF.~.A~.C~~~~.S~C..~~(1960) 852_ 2 IL. GIERST. ElectrcchemicaZ Society Sym$osium on EZecirode Processes. rgsg, Abstr- No. 176. 3 J-JTONDEUR,A.DOX-EIERTAND L_G=RsT,]. EZedroa~LCAcm.,3 (rg62) 225. 4 A. N. FRUXKIN.~. EZektr~chem-. 59 (1955) 807. 5 A. N_ FRUMFCIN. J_ Eledrochem. Sot., 107 (1966) 46x_ 6 J. W_ GSUZENS A.SD k WAL~~EY~ AurZraZiali /_ Chem.. 8 (1955) gr_ 7 D. BACH XUD I. R. MILUZR, Bioc&m_ SiOpkys_ Ackz, xs4 (rg66) 3x1. 8 E. Dmrrr. AND Z. ALEU,IUDROWICZ. Bio-poZymc7s. I (1963) 473. g A- I. VOGEL, A Twibod of Quantitative Inorganic Analysis, Longmans Green, London, 3rd ed..

rg6r. LO J_ K0FtYTA.~oZZe~%onCzeck_

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