Potential sweep voltammetries of Na(2)O·SiO(2) and Na(2)O·2SiO(2) at 730-1200°C were performed with the stationary platinum wire electrode and with the sweep rates of 40 and 120mV/sec. Although current-potential relation of Na(2)O . SiO(2) at 1200°C gave stationary S-shape pattern, those of the rest were transient modes with hysteresis. From the voltammetric considerations, a reversible oxygen electrode process where diffuion of free oxygen anion or silicate anion may be a rate-determining stage was tentatively proposed. Approximate estimations of decomposition voltage supported that a sole reaction process such as the decomposition of Na(2)O or indirect decomposition of
SiO(2) in the melt was most probable

Hirai, Taketsugu

Takahashi, Katsuaki

Okayama University Digital Information Repository

Potential Sweep Voltammetry of Na(2)O-SiO(2) Glass Melt by Stationary Platinum Electrodes

Okayama University Scientific Achievement Repository

MEMOIRS OF THE SCHOOL OF ENGINEERING, OKAYAMA UNIVERSITY, Vol. 3, 1'\0. I, SEPTEMBER, 1968Potential Sweep Voltammetry of Na20-Si02 GlassMelt by Stationary Platinum Electrodes*Taketsugu HIRAI, Katsuaki T AKAHASHI**Department of Synthetics Chemistry(Received March 15, 1968)Potential sweep voltammetries of Na20· Si02 and 1'\a20·2Si02 at 730~1200°Cwere rerformed with the stationary platinum wire electrode and with thesweep rates of 40 and 12C mV/sec. Although current-potential relation ofNa20 . Si02 at 1200°C gave stationary S-shape pattern, those of the restwere transient modes with hysteresis. From the voltammetric considerations,a reversible oxygen electrode process where diffuion of free oxygen anion orsilicate anion may be a rate-determining stage was tentatively proposed.Approximate estimations of decomposition voltage supported that a sole reac-tion process such as the decomposition of Na20 or indirect decomposition ofSi02 in the melt was most probable.§ 1. IntroductionVoltammetry, in which current (i)-potential(E) relations during electrolysis are deter-mined, is an essential and fundamental pro-cedure in electrochemical and electroanaly-tical studies. Because both stationary (timeindependent) and transient (time dependent)voltammetric techniques can be applicable toan electrolytic cell where mass transfer modeis controlled by sole or mixed process of diffu-sion, migration, convection, charge transferor adsorption, the discussion of voltammetryrequires a clear and quantitative understand-ing of mass transfer process. Well-knownpolarography, which is a form of stationaryvoltammetry with dropping mercury electrode,is based on the fact that, during electrolysis,the limiting current depends on the concentra-tion of the solution. In accordance with theimprovement in fused salt polarography usingsolid electrode, several polarographic investi-gations on various transition metals in moltensilicates as the supporting electrolyte have* Presented at the Chugoku-Shikoku Meeting ofthe Chemical Society of Japan, Okayama, October11, 1967.** Department of Industrial Chemistry, School ofEngineering, Okayama Universitybeen performed by Lyalikov,l) Delimarskii2)and Baak,3) and made clear the concentrationsand valence states of metal oxides in the melt.On the contrary, recently quasi-steady stateand transient voltammetries with linearlychanging potential, namely "potential sweepmethod" have been highly developed in res-ponse to the remarkable improvement in ele-ctronic devices for electrochemical studies.This method is characterized by a wide availa-bility and high sensitibity for some kinds ofmass transfer modes under the adequate ex-perimental conditions. On the other side, ourpreceding work4) concerning decompositionvoltage of Na20-Si02 system by commutatortechnique indidated that observed values con-tained some irreversible polarization such asconcentration polarization in addition to re-versible electromotive force. Therefore, vol-tammetry of alkalisilicate itself, in which cur-rent is carried completely by alkali cations,would be expected to be available for investi-gations on the diffusive behaviors of freeoxygen anions or silicate anions and furtheron electrode processes of sodium cations in themelt. The present paper contains the result ofseveral voltammetries of both sodium mono-silicates and disilicates with stationary pla-tinum wire electrode, and with comparatively101102 T. HIRAI and K. TAKAHASHI (Vol. 3,Fig. 1 Schematic representation of the measuring apparatusslow potential sweep such as the rates used inconventional polarography.§ 2. Experhnental procedurea) Sanlple glass used Na20· Si02 andNa20 ·2Si02 sample glass was prepared fromsodium carbonate (chemical reagent grade)and silica sand (99.896 purity). An uniformmixture of dry reagents was melted at 1200°Cuntil well fined, quenched in air on the stain-less steel plate, and crushed.b) Apparatus and procedure i-E curveswere determined using both conventionalpotentiostat and auxiliary electronic equip-ments of ultra-low frequency triangular wavegenerator, cathode-ray oscilloscope and ele-ctronic voltage recorder. In Fg. I is shown theschematic arrangement of these equipments,electrolytic cell and furnace. In the figure,electrolytic cell consists of platinum crucibleas container, sample glass melt as electrolyteand three same smooth platinum wires (0.4mm in diameter) designated as test electrode(T), counter electrode (C) and reference ele-ctrode (R) respectively. These three platinumF[mffiCT: Test electrodeR·: Reference electrodeC; Counter electrodeElectrode configurationTC: ThermocoupleV: Precise milli\"oltmeterCRO: Cathode·ray oscilloscopewires, covered with fine ceramic tubes separa-telyexcept each end of about I cm in length,were inserted into a larger ceramic tube andfixed tightly with alumina cement adhesives.Electrode configuration was shown schemati-cally in Fig. I, where ohmic loss between T andR electrodes is intended to be minimized. HereR electrode is expected to act as an oxygenelectrode. Potentiostat with current capacityof 150 mA supplies extermally programmedpotential across T and R electrodes. Outputof ultra-low frequency triangular wave genera-tor with frequency range of 10-3 -103c/s iselectrically connected with auxiliary terminalsnamed externally setting terminals on thepotentiostat through the manual change-overswitch. Cathode-ray oscilloscope, conventionaltype synchroscope, is used to monitor thewave form and to set a time when externallyprogrammed triangular wave is introduced topotentiostat. Electronic voltage recorder givesthe electrolytic current through T and C ele-ctrodes.The order of experimental procedure is asfollows: Platinum crucible containing samplecullet is heated in the furnace to 1200°Cor more where no volatilization substan-tially occurs, followed by inserting as-sembled platinum wire electrodes verti-cally into the melt to a certain depth,determined experimentally as mentionedbelow, along the vertical steel guideattached to the furnace wall. After thecell is allowed to cool to a given tem-perature, it is checked that the spon-taneous potential across T and R ele-ctrodes is to be essentially zero bymanipulating the knob of the rest poten-tial reading. Then monitoring the trian-gular wave form on the Braun tube ofthe oscilloscope, half cycle of the wavehaving given frequency and magnitudein peak-to-peak voltage was introducedto the potentiostat by the timely change-s: I.lanual change·over switchover of the switch. The potential sweeprates of 40 and 120 m V jsec used in thisR: Electronic voltagc recorder experiment, corresponded to the opera-tion in the frequency of 0.0033 and 0.0 Icis respectively for the same peak-to-peak voltage of 6V. At the same time,time dependence of electrolytic currentthrough T and C electrodes was observedby the recorder. Because elapsed time isRCROPS,- ---.,I 15, I1._ _ Jtriangular wave generatorPC: Platinum cruciblePS: PotentiostatU: Ultra·low frequencyC: Ceramic tuheG: Glass meltE: Platinum wire electrode1968) Voltammetry of Na20.SiOz Glass Melt 103directly proportional to the applied potential,voltammetry with linearly changing potential,in other words, i-E curve by potential sweepmethod was finally obtained from above i-tcurve. Appropriate operation below the cur-rent capacity of the potentiostat can be at-tained by adjusting the immersion depth ofplatinum electrodes. Experiments were con-ducted by the following combined conditions,i. e., sample glasses of NazO·SiOz and NazO·2SiOz, potential sweep rates of 40 and 120mV/sec and temperatures of 730, 840, 980 and1200°C.§ 3. Experhnental resultIn the couse of the experiments was therefound the deposition of dark grey substanceon the T electrode after cathodic half cyclesweep as well as on C electrode after anodicsame sweep, and innermost melt layers adja-cent to these electrodes were also faintly conta-minated in foggy state. These deposits may berecognized as metallic silicon or further alloyof Pt-Si from the analogous feature in thepreceding work.4) As a result, each experi-ments were carried out by using fresh platinumwire electrodes except R electrode. In Fig. 2and 3 are shown summarily typical i-E curvesof NazO· SiOz and NazO·2SiOz at 840 and1200°C. In the figures, effect of potentialsweep rates was omitted due to the less diffe-rence in the patterns as compared with thereproducibility in the same repeated experi-ments. The patterns at 980°C were almostsame as those at 840°C. The patterns at730°C were fairly obscure because of the lesssensitivity of the recorder. In Fig. 2 only i-Ecurve of NazO· Si02 at 1200°C showed polaro-graphic stationary S-shape pattern with aslight maximum wave, but the rest curvesshowed transient patterns with hysteresis,having one anodic peak in the latter half res-pectively. Wholly same tendency was detectedin the case of cathodic half cycle sweep asshown in Fig. 3, although the shape of eachcathodic peak in the first half of the half cyclesweep becomes generally more sharp. Thusvoltammetry of relatively high NazO contentsample at higher temperature transfers tostationary mode from transient one.§4. DiscussionRecently Gileadi5) predicted that the slowmethod with sweep rates in the range of 1-100 mV/sec is a quasi-steady state method andaim of such measurements is to evaluate thesteady state mechanism of the reaction con-cerned. Therefore, if now interesting reaction"21~...-----+---.----~---rE(Y) 1oeiI'-==----,--------,-----, E(Y) (1200 aC)3 1 2 3(810(')Fig. 2 Anodic voltagrams of the Pt INazO·Si02 and Pt1NazO·2SiOz half.cell systems at840°C and 1200°C104 T. HIRAI and K. TAKAHASHI (Vo\. 3,E(V)_r:-3--~-'--')-----Ir---,.-.....-II E(\·~~>--~----~-7.F'-.....,(t: !()'C)Fig. 3 Cathodic voltagrams of th~ Pt!NazO·SiOz and Pt!NazO·2SiOz half-cell systems at840°C and 1200°Cis partially or completely controlled by diffu-sion, the corresponding theory given by Dela-hay as well as that in polarography would beavailable to evaluate above experimentalresults.In the anodic sweep, free oxygen anion orsilicate anion, which participates oxygen ele-ctrode reaction, may move under the diffu-sion-controlled mode, because sodium ion, amajor charge carrier in the melt, acts as thesupporting electrolyte. Now S-shape i.E rela.tion obtained experimentally in the anodicsweep of NazO· SiOz at 1200°C is satisfied byeither of following two equations, namelyE =,-E1/2 +_1!.pT In ii,,_~i)n I(reversible process) (l)E E RT I (~,,--:Jl~ =;,- 1/2+,1nF n I(totally irreversible process) (2)E: potentialrEIf2 , irEI!2: half-wave potentials for rever-sible and irreversible process respectivelyi: currentI,,: limiting currentT: absolute temperatureR: gas constantF: Faraday's constantn : number of electrons involved III ele-ctrode process{3: transfer coefficientAs above equations are of same form, n forreversible process or {3n for totally irreversibleprocess is easily calculated from the gradientof E-ln (i,,--i)ji plot as shown in Fig. 4. But1.0~v O.t:'"0bu<l) (Uiv»::1 0.4i'.:l 10 , ](I , 10" 10' 102Ci,,-i)/iFig. 4 E-In (id-i)/i plot of Pt\NazO· Si02half-cell system at 1200°Cthese experimental values were extremely lessin reproducibility. For example, several valuesbetween 2.5 and 1.2 were obtained as shownin Table 1. Taking round number of 2 for n,a reversible oxygen electrode, where diffusionof oxygen or silicate anion is a rate-determin-ing stage, may be proposed. On the contrary,for the irreveriible process further discussion is1968) Voltammetry of Na20-Si02 Glass Melt 105impossible for lack of {3 value. It should benoted, however, that obrerved half.wave po-tential irEJf2 is more anodic than rE1/2' Thusoxygen electrode reaction of Na20· Si02 at1200 u C would be explained as a reversible oran irreversible process having n or {3n of 2 andrE1/2 or irE1/2 of approx. +O.5V us R electroderespectively. In the same manner, wholetransient i-E relations having on peak in cur-rent may also be explained as the diffusion-controlled process. The potential E p corres-ponding to the maximum current is theoreti-cally introduced by Randles,B) Sevik7) andDelahay.S)RTrEp = ,.EI / 2 + 1 . I-F-- (reversible process)n (3)RT [ 1 {JnF ]irEp = Ei+(inF Q+2InRT v(totally irreversible process) (4)Ei : initial poten tiall' • rate of potential sweepQ: constantAccording to the equation (3) and (4) rEp isRT . h . 1I.I-liF volt more anodic than t e potentia,E1/ 2, but quantitative comparison of irEp with;,.EI / 2 is obvously impossible. Thus observedpeak potential E p of approx. +O.5V I'S R ele-n or fin values of the several half-cellsystems.TableSample Glass(mole %)50 Na20·50 Si0240 Na20·60 Si0240 Na20·60 Si0235 Na20·65 Si0235 Na20·65 Si0235 Na20·65 Si0233 Na20·67 Si021200107095012801280125012501.31.81.31.91.21.72.4ctrode may be slightly more anodic than rEl/2for the reversible process.In the cathodic sweep, i-E relataon of Na20·SiOz at I200°C was same S-shape pattern asshown in the anodic sweep and those of therest were also transient patterns with hys-teresis. But in this case it is obviously impos-sible to explain approximate value of -1.3Vus reference electrode as theoretical E1/ 2 or Epfrom the standpoint of the diffusion-controlledprocees, because the sodium ions transfer bymigration as well as by diffusion.As a result, it was concluded that oneanodic reaction and one cathodic reactionwere detected in the electrolysis of the meltby stationary platinum electrode. Further-more, the value of 1.8V, potential differencebetween anodic and cathodic values of E1/ 2 orE p , proposed that decomposition reaction ofNa20 or the indirect decomposition of Si02 inthe melt was most probable, although theymay be less in physical meaning.AcknowledgeInentThe authors wish to thank Mr. M. Fujimotofor his contribution in the experimental work.ReferencesI) Yu. S. LYALlKOV, Zavodsk. Lab., 18, (1952) 1070.2) Yu. K. DELlMARSKII, S. S. OGNYAN1K, Zh. Neorgall.Khim., -r, (1962) 1757.3) T. BAAK, R. L. FREDERICK, J. Amer. Ceram. Soc.,50, (1967) 38.4) T. HIRAI, Y. MIURA, T. YOSHlO, K. TAKAHASHI,This Memoirs 2, (1967) 62.5) S. SRINIVASAN, E. GILEADI, Electrochim. Acta, 11,(1966) 321.6) J. E. B. RANDLES, Trans Faraday Soc., 43, (1948)327.7) A. SEVIK, Collection Czechoslov. Chern. Com-muns., 13, (1948) 349.8) P. DELAHAY, J. Am. Chern. Soc. 75, (1953) 1190.

Potential Sweep Voltammetry of Na(2)O-SiO(2) Glass Melt by Stationary Platinum Electrodes

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