Rate of hydrogen evolution on a liquid gallium electrode in partially miscible mixtures

. 1 ~

ELSEVIER

Ole

Journal of Electroanalytical Chemistry 436 (1997) 257-260

Preliminary note

Rate of hydrogen evolution on a liquid gallium electrode in partially miscible mixtures Kfilmfin Szab6 *, J6zsef Mika, Lfiszl6 Budai D,,l~artment ~!l'Physical Chemistry l~Mnd EiJtvi;s Uni,,ersity, P,O,B, 32, 1518, Budapest H-1117, Hungao,

,.,.... .... ,%]7., t~?~yivedit~ revised term 24 June 1997 Pe~,,i': ~,t 14 F~:,,,,,,~ap

Abstract Tile overpoteultal of Itydrogen on it liquid galliunt electrode hits been studied in the equilibrium phases ot' isobutanolowater~hydroo chloric acid systems. AccoMing to o u r investigations, the ~l-olgj curves measured in equilibrium ttqueous (W) and Ol'gltnic (O) phases practically coincide, This can probably be explained by the Ihet tlmt the rate of hydrogen evolution in the equilibrium phases studied is determined by the quasioindependence of the structure of W[Ga or O[Ga boundary layers, as well as of the correlation between the qJo potential and the acid concentration of the composition of phases. © 1997 Elsevier Science S.A. Eeywords: Gallium: Hydrogen overpotential; Partially miscible mixtures: Equilibrium plmses; lsobutanol-water-hydrochloric acid systems

!. Introduction The electrochemical reduction of hydrogen ions has been investigated in organic solvent-water mixtures of different compositions [I-3]. It has been established that, depending on the nature of the organic component and of the electrode metal, the hydrogen overpotcntial (~1) changes differently with the composition of the mixture. For exam° pie, in methanol-water-hydrochloric acid mixtures on copper [ I ] and on liquid gallium electrodes [2], the overpotential (at constant current density) passes through a maximum or minimum with the increase of the molar fraction of the organic component (X,,r~), while on mercury in acid mixtures, it changes in a much simpler way with X,,rt~ than in the case of Cu and Ga electrodes. The electrochemical reduction of other ions. i.e.. Zn 2. [4,5]. Cd ~+, V ~*, HPbO~* [6] in various organic solvent-water mixtures has been investigated on a mercury electrode. Considering literature data [I-7]. it seemed interesting for us to investigate the hydrogen evolution in partially miscible special mixtures like the aqueous (W) and organic phases (O) of equilibrium systems. Since it is well known that the contents of the organic component and of water in these

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o ~ and X ~., o * -r °a:o,. phases are different (x,,W~~# xor whereas the activities of water and organic components are identical in the aqueous t (W) and organic z (O) phases (a~:o = aH°:o and aor~w_ a,,r ~o ) it seemed to be interesting to study the hydrogen evolution in the case of equilibrium systems. We chose the isobutanol-water system because some electrochemical investigations had beet) performed in this system [7,8]. The gallium electrode was chosen because we have already performed numerous measurements on this electrode in both aqueous and nonaqueous solutions

[91. 2. Experimental Experiments have been performed in a set-up described in an earlier paper [10], at 32°C, Electrodes consisted of 99.9999% pure gallium, and they were prepared by the method described in paper [lO], Sohttions were prepared from suprapur hydrochloric acid by Merck, bidistilled

I Aqu~ou~ p l l ~ (W): in th~ mixt~r~ conlainin~ water° i~obuta,ol ~fid hydrochloric acid. the ph~¢ richer Jn water. Organic pha~e {0): in the ~am¢ mJy.ture, the pha~e tidier in i~obuo tat|ol.

K. Stab6 et al. / Journal of Electnmnalyticai Chemisto' 436 (1997) 257-260

258 Table I

Table 2

Water Hydr~x:hloric acid l~utanol

Aqueous phase (%) ( m / m ~;~)

Organic phase (0) (m/m ~)

83.56 5.68 8.97

3"1 ~,.60 2.18 75.21

At T =--.'t05 K.

water and bidistiiled isobutanol dried above CaSO4. Isobu. lanol-.water=hydrt~zhloric acid equilibrium pha~s were produced at 32°C in a doub!e-wall thermostatted separating funnel, and the appropriate phases were placed into the measuring cell. The composition of equilibrium phases was ~termined by dropping isobutanol from a microbuo rette to a hydn~zhloric acid solution of known concentrao lion and mass, placed in a titrating vessel and thermostat° ted to 32%~, until the mixture remained turbid even after multiple shaking. The saturation p~wameters of the organic pha~ were detected by adding an amount of hydrochloric acid of known mass and concentration to a given mass of alcohol, and then adding distilled water dropwi~ to the system until turbidity appe.;¢ed. The hydrochloric acid content of tb-" phases was choked by tim, ion. Measurements were carried out in aqueous and organic phases of the equilibrium systems with different hydrochloric acid contents. The HCI concentration in the aque~ ous phase (W) was !.76 M. whe~as in the organic phase (O), being in equilibrium with it, the HCI concentration was 0,58 M. _The composition of the phases of the equilitb rium system are summarized in Table I. The overpotential ~1 was measured relative to an equilibrium hydrogen dectr~le of identical coml~sition and teml~rature as those of

J /,Oi¢

Aqueous p h a ~ (W) aw bw a~ r oHcl

0.924 0.08 ! 9 0.73 0.9928 1.76 M

Orga.ic phase (O)

Sd,, Sd i,

0.0050 0.00165

SD

0.00557

ao bo ao r cuc ~

0.914 0.0802 0.75 0,9722 0,58 M

Sd,, Sd i,

0.0148 0.00432

SD

a (V) and b (V): Tal~i coustants. a: transfer ¢oeffici~:m. clic~ ¢on¢¢nlratirolls of HCi ill the mixtures (tool d m -~), Sd., Sd h and SD corrcspondi~g values of sta,dard deviations. r: cof'relalion co¢fficienL

the solution to ~ studied. Polarization curves were de° tected in the ~tentiodynamic mq.~le by a i~tenlioslal of the EF 450 l y ~ coupled online to an MC 86 W TI"L 14 computer, by using a sweep z~zte of 1~2 mV s =. The prognam allowed also the measurement of the ohmic I~)~ tential d~x)p.

3, Results Polarization curves constructed from the rl-lg j pairs measu~d in the aqueous and organic phases of investigated system are shown it) Fig, I, As it is seen, the rl-lg j Tafel straight lines determined in the ,~queous and organic pha~s of the syslem practically coincide. The parameters of the straighl lines are summarized in Table 2. It is obvious that t ~ a and b parameters of the Tafel straight lines measu~d in the aqueous and organic phases of the system coincide within the erpt~r limits of the experiment. The resistance in the organic phase is about 100 times higher, causing a ~duction in the accuracy of measure~ ments; standard deviation (Sd) values are higher, and ~gression values (r) are wor,~ (s~e Table 2). As seen in Fig, I, ~ l g j curves are pre,~nted in a narrower ra~tential interval for the organic pha.~e~ than for the aqueous ones, since in the organic phases the higher resistance of the mixture and the adhesion of hydrogen bubbles to the electrode surface impair the measurability at more negative potentials,

4. Discussion of results

(h6

=3,5

-2~5

~g(j~Acm )

Fig. l, ~l-lg j carves in the aqueous (XXX) and organic ( ~ ) phases of the i ~ l a n o l - ~ a l e r - h y ~ 4 r t ~ h i ~ i ¢ acid equilibrium x y l e m on a liquid galliura electrode.

The fact thai the +-Ig j curves measured in the aqueous (W) and organic (O) phases of equilibrium isobutanolwater-hydrochlorid acid systems p~actically coincide, indicates that the rate of hydrogen evolution in the case investigated in our equilibrium system does not depend on the X~e" as in the ca+ of other (not equilibrium) waterorganic solvent systems [!-3]. This phenomenon can be explained by assuming that the structures of W IGa and OlGa boundary layers may be similar, and these structures determine the rate of the electrode process. This assump-

K. Szabd et aL / Jmmtal of Electrmmalyth'al Chemist~" 436 ¢1997~ 2 5 7 - 2 6 ~ )

tion of similarity of boundary layers seems to be supportable by our earlier studies on the Hg electrode dipping into the equilibrium phases of the same system [8]. In equilibrium phases, the activities of water and isobutanol are identical in the aqueous and organic phases: (l::(w _: aO ,O and aWi-hut ¢/i-b,,,O• thus on the electrode suit:ace, coverage O is alsq identical for water and isobutanol in both the W[Ga and OlGa boundary layers. The activity of HCI in the equilibrium phases of the isobutanol-water-hydrochioric acid system is not the ° [8], but the chemical potentials of HC! same: a HwC I # : cq,tct in the two phases are necessarily identical btWcl= /'&dCl' o The mixture/air and mixture/Hg surface tension measuremeats show that the structure of the boundary layer can be imagined in both the aqueous (W) and organic (O) phases so that both the W IHg and the O}H8 boundary layers are saturated with isobutanol: O~,~,,, is maximum. With the change of HCI activity, the amount of both isobutanol and water remains constant in both boundary layers, since the surface tension is practically klentk'al in both phases of the equilibrium system independently of their composition (amount of isobutanol, water and hydrochloric acid) [8], The Ep,~ values in water-alcohol mixtures are determined by the dielectric parameters (relative pennittivity, dipole moment) of the species in the mixture/metal boundary phase, together with their amounts and coverage [11]. In the case studied, in the equilibrium aqueous phase/metal and organic phase/metal boundary layers. the coverage for the organic material, isobutanol, O..,,. is identical and maximum, as it has been shown earlier [8]. in the isobutanol-water system, or in its two equilibrium phases, /:'.,,~, is determined only by Oi..,,,:oiW,,,=--i-..,. . W ~. ~O i ' , /:~t,. ~/:t,~¢ thus is also valid. Accepting the validity of experimental i~su!ts and their interpretation shown for mixture/Hg also for the mixture/Ga boundary layers, in the case of the equilibo rium systems studied, in the (W)/Ga aqueous and (O)/Ga organic boun&ry phases, the identity of the kinetic parameters of hydrogen evolution can b~ explained by the same or at least very similar structure of the (W)IGa aqueous and (O)1¢3a organic boundary layers. The rate of hydrogen evolution, in turn, is determined by the structure of the boundary layer, i.e.. inner and outer parts of the electric double-layer. Our calculations from literature data [12,13] show that the Gibbs energy of adsorption lbr Br- ions at the pzc on a gallium electrode is much smaller than for the molecules of butanol. Since the adsorption energy tbr Cl- ions is even smaller than for the B r ions [13], we suppose that the coverage of CI ~ ions in the case of isobutanol adsorption is rather low: Oct < I. In our opinion, the water and isobutanol molecules occupy different positions in the boundary layer in such a way that the water molecules are situated above the isobutanol molecules, similarly to the results found for pentanol and hexanol adsorption [14]. This is why the structure of the dense part of the double ~--"

259

layers in the both phases is mainly determined by the arrangement of isobutanol molecules, Experimental results show that in the aqueous (W) and organic (O) phases of this equilibrium system, where the concentrations and activities of HCi are different, the values of ~7 differ only by 4-5 mV in the current density interval studied (see Table 2), indicating that the value of overpotential (at j = constant) for mixtures seems to be independent of the acid concentration, similarly to aqueous acid solutions without any foreign electrolyte [15]. We suppose that this phenomenon can also be explained, as it is known, on the basis of the Tafel equation and by the ql,,-acid concentration (c,,~.i,l) correlation °~ [8]. The Tafel equation can he given for the aqueous phase (W) as: RT

I - ~ RT

flw '~ a'w + ~

In j

t:~

F in cw lit3

I + ~

tit

(la) The same for the organic phase is: RT

I oo tv R T

~/.. ~ d,, + ~ l n j

a

F In ' '".c, +

I ~o tv

- tl'o"

(In)

On the other hand, for electrolytes with c total concentration in the aqueous pha:~e: RT

%w =/~ + ..7.1 n ,.w

(2a)

in the organic phase: RT

11,o = l~ + " 7 In ."°

(2b)

Here we suppose that constant B is the same in both the aqueous and organic phases.In the cas~ studied, c w ~ cw HUt and t '~ ~ t ' t l ~ t , l . ~ r o l n Eqs. (]a). (2a). (Ib) and (210 follows RT

~w ~ "w + ~ l n

j

(3a)

and RT

71. = "o + ~

In j

( 3b )

Since ao = aw (see Table 2), according to Eqs. (3a) and (3b), at j ~ constant, the value of 7/is independent of the acid concentration, i.e., of the composition of the aqueous (W) and organic (O) phases of the equilibrium system, i.e., 77w = % . with an accuracy of 4=5 inV, This indicates that besides the structures ot' the boundary layers WIGa and OlGa in the aqueous (W) and organic (O) phases of the equilibrium mixture, the structures of the bulk phases are also identical, or at least ~imilar, This is

t¢ ii~1

~()n liquid gallium el¢ctrode~ i. ~.]ueou~ IICI ~,olution~ [ ~ J ,

~0

even ralllcr dome to the pzc [16] +how+ tile validity ot' lh+ futtclion +IG~ +~, Ig c,,.id. Thai is why we mUpl~)Sethat -~lS. (2a) mild (2b) are alma valid i. the case o1"our present investigation.

200

K. Szabt~et aL / Journal of Electroanalytical Chemistr)."436 (1997) 257-260

why c o n s ~ t B in Eqso {2a) and (2b) is independent of the composition of the mixture. On ~ ~ i s of the former, it seems probable that the ra~ of hydrogen evolution in the equilibrium phases studled is determined by the quasi-independence of the structure of WlGa and OiGa boundary layers, and of the correlation between ¢,~ and c~,d of the composition of aqueous (W) and organic (O) phases. experimental results allow for the conclusion that mixtures in equili~um can be considered as simple, com~lvents, some properties of which, such as the st~ctu~ of the mixture/air and mixture/metal boundary layers, the con~.~ntrafion deper,dence of q'o and. consequrnl~, the kinetic:s of hydrogen evolution and the poten~ ti~ dependenceof the ~action rate are identical or at least ~imila~'. At the s~ame time. the electric c~~ducfivity of the equilibrium phases differ to a great extent (thai of the W pha~ is !00 times higher than that of the 0 phase),

Aeknowledgemenrs This work was carried out with the help of the OTKA grant I/7o014849.

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[I i] P. Nikitas, A. P~pado~misi, J. FJeclr~mal. Cheln. 385 (199~).~57. [I 2] W,R, Fawc~ll, AJ, M¢~heo, I~leclrochlm, Ac|a ~f~ (I~)1) 1971, [13] I.A+ Bagt~:sk~ya, BB+ Dam~sktn, V,~+ Ka~arinov+ I~lek|mkhimlya [14] S. Romanow~ki. K. Maksymiuk+ Z, GMum,J, I~leclnmnml+Clam, 38~ [I ~] B,B. D'amaskin, O,A. Pelt|i, Osnovy Te~weliche.~kojI~leklmkhln|it, Mo~ow. 1078, p, I~}? [16] St,G. Christov. S, R~ireva, 7~, I~lekm~'hcm, 66 (P~2) 484,