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Journal of Non-Crystalline Solids 352 (2006) 4082–4087

The effect of fluoride on the Fe2+/Fe3+-redox equilibrium in glassmelts in the system Na2O/NaF/CaO/Al2O3/SiO2

Luciana Maia, Christian Russel *

Otto-Schott-Institut, Universitat Jena, Fraunhoferstr. 6, 07747 Jena, Germany

Received 21 June 2006Available online 24 August 2006

Abstract

Liquids with the base compositions (16 � x/2)Na2O Æ xNaF Æ 10CaO Æ 74SiO2 (x = 0, 1, 3, and 4) and (10 � x/2) Æ Na2O Æ xNaF Æ 10-CaO Æ yAl2O3 Æ (80 � y)SiO2 (x = 0, 1, 3, 5 and y = 5 and 15) doped with 0.25 mol% Fe2O3 were studied by means of square-wave vol-tammetry in the temperature range from 1000 to 1500 �C. With increasing temperature, the redox equilibria were shifted to the reducedstate. Also while increasing the alumina concentration, the Fe2+/Fe3+-redox equilibrium is shifted to the reduced state. In the soda-lime–silica melt the addition of fluoride shifts the equilibrium to the oxidized state, while in the aluminosilicate melts with 15 mol% Al2O3, theequilibrium is shifted to the reduced state. In the aluminosilicate melts with 5 mol% Al2O3, the equilibrium was not affected by the fluo-ride concentration. This is explained by the structure of the respective glass compositions.� 2006 Elsevier B.V. All rights reserved.

PACS: 81.05.Kf; 82.45.Rf

Keywords: Electrochemical properties; Voltammetry

1. Introduction

In the past few years, the effect of the chemical compo-sition in the Na2O/CaO/MgO/Al2O3/Fe2O3 system on theredox equilibrium Fe2+/Fe3+ has intensively been investi-gated [1–11]. The concentrations of each of these compo-nents were varied and empirical correlations have beenestablished in order to enable the calculation of the respec-tive equilibrium constants from the chemical composition[8,10,11]. The Fe2+/Fe3+-redox equilibrium is the mostimportant in glass technology and widely determines thetransmittivity of light in technical glasses. At high temper-ature, an equilibrium of Fe2+, Fe3+ and physically dis-solved oxygen is formed:

4Fe3þ þ 2O2� () 4Fe2þ þO2 ð1Þ

0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.06.030

* Corresponding author. Tel.: +49 3641 948501; fax: +49 3641 948502.E-mail address: ccr@rz.uni-jena.de (C. Russel).

The attributed equilibrium constant K depends on thetemperature.

KðT Þ ¼ aFe2þ

aFe3þ

� �PO1=4

2 ð2Þ

with aFe2þ , aFe3þ : activities of Fe2+ and Fe3+, respectively. Ifthe activity coefficients are equal to unity, activities can bereplaced by the respective concentrations. In the case ofiron, Eq. (1) is shifted to the right with increasing temper-ature. Since the physical solubility of O2 in a glass melt isvery small, in most cases much smaller than the iron con-centration, during and after quenching the melt, theFe2+/Fe3+-ratio of the glass is the same as that of the meltat the temperature it is equilibrated. In the past, this meth-od has been applied in numerous studies on redox equilib-ria [12–20]. However, it is time-consuming, and it alwayshas to be ensured that the melt is really in equilibrium withthe atmosphere. Both limitations are of special importanceif high-viscosity melts are studied because then the equili-bration is less enhanced by convection.

Fig. 1. Square-wave voltammograms recorded in a melt with the basecomposition 9.5Na2O Æ 1NaF Æ 10CaO Æ 5Al2O3 Æ 75SiO2 doped with0.25 mol% Fe2O3 (s = 100 ms). Curve 1: 1500 �C, curve 2: 1400 �C, curve3: 1300 �C, curve 4: 1200 �C, curve 5: 1100 �C and curve 6: 1000 �C.

Fig. 2. Square-wave voltammograms recorded in melts with the basecompositions 10Na2O Æ 10CaO Æ 15Al2O3 Æ 65SiO2 (curve 1), 9.5Na2O Æ1NaF Æ 10CaO Æ 15Al2O3 Æ 65SiO2 (curve 2), 8.5Na2O Æ 3NaF Æ 10CaO Æ15Al2O3 Æ 65SiO2 (curve 3) and 7.5Na2O Æ 5NaF Æ 10CaO Æ 15Al2O3 Æ65SiO2 (curve 4). T = 1500 �C, s = 100 ms, 0.25 mol% Fe2O3.

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In the past decade, redox equilibria have frequently beenstudied using electrochemical methods [21–24], especiallythe square-wave voltammetry [1–9,25–27]. These measure-ments are carried out directly at high temperatures (i.e. inthe range from 1000 to 1600 �C) using electrodes insertedinto the melt. Besides thermodynamic data, also diffusioncoefficients can be measured and have been reported alsoas an effect of temperature and melt composition.

Up to now, only the effect of cationic compounds hasbeen studied. In this paper, however, the influence of anexchange of the anionic component, O2� against F� isreported. If simultaneously O2� and F� occur in the melt,also the first coordination sphere of Fe2+ or Fe3+ mightchange.

This paper provides a study of the Fe2+/Fe3+-redoxequilibrium in glasses with the base compositions(16 � x/2)Na2O Æ xNaF Æ 10CaO Æ 74SiO2 (x = 0, 1, 3, and4) and (10 � x/2)Na2O Æ xNaF Æ 10CaO Æ yAl2O3 Æ (80 � y)SiO2 (x = 0, 1, 3, 5 and y = 5 and 15) doped with0.25 mol% Fe2O3.

2. Experimental procedure

The measurements were carried out in soda-lime–silicateand soda-lime–aluminosilicate melts with the base compo-sitions (16 � x/2)Na2O Æ xNaF Æ 10CaO Æ 74SiO2 (x = 0, 1,3, and 4) and (10 � x/2)Na2O Æ xNaF Æ 10CaO Æ yAl2O3 Æ(80 � y)SiO2 (x = 0, 1, 3, 5 and y= 5 and 15) doped with0.25 mol% Fe2O3. The temperatures used were in the rangefrom 1000 to 1500 �C.

The experimental equipment used and the procedureapplied have already been described in detail [26,27]. Aresistance-heated furnace (MoSi2) with a dc power supplywas used. In its middle, a platinum crucible containingthe melt was located. From the top, three electrodes wereinserted into the melt: a platinum wire (diameter: 1 mm)as the working electrode, a platinum plate as counter elec-trode and a ZrO2 probe flushed with air as reference elec-trode. The electronic equipment was self-constructed. Thepotentiostat, the main part of the electronic equipment,controls the potentials in such a manner that the potentialbetween working and reference electrode is always equal tothe required value. A computer gave the potential–timedependence to the potentiostat and recorded the current–potential curve.

The method used was square-wave voltammetry; thepotential–time dependence a staircase ramp superimposedon a rectangular wave. The potential amplitude, DE, was100 mV and the step times used were in the range of 10–400 ms. At the end of every half-wave, the current wasmeasured and differentiated. The current–potential curvesobtained were deconvoluted using experimentally recordedmatrix currents and theoretically calculated current–poten-tial curves. The fundamentals of the method applied andthe deconvolution procedure used have already beenreported in detail [28–30]. The peak potentials observedin the voltammograms are equal to the standard potentials,

if the currents are only controlled by diffusion. The pres-ence of iron gives rise to a well pronounced maximum inthe current–potential curves. Its peak potential is equal tothe standard potential, E0, of the Fe2+/Fe3+-redox pairand correlated with the equilibrium constant, K(T), thestandard free enthalpy, DG0, the standard enthalpy, DH0,and the standard entropy, DS0,

DG0ðT Þ ¼ DH 0 � TDS0 ¼ �nFE0ðT Þ ¼ �RT ln KðT Þ ð3Þwith n = number of electrons transferred, F = Faradayconstant.

The peak currents, IP, depend on the total concentra-tion, C0, of the polyvalent ion, the diffusion coefficient,

Fig. 3. Square-wave voltammograms recorded in a melt with the basecomposition 9.5Na2O Æ 1Na2O Æ 10CaO Æ 15Al2O3 Æ 65SiO2 (T = 1500 �C,s = 100 ms). Curve 1: doped with 0.25 mol% Fe2O3, curve 2: undopedmelt, curve 3: difference curve 1 minus curve 2, curve 4: theoreticallycalculated current–potential curve.

Fig. 4. Peak currents recorded in a melt with the base composition7.5Na2O Æ 5NaF Æ 10CaO Æ 5Al2O3 Æ 75SiO2 doped with 0.25 mol% Fe2O3

at m: 1500 �C, d: 1300 �C and j: 1100 �C using different step times.

Fig. 5. Peak potentials (error ±10 mV) measured in melts with the basecompositions: �: 16Na2O Æ 10CaO Æ 74SiO2, h: 15.5Na2O Æ 1NaF Æ10CaO Æ 74SiO2, n: 14.5Na2O Æ 3NaF Æ 10CaO Æ 74SiO2, ,: 13Na2O Æ4NaF Æ 10CaO Æ 74SiO2 as a function of the temperature. All melts weredoped with 0.25 mol% Fe2O3.

Fig. 6. Peak potentials (error ±10 mV) measured in melts with the basecompositions: �: 10Na2O Æ 10CaO Æ 5Al2O3 Æ 75SiO2, h: 9.5Na2O Æ 1NaF Æ10CaO Æ 5Al2O3 Æ 75SiO2, n: 8.5Na2O Æ 3NaF Æ 10CaO Æ 5Al2O3 Æ 75SiO2,,: 7.5Na2O Æ 5NaF Æ 10CaO Æ 5Al2O3 Æ 75SiO2 as a function of the tem-perature. All melts were doped with 0.25 mol% Fe2O3.

4084 L. Maia, C. Russel / Journal of Non-Crystalline Solids 352 (2006) 4082–4087

D, and the experimental parameters DE and s (pulse time)[28,29].

IP ¼ const: � A � D1=2 � C0 � n2 � s�1=2 ð4Þwith A = surface area of the working electrode, con-st. = 0.31p�1/2 Æ F2 Æ DE/RT, F and R have their usualmeaning.

3. Results

Fig. 1 shows square-wave voltammograms recorded in aliquid with the base composition of 9.5Na2O Æ 1NaF Æ 10-CaO Æ 5Al2O3 Æ 75SiO2 doped with 0.25 mol% Fe2O3. The

temperatures were in the range from 1000 to 1500 �C andthe step time was 100 ms. All curves exhibit distinct max-ima. The currents increase with increasing temperature.While decreasing the temperature, the peaks are shiftedto more negative potentials. The peaks are caused by thereduction of Fe3+ to Fe2+ as previously described in the lit-erature (see e.g. [1–4,25]).

Fe3þ þ e��Fe2þ ð5ÞFig. 2 shows square-wave voltammograms recorded inmelts with the base compositions of 10Na2O Æ 10CaO Æ15Al2O3 Æ 65SiO2 (curve 1), 9.5Na2O Æ 1NaF Æ 10CaO Æ

Fig. 7. Peak potentials (error ±10 mV) measured in melts with the basecompositions: �: 10Na2O Æ 10CaO Æ 15Al2O3 Æ 65SiO2, h: 9.5Na2O Æ1NaF Æ 10CaO Æ 15Al2O3 Æ 65SiO2, n: 8.5Na2O Æ 3NaF Æ 10CaO Æ 15Al2O3 Æ65SiO2, ,: 7.5Na2O Æ 5NaF Æ 10CaO Æ 15Al2O3 Æ 65SiO2 as a function ofthe temperature. All melts were doped with 0.25 mol% Fe2O3.

L. Maia, C. Russel / Journal of Non-Crystalline Solids 352 (2006) 4082–4087 4085

15Al2O3 Æ 65SiO2 (curve 2), 8.5Na2O Æ 3 NaF Æ 10CaO Æ15Al2O3 Æ 65SiO2 (curve 3) and 7.5Na2O Æ 5NaF Æ 10CaO Æ15Al2O3 Æ 65SiO2 (curve 4), all doped with 0.25 mol%Fe2O3 and all recorded at the same temperature(1500 �C) using the same step time (100 ms). While thepeak currents do not differ much, the peak potential isshifted from �260 to �218 mV while increasing the NaFconcentration from 0 to 5 mol%.

The square-wave voltammograms were carefully decon-voluted as illustrated in Fig. 3 for the composition9.5Na2O Æ 1NaF Æ 10CaO Æ 15Al2O3 Æ 65SiO2. The experi-mental current–potential curve, recorded in a melt withiron at 1500 �C (s = 100 ms) is shown as curve 1. Thevoltammograms recorded in a melt without iron usingsame parameters is shown as curve 2. The latter curvewas subtracted from curve 1, the difference is shown ascurve 3. Curve 3 was simulated by a theoretically calculated

Table 1Standard enthalpies, DH0 and standard entropies, DS0, of the redox reaction

[Na2O] (mol%) [NaF] (mol%) [Al2O3] (mol%) DH0 (kJ m

16 0 0 10215.5 1 0 101.714.5 3 0 100.814 4 0 103.110 0 5 89.59.5 1 5 88.68.5 3 5 907.5 5 5 92.6

10 0 15 65.19.5 1 15 61.18.5 3 15 60.27.5 5 15 67.1

Additionally, Fe3+/Fe2+-ratios at 1300 �C (for equilibrium with air) are given

current–potential curve (see curve 4) using a least squareapproximation [30].

The peak potentials are not an effect of the step time,whereas the peak currents strongly depend on s. Fig. 4shows peak currents as a function of s�1/2 for a melt withan alumina concentration of 5 mol% studied at 1100, 1300and 1500 �C. According to Eq. (4), the graph should exhi-bit a proportionality between IP and s�1/2. While in the srange from 50 to 400 ms, this is fulfilled (see the full line),peak currents for smaller step times are lower thanexpected from theory. This behavior is qualitatively thesame for all temperatures and sample compositionsstudied.

Fig. 5 shows peak potentials measured in a melt with16 mol% Na2O as a function of temperature. With increas-ing temperature, a shift to less negative potentials isobserved. Within the limits of error (error in Ep:±10 mV), a linear correlation is observed for all studiedcompositions. The potentials of the melt without fluorideare much less negative than those observed in the fluoridecontaining glasses. While increasing the fluoride concentra-tion from 1 to 5 mol%, the potentials get slightly more neg-ative. In Figs. 6 and 7, the voltammetric peak potentials ofthe melts with 5 and 15 mol% Al2O3 are respectively pre-sented. In analogy to Fig. 5, the peak potentials decreaselinearly with decreasing temperature. In melts with5 mol% Al2O3 (see Fig. 6), the peak potentials are the samewithin the limits of error for all studied fluoride concentra-tions. As shown in Fig. 7, for the melts with 15 mol%Al2O3, the peak potentials are most negative for the samplewithout fluoride. In the samples with 1 and 3 mol% NaF,the peak potentials are 40–50 mV less negative and areequal within the limits of error. At 5 mol% NaF, the peakpotentials are more negative again. Table 1 summarizes thevalues of the standard enthalpies, DH0, and the standardentropies, DS0, for each studied glass composition calcu-lated from the linear dependencies of the peak potentialsupon temperature, as well as the redox ratios [Fe2+]/[Fe3+] calculated for an equilibrium with air at 1300 �C.

according to Eq. (1)

ol�1) DS0 (J mol�1) K�1 log[Fe3+]/[Fe2+] (at 1300 �C)

33 1.4929.1 1.6928 1.7129.2 1.7332.2 1.1230.8 1.1732.4 1.1333.3 1.1622.2 0.8320.9 0.7720.2 0.7722.8 0.87

.

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4. Discussion

As shown in Fig. 4, a deviation from the theoreticallypredicted proportionality between IP and s�1/2 is observedat step times 620 ms. From the proportionality (error in IP:±5% [26]), it can be concluded that at step times P20 ms,the currents are only controlled by diffusion and, hencepeak potentials are equal to standard potentials. In allmelts studied, within the error limits (±10 mV [14]), a lin-ear correlation of the peak potentials with temperaturewas observed within the temperature range studied. Theslopes observed (see DS0 in Table 1) were in the range of20–33 J mol�1 K�1. In the melt with 16 mol% Na2O, DS0

varies between 28 and 33 J mol�1 K�1. The standardenthalpies of melts with 10% Na2O were larger if 5 mol%Al2O3 was present than in those with 15 mol% Al2O3. Allstandard potentials were shifted to less negative values withincreasing temperature, as observed in any investigation onFe2+/Fe3+ equilibria reported up to now in the literature(see e.g. Refs. [1–9,25–27]). This means that the reducedstate, Fe2+ is favored at higher temperatures. In Fig. 8,the effect of fluoride on the peak potentials is shown forall three melt series for an arbitrary temperature of1300 �C. The main effect is that of alumina. The peakpotentials of the series with 16 mol% Na2O are most nega-tive, those of the series with 10 mol% Na2O and 15 mol%Al2O3 are least negative and those with 10 mol% Na2Oand 5 mol% Al2O3 are in between. This behavior can beexplained as follows: in a soda–silica or soda-lime–silicamelt, Fe3+ is incorporated in fourfold coordination, i.e.as FeO�4 , which has a formally negative charge. This chargemust be compensated by a cation, preferably by Na+. Asknown from the literature [9], the peak potentials get morenegative with increasing Na+-concentrations which is dueto a better stabilization of the FeO�4 -tetrahedra. If nowAl2O3 is added, alumina is also incorporated in fourfold

Fig. 8. Effect of the NaF-concentration on the peak potentials in meltswith the base compositions: (T = 1300 �C and 0.25 mol% Fe2O3) d:16Na2O Æ 10CaO Æ 74SiO2, m: 10Na2O Æ 10CaO Æ 5Al2O3 Æ 75SiO2 and .:10Na2O Æ 10CaO Æ 15Al2O3 Æ 65SiO2.

coordination, i.e. as AlO�4 which also needs Na+ to com-pensate the negative charge. Hence, the effect of increasingAl2O3-concentration is similar to that of decreasing Na2Oconcentrations – the peak potentials will get less negative.This readily explains the effect of Na2O and Al2O3 on thepeak potentials observed in the three studied compositionseries. It should be noted that the glass structures in thesethree series are notably different. The glass with 16 mol%Na2O does not contain alumina but a large number ofnon-bridging oxygens, the series with 10 mol% Na2O and5 mol% Al2O3 contains approximately equal quantities ofnon-bridging oxygens and AlO�4 -tetrahedra which negativecharge is compensated by Na+. The glass with 10 mol%Na2O and 15 mol% Al2O3 should not contain non-bridgingoxygen, all Na+-ions are needed to compensate the chargeof AlO�4 -tetrahedra. However, the Na2O concentrationavailable is not sufficient and hence alumina also occursin octahedral coordination or as triclusters [31,32].

In the three composition series, the effects of fluorideadditions are fairly different as seen in Fig. 8. In the glasswith 16 mol% Na2O, the peak potentials decrease whenfluoride is added while in the glass system with 10 mol%Na2O and 15 mol% Al2O3, an increase in the peak poten-tials is observed. In the system with 10 mol% Na2O and5 mol% Al2O3, the peak potential does not depend uponthe fluoride concentration. In the soda-lime–silica glass,the fluoride addition leads to an increasing stabilizationof Fe3+, in comparison to that of Fe2+. By contrast, inthe alumosilicate glass with 15 mol% Al2O3, the oppositeis the case, the peak potentials increase and hence the sta-bilization of Fe3+ decreases if fluoride is added.

These observations should directly be related to thestructure, i.e. to the incorporation of fluoride into therespective glasses. In Ref. [33], studies on the structure offluoride containing soda-lime–silicate and aluminosilicateglasses by magic angle spinning nuclear magnetic reso-nance have been reported. In alumina containing glasses,fluoride is reported to be closely connected to aluminium,while in soda-lime–silicate glasses, calcium is coordinatedwith fluoride. Oxyfluoride glasses have also been studiedby thermodynamic modeling [34], which resulted in theconclusion that more complex species such as NaCaF3,CaSiF6, Na3AlF6 or NaAlF4 may play a part and explainboth the results from thermodynamic modeling andNMR-spectroscopy. The coordination of Fe3+ and Fe2+

in an oxyfluoride melt up to now (as far as the authorsknow) has not been the subject of spectroscopic studiesor thermodynamic calculations. Since both Al3+ andFe3+ are incorporated in similar sites in the glass structure,i.e. as AlO�4 - or FeO�4 -tetrahedra, also Fe3+ might be coor-dinated with F�. Then, the formed Fe3+–F complex shouldbe thermodynamically more stable than the FeO�4 -tetrahe-dra and hence shift the standard potential to more negativevalues in a soda-lime–silica glass. Since the fluoride concen-tration in any glass studied was larger than the Fe3+ con-centration, in a soda-lime–silica glass also a coordinationof Ca2+ with fluoride should occur as known from

L. Maia, C. Russel / Journal of Non-Crystalline Solids 352 (2006) 4082–4087 4087

NMR-spectroscopy [33]. This should have a similar effectas the decrease in the CaO concentration of the glass. Asalready reported in Ref. [35], a decrease in the CaO-concentration has the same effect as an increase in theNa2O-concentration and results in more negative standardpotentials. Hence these two effects may run parallel andboth should lead to more negative standard potentials asexperimentally observed (see Figs. 5 and 8).

In alumina containing glasses, fluoride according to theNMR studies mentioned above should be connected toAl3+. In the following, it is assumed that also Fe3+ is coor-dinated with fluoride. Since in the glasses studied, the alu-mina concentration is much larger than the ironconcentration, most fluoride should be connected to alu-mina. Then alumina coordinated with fluoride occupiessites different from those of AlO�4 but similar to those offluoride coordinated Fe3+. Since the concentration of theAl–F species is much larger than that of the Fe–F species,the latter are forced to occupy energetically less advanta-geous sites. This should lead to less negative standardpotentials. As shown by thermodynamic modeling [34]and also by Raman spectroscopy [36] also Na+ participatesin the Al–F complexes. Hence, the introduction of fluorideinto the aluminosilicate glass also leads to less Na+ avail-able to stabilize Fe3+. This should also lead to less negativestandard potentials. In summary, the effect of compositionand fluoride concentration on the thermodynamics of theFe2+/Fe3+-equilibrium can readily be explained by therespective structure of the glass obtained from spectro-scopic investigations and thermodynamic modeling.

5. Conclusions

Square-wave voltammograms were recorded at temper-atures in the range from 1000 to 1500 �C in soda-limeand aluminosilicate melts doped with 0.25 mol% Fe2O3.The effect of fluoride on the thermodynamics of theFe2+/Fe3+-equilibrium was studied. In the soda-lime–silicamelt, the equilibrium was shifted to the oxidized state whenadding fluoride, while in the case of a soda aluminosilicateglass with 15 mol% Al2O3, the equilibrium was shiftedtowards the reduced state. In the aluminosilicate glass with5 mol% Al2O3, the equilibrium was not affected by the fluo-ride concentration. Generally, an increase in the aluminaconcentration shifts the equilibrium towards Fe2+. This

behavior was explained by the structure of the respectiveoxyfluoride glasses, known from both NMR spectroscopicstudies and thermodynamic modeling.

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