- Email: [email protected]
Electrochemical determination of reduction potentials in glass-forming melts
Journal of Non-Crystalline Solids 240 (1998) 193±201
Electrochemical determination of reduction potentials in glass-forming melts Michael W. Medlin, Karl D. Sienerth, Henry D. Schreiber
*
Department of Chemistry, Virginia Military Institute, Lexington, VA 24450, USA Received 6 November 1997; received in revised form 18 February 1998
Abstract Square-wave voltammetry (SWV) has been used to determine the reduction potentials of several multivalent elements in situ in an alkali borosilicate melt. Such reduction potentials measured at melt temperature were found to be proportional to the relative reduction potentials obtained from analyses of the quenched glasses. Accordingly, the redox state of the glass typically re¯ects the redox state of the melt. Although SWV is a sensitive tool for monitoring the total concentration of speci®c multivalent elements in the melt, the method does not directly monitor the redox state of the melt. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction 1.1. Redox state of glass-forming systems Many elements included in glass batches can exist in more than one redox state. As one example, iron is present as a mixture of Fe2 and Fe3 ions in most glass-forming systems; as another example, chromium can be stabilized as Cr6 3 2 (CrO2ÿ ions in glasses. The 4 ), Cr , and/or Cr concentrations of speci®c redox ions of a multivalent element in a glass-forming system are controlled principally by composition, temperature, atmosphere (oxygen partial pressure), and/or addition of oxidizing or reducing agents [1]. Multivalent elements in the glass batch play key roles in the processing of the melt as well as in controlling the properties of the resulting glass.
* Corresponding author. Tel.: +1-540 464 7416; fax: +1-540 464 7261; e-mail: [email protected].
For example, redox components impose constraints on ®ning, thermal conductivity, refractory corrosion, and volatile loss during processing. The optical, electrical, and magnetic properties of the glass product are also related to the nature and the amounts of redox ions contained therein. Thus, a knowledge of the redox states of the multivalent elements and how to control their concentrations in the melt and/or glass is necessary to manufacture glass with desired target characteristics [2]. Because iron is present, either as a batch additive or impurity, in most glass-forming systems, the Fe2 /Fe3 ratio is commonly used as a measure of the redox state of the glass. This iron redox ratio is typically measured by analyzing the glass by chemical or spectroscopic methods [1]. Identi®cations and concentrations of speci®c redox ions of other multivalent elements are similarly obtained through analyses of the glass [3]. However, analyzing the glass for its redox state is not timely, especially if the redox state of the melt needs to be re-adjusted through batch additives in
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 6 9 7 - 8
194
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
real time in order to obtain the target redox state. It would be advantageous to monitor the redox state on-line, that is, in situ in the melt during processing; but most analytical methods have limitations that do not allow their application to high temperatures. Electrochemical methods have shown promise for such on-line measurement and, thus, control of the redox state of the melt. In particular, square-wave voltammetry (SWV) has been proposed as a potentially useful in situ analytical tool to determine the system's redox state during the processing of commercial glasses [4,5] as well as for waste vitri®cation [6]. 1.2. Redox chemistry of glass-forming systems Electrochemical methods can also supply information that will enhance the fundamental understanding of the redox chemistry of melts. Much has been reported on the redox chemistry of multivalent elements when measured in glass, but electrochemical methods allow the redox chemistry to be investigated in situ in the melt as the precursor to glass. Of particular interest is the relationship between the redox state of the melt versus that of the glass. Because quench eects on the redox state have been identi®ed during cooling from melt to glass [7], it is uncertain how accurately the redox state of the glass re¯ects the actual redox state of the melt. In situ electrochemistry provides a means for determining the reduction potentials of individual multivalent elements in the glass-forming melt. The concept of reduction potentials is central to understanding and organizing redox reactions in any solvent, whether aqueous or glass-forming melt [8,9]. Reduction potentials of redox couples are eectively used to predict reaction spontaneity in that solvent. The reduction and oxidation potentials of the two reacting components can simply be added; and if the net potential is positive, then a redox reaction is expected from a thermodynamic perspective. Thus, de®ning which reactions do or do not occur in the glass-forming system can identify what oxidizing or reducing agents might be added to the batch to achieve a desired product. A comprehensive electromotive force series, or table of reduction potentials, has been previously
established for multivalent elements in an alkali borosilicate glass [8±11]. This series was based on analyses of glasses produced by equilibrating glass melts containing individual multivalent elements at 1150°C at dierent imposed oxygen fugacities. The x-intercept of a resulting plot of ÿlog fO2 as a function of log([reduced ion]/[oxidized ion]) was then related to the relative reduction potential, E0 , of that couple at that temperature and composition [11]. Two limitations of this series were that (1) it provided only a relative scale of reduction potentials, and (2) experimental measurements were done on glasses which were assumed to re¯ect melt conditions. 2. Objectives One goal of this study is to use SWV to determine the in situ reduction potentials for several multivalent elements in a borosilicate melt. The position and the intensity of the electrochemical signal are correspondingly tested as monitors for the identity and concentration of individual redox components. The second goal is to correlate such in situ reduction potentials with relative reduction potentials characteristic of the same multivalent elements in the quenched glass. This then shows whether the redox state of the glass is indeed representative of the redox state of the melt during its processing at much higher temperature. The ®nal goal is to evaluate the applicability of SWV as a monitoring sensor for redox in glass processing. In particular, the results should show whether SWV can be routinely used for on-line analysis and control of the redox state. 3. Experimental procedures 3.1. Base composition The base composition used in this study has been previously identi®ed as SRL-131, an alkali borosilicate composition supplied by the Savannah River Site as a homogeneous glass frit. Weight percentages of its components are 57.9% SiO2 , 1.0% TiO2 , 0.5% ZrO2 , 14.7% B2 O3 , 0.5% La2 O3 ,
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
2.0% MgO, 17.7% Na2 O, and 5.7% Li2 O [10]. This composition has not only been used as a model glass composition for redox studies for nuclear waste immobilization but also for commercial glasses [10]. It is a well-characterized glass composition for which a comprehensive series of relative reduction potentials of multivalent elements has already been prepared from glasses quenched from 1150°C [8±11]. Glass frits were prepared containing a single multivalent element at a concentration of 1 wt% in the SRL-131 base composition. Dopants used were cerium, chromium, copper, europium, iron, manganese, nickel, tin, uranium, and vanadium; and each was added to the SRL-131 base glass as a high-purity oxide. After being mechanically mixed, 30 g batches were melted in platinum crucibles at air and 1150°C for 24 h. The melts were rapidly quenched to glasses and then powdered for use in the electrochemical experiments. The reference concentration, which is not a standard condition in the thermodynamic sense, was de®ned as 1 wt% of the redox component dissolved in the glassforming melt, simply because this is the dopant level used in prior redox studies of this system. Additionally, glass frits were prepared without any dopant as well as with 5 and 10 wt% iron. Although SRL-131 contained the multivalent element titanium as a component in its base composition, the presence of titanium did not interfere with the SWV analyses of other multivalent elements in the melt. Extremely reducing conditions are needed to stabilize Ti3 in this system [11]. In addition, titanium's presence in this alkali borosilicate glass did not hinder prior redox studies in this reference composition [8±11]. 3.2. SWV analyses of melts All SWV trials were conducted in a DelTech (model DT31-VTOSD) furnace dedicated for electrochemical experimentation. Furnace temperature was controlled by a programmable microcomputer unit that allowed the furnace to be heated to temperature on a ramp and to be maintained at that temperature for the desired length of time. Glass frits were melted in the furnace in a 40 ml platinum crucible positioned on a
195
pedestal and loaded from the bottom into the furnace. The electrode assemblage was lowered into the melt from the top of the furnace. The fundamental design of the electrode assemblage has been described previously by R ussel [12,13]. At the center was an oxide±oxygen reference electrode. An alumina tube was tipped with a yttria-stabilized zirconia plug; the interior of the tube contains a platinum thermocouple junction contacting the zirconia plug. Air was employed as the reference gas in the probe. The working electrode was made from 0.5 mm diameter platinum wire which was ¯ame polished to produce a spherical tip. This spherical bead protruded from the bottom of the assemblage by about 1 cm. The design aorded a reproducible operational surface area of the working electrode from melt to melt as well as a durable assembly. The counter electrode was a heavy gauge platinum post which made contact with the crucible bottom, resulting in the entire platinum crucible serving as part of the counter electrode and ensuring a reproducible depth in lowering the electrode assemblage. The amount of glass frit and the counter electrode's post length were adjusted so that (1) the reference electrode would be just wetted by the melt and (2) the spherical tip of the working electrode would be fully engulfed by the melt. The electrodes were allowed to soak in the molten system for about 30 min. The exact time was unique for this particular sample con®guration, and was needed in order for the oxide±oxygen equilibrium to be established across the zirconia junction of the reference electrode. Without this lag time for wetting, the resulting voltammogram exhibited an unacceptable amount of noise. Each electrode in the assembly was connected via platinum wire to an EG&G Princeton Applied Research (PAR) electrochemical unit which was interfaced to a personal computer for collection, storage, and manipulation of the experimental data using the PAR M270 software package. The PAR was used in the SWV mode with a pulse height of 50 mV, a frequency of 60±540 Hz, a scan increment of 2.00 mV, and a potential range limited from +1.00 to )1.00 V. While the pulse height and scan increment were the same for each melt,
196
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
the frequency and potential range were varied and optimized for each species under study. The theoretical basis for SWV as applied to silicate melts has been previously discussed [12±15]. Once a SWV scan was obtained on a given melt, a 5 min rest period was allowed. Without this rest period, the reduction potential for the signal tended to shift. After an optimized signal for a particular melt was obtained, the melt was tested several times with the same parameters (pulse height, frequency, scan increment, and potential range) to ensure reproducibility of the results. The output data were plotted as the cell current versus the applied potential; prominent peaks in each of the scans were smoothed by the sliding average method. One limitation of the electrode assemblage was that melts doped with 5 and 10 wt% iron were extremely corrosive to the reference electrode. Although distinct signals were achieved for the iron reduction process, the zirconia tip of the reference electrode was rapidly destroyed upon minimal exposure to the melts. Thus, the reference electrode would only be useful on a long-term basis for the analysis of melts with relatively low iron concentrations (1 wt% or less). This would present a signi®cant problem for on-line monitoring of nuclear waste processing by SWV, because some high-level nuclear waste vitri®cation streams may contain upwards of 10±15 wt% iron.
Fig. 1. SWV for the background of reference alkali borosilicate melt at 1150°C in air.
electrode and/or residual amounts of ®ning agents (for example, sulfur) left in the frit from its production. The lower and upper limits of the potential range were constrained by both melt and electrode properties.
4. Results 4.1. Reduction potentials The SWV scan (current versus potential) without any intentional dopant included in the SRL131 melt at 1150°C is shown in Fig. 1. It features a residual current of about 10 mA over its entire potential range, but over any particular region has a somewhat sloping background. The characteristics of the background scan with weak current signals evident between +0.5 and )0.5 V are unique to the SRL-131 composition and have not been observed in other less-complex compositions. Possible explanations for such features in the background may be oxide±oxygen reactions at the
Fig. 2. SWV of the reference alkali borosilicate melt containing 1 wt% Cr at 1150°C in air.
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
197
Fig. 3. SWV of the reference alkali borosilicate melt containing 1 wt% Eu at 1150°C in air.
Fig. 5. SWV of the reference alkali borosilicate melt containing 1 wt% Mn at 1150°C in air.
Figs. 2±6 show representative SWV scans for SRL-131 melts containing 1 wt% of a sole multivalent element; the signal peaks on the scans are assigned to speci®c reductions of each additive. Scans in these ®gures are shown without back-
ground subtraction. However, background corrections to yield a net SWV scan were performed to obtain the values reported in Tables 1 and 2, which relied on the quantitative determination of the potential and the current at the identi®ed
Fig. 4. SWV of the reference alkali borosilicate melt containing 1 wt% Fe at 1150°C in air.
Fig. 6. SWV of the reference alkali borosilicate melt containing 1 wt% U at 1150°C in air.
198
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
Table 1 Compilation of reduction potentials determined for redox couples in situ by SWV, E, (this study) and in the quenched glasses by chemical analyses, E0 [8±12] Reduction 3
2
Ni ® Ni Mn3 ® Mn2 Ce4 ® Ce3 Fe3 ® Fe2 U6 ® U5 Cr6 ® Cr3 Cu2 ® Cu1 V5 ® V4 U5 ® U4 Cu1 ® Cu0 V4 ® V3 Cr3 ® Cr2 Eu3 ® Eu2 Ni2 ® Ni0 Sn4 ® Sn2 Fe2 ® Fe0
Emelt (V)
0 Eglass V
+0.59 +0.16 +0.16 )0.10 )0.11 )0.15 )0.20 )0.38 )0.59 )0.60 )0.72 )0.76 )0.80 )0.83 )0.84 )0.90
+1.7 +0.8 )0.1 )1.7 )1.5 )0.3 )0.8 )1.9 )2.2 )3.3 )4.0 )3.4 )4.3 )5.3 )5.5 )6.3
System: alkali borosilicate (SRL-131) at 1150°C. Reference oxygen fugacity for E is air, while that for E0 is pure oxygen (both at 1 atm pressure). Error in measuring Emelt is 0.01 V, 0 is 0.2 V. while that in determining Eglass
peaks. Depending on the multivalent additive, the intensity of the peak current for a reduction ranges from a signal just barely above the background to a signal where the background correction is negligible. Even though certain features, for example the Fe3 ® Fe2 reduction in Fig. 4, in the scans Table 2 Diusion coecients in alkali borosilicate (SRL-131) melt at 1150°C Diusing ion 4
Ce Cu1 U5 Eu3 Mn3 Cu2 Cr3 U6 Sn4 V4 Ni2 V5 Ni3 Fe3
Diusion coecient (cm2 /s) 8.7 1.8 1.5 1.2 9.8 1.2 1.2 8.2 4.5 3.2 4.8 2.8 2.2 1.8
´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´
10ÿ5 10ÿ5 10ÿ5 10ÿ5 10ÿ6 10ÿ6 10ÿ6 10ÿ7 10ÿ7 10ÿ7 10ÿ8 10ÿ8 10ÿ8 10ÿ8
Error in determining each diusion coecient is estimated at an order of magnitude
are less intense than the background features identi®ed in Fig. 1, such signals are real and reproducible, as has been reported in prior studies [12±15]. Further, the intensities of such signals are proportional to the concentration of the dopant multivalent element. The peak potential unique for that multivalent element was read directly from the scan (after background subtraction) via the tracing cursor, and was found to be reproducible to about 0.01 V. For ion-to-metal reductions, the peak potentials were further clari®ed by the stripping potentials, because in such cases the reductions were obscured by the background in the normal SWV scans. The SWV scan in Fig. 4 was obtained at a frequency of 60 cps. At higher frequencies, the Fe3 ® Fe2 signal tended to separate into two distinct signals, indicative that faster reductions involving intermediates were operational. As has been explained by R ussel [5], the iron ions cluster when present at 1 wt% total iron, with the separated signals indicating the stepwise reduction of a ferric iron cluster to a mixed ferric±ferrous iron cluster followed by the reduction to a ferrous iron cluster. The position of its reduction potential was unique for each multivalent element under the conditions of the analyses (SRL-131, 1150°C, air in the reference electrode). This reduction potential or peak potential was de®ned as E for that reduction; it was not E0 for this melt and temperature because measurements were not done under standard conditions. This E value is independent of the relative concentrations of the redox ions of the multivalent element. That is, the measured reduction potential by SWV is the same no matter whether the melt contains 20% Fe2 and 80% Fe3 or contains 20% Fe3 and 80% Fe2 at equilibrium in the melt. The SWV procedure experimentally oxidizes all the iron (or other multivalent element) within the diusion sphere of the electrode to Fe3 before the SWV scan is initiated. Thus, the current intensity as well as potential of the reduction signal is attributed to the reduction of the total concentration of the multivalent element and not to the reduction of a speci®c redox ion's concentration at equilibrium [16]. The SWV scans for chromium (Fig. 2), uranium (Fig. 6), and vanadium contained multiple peaks.
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
These could readily be attributed to the multistep reductions of Cr6 ® Cr3 and Cr3 ® Cr2 , of U6 ® U5 and U5 ® U4 , and of V5 ® V4 and V4 ® V3 . However, several additional SWV peaks were observed in the chromium and vanadium SWV scans, as illustrated in Fig. 2. These two elements in particular are notorious for existing in numerous oxidation states and/or oxyanionic species in solvents. Thus, one reasonable explanation for the extra peaks in these SWV scans is to attribute them to the formation of intermediate oxo-complexes by the multivalent chromium and vanadium species [17]. These complexes, even though they are short-lived, register a reduction peak when they undergo reductions to lower redox states. This behavior has also been observed for refractory and transition elements in molten salt electrochemistry [18]. Table 1 compiles the in situ reduction potentials, E, for the multivalent elements studied in SRL-131 at 1150°C. The redox couples are arranged in order of their electromotive force, with the diculty of the reduction increasing as the potential decreases or becomes more negative. As such, the listing de®nes the ease of reduction of the redox couples in the SRL-131 melt with respect to one another. The values (and relative order) of the measured reduction potentials are generally consistent with those reported in situ by R ussel [12± 15] and others [19] for silicate and borate melts. 4.2. Diusion coecients Diusion coecients of the oxidized ions through the melt have been calculated from information gleaned from SWV scans [20±22]. The peak height of the current's signal, DIpeak , is governed by the equation 1=2
DIpeak nFAD0 C f 1=2 DWpeak = p1=2 ;
1
where n is the number of electrons transferred in the reduction, F Faraday's constant, A the surface area of the working electrode, D0 the diusion coecient of the ionic species (in this case the oxidized ion), C the concentration of the species, f the frequency of the scan, and DWpeak a function of the scan increment, pulse height and temperature.
199
Because D0 is the only unknown in the equation, the diusion coecient of the oxidized ion of the multivalent element through molten SRL-131 at 1150°C can be calculated. However, only a determination to within about an order of magnitude is possible, because parameters such as the surface area are only approximately the same from sample to sample. The diusion coecients for the doped SRL-131 series are compiled in Table 2; values are compatible with those determined in other studies [20±22].
5. Discussion 5.1. Melt±glass correlations SWV provides the reduction potentials, E, for redox couples in the SRL-131 melt at the temperature of analysis (1150°C). Previous studies of the quenched SRL-131 glass have provided a comprehensive compilation of relative reduction potentials, E0 , for the same redox couples in the quenched glass. Such values of E0 are listed in Table 1 and were determined from samples quenched from melts prepared at varying oxygen fugacities at 1150°C. Fig. 7 shows the correlation between these two reduction potential scales. Indeed, the correlation shown in Fig. 7 also provides con®dence in the identi®cation of the reduction potentials for the vanadium and chromium systems, both of which contained multiple peaks in their original SWV scans. It is comforting that the strong correlation exists between E and E0 ; that is, the redox state of the glass is representative of the redox state of the melt. Correlations are also present among the reduction potentials in the SRL-131 glass versus those in other glasses as well as in other solvents (including water) [8]. The over-riding factor determining the reduction potential of a particular redox couple is its inherent ability to gain an electron. Solvation factors seem to be approximately equal for all redox couples, albeit with several exceptions. This property can further be extended to imply that solvations of the redox couples in the melt are similar to those in the glass.
200
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
Fig. 7. Correlation of reduction potentials in the reference alkali borosilicate system at 1150°C. E0 , the relative reduction potential from analyzing the glass, versus E, the in situ reduction potential from analyzing the melt via SWV. Error bars, based on uncertainties stated in Table 1, are within the size of the symbol drawn.
However, the series of reduction potentials from this study does not completely agree with the recent compilation of reduction potentials in several glass melt compositions by Claussen and R ussel [23]. Their general trend in reduction potentials for molten glass is not consistent with the ordering in aqueous solutions or in SRL-131 melt (despite the similarity in their borosilicate composition). One source for this discrepancy may be that the positions of the peaks in the SWV scans determined in this study did not rely on deconvoluting peaks to ascertain reduction potentials, unlike the study of Claussen and R ussel [23] which used larger voltage steps. 5.2. Diusion of oxidized ions In a sense, the diusion coecient of the oxidized ion of a redox couple controls its peak intensity (current) of the SWV signal. SWV scans are `analogous' to optical spectra in which some redox ions have strong while others have weak absorptivities. Unfortunately, one of the more important
redox components, Fe3 , has a very weak SWV signal due its low diusion coecient as shown in Table 2. The SWV analysis for total iron can nevertheless be enhanced by computer modeling [16]. Oddly, the ions with the highest diusion coef®cients through the SRL-131 melt at 1150°C are larger ions such as Ce4 and Eu3 . The slowest diusion coecients are possessed by smaller ions such as Fe3 and V5 . Whereas the ions with higher diusion rates through the melt are relatively unencumbered by direct bondings to oxygen species, the oxidized iron and vanadium species form oxyanions such as the ferrite (FeOÿ 2 ) and 4 and U6 vanadate (VOÿ 3 ) ions. Ions such as V with intermediate diusion coecients have a tendency to form oxycations such as VO2 and UO2 2 . The diusion of the oxyanions is impeded by their apparently larger size as well as charge repulsions with the borosilicate network. 5.3. Limitations of SWV for on-line control of redox state of iron The ideal technique for redox state measurement and control on-line is one that analyzes selectively for one of the two (or both) iron redox states in situ. Less important is for the procedure to measure the total iron concentration, because that can usually be obtained reliably by analyses of the glass. Knowledge of either Fe2 or Fe3 concentration along with the total iron concentration would then de®ne the redox ratio, or the redox state, of the melt. Alternatively, speci®c redox ions of another multivalent element, such as chromium or manganese, could be analyzed to provide an indicator of the redox state of the melt. A crucial limitation for the application of SWV as an eective in situ monitor of a melt's redox state is its inability to determine the equilibrium concentrations of speci®c redox ions. With SWV, sampling of the melt occurs only in the region directly surrounding the working electrode. At the start of a SWV experiment, all ions near the working electrode are oxidized at the initial potential; these diusion sphere ions are then reduced during the subsequent sweep to a lower potential, giving rise to the familiar current-potential peak.
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
Because SWV trials are relatively short in duration, there is not sucient time for other ions to migrate into the sampling region. Thus, SWV is limited to determining the total concentration of a particular multivalent element and not its redox ratio [16]. In the case of iron, the SWV signal is proportional to the total iron concentration, not equilibrium concentrations of Fe3 or Fe2 . SWV must therefore be used in conjunction with another method, for example an oxygen electrode, for it to be employed as a monitor of the redox state of the melt [16,24]. This results in an indirect measurement of the redox state because it relies on theoretical relations between the oxide ion activity of the melt and speci®c redox ion concentrations.
6. Conclusions SWV can be used to analyze a wide scope of multivalent elements in a glass-forming melt. The position of a signal in terms of applied potential can be used to identify the element, while the peak's current can be related to the element's concentration. The reduction potential of the redox couple in the melt correlates with reduction potentials previously measured in the glass, meaning that the redox state of the glass is representative of redox processes occurring in the melt. The limitation of SWV for on-line processing is that it monitors the concentration of speci®c multivalent elements (for example, total iron concentration) and not speci®c redox ion concentrations (for example, Fe3 and Fe2 ion concentrations, or their redox ratio).
Acknowledgements This research was supported by the NSF Industry-University Center for Glass Research
201
located at the New York State College of Ceramics at Alfred University. The electrochemical analyzer and electrodes were furnished through Dennis Bickford of the Westinghouse-Savannah River Site. References [1] I. Joseph, Glass Res. 3 (1993) 13. [2] H.D. Schreiber, J. Non-Cryst. Solids 84 (1986) 129. [3] H.D. Schreiber, in: D.R. Rossington, R.A. Condrate, R.L. Snyder (Eds.), Advances in Materials Characterization, Plenum, 1983, p. 647. [4] M. Zink, C. R ussel, H. M uller-Simon, K.W. Mergler, Glastech. Ber. 62 (1992) 25. [5] C. R ussel, Ceram. Trans. 29 (1993) 259. [6] D.F. Bickford, Ceram. Trans. 23 (1991) 607. [7] C. R ussel, Glastech. Ber. 62 (1989) 199. [8] H.D. Schreiber, M.T. Coolbaugh, J. Non-Cryst. Solids 181 (1995) 225. [9] H.D. Schreiber, Glass Res. 3 (1993) 6. [10] H.D. Schreiber, G.B. Balazs, B.E. Carpenter, J.E. Kirkley, L.M. Minnix, P.L. Jamison, J. Am. Ceram. Soc. 70 (1984) C106. [11] H.D. Schreiber, J. Geophys. Res. 70 (1987) 9225. [12] C. Montel, C. R ussel, E. Freude, Glastech. Ber. 61 (1988) 59. [13] C. R ussel, E. Freude, Phys. Chem. Glasses 30 (1989) 62. [14] C. R ussel, J. Non-Cryst. Solids 119 (1990) 303. [15] C. R ussel, G. Sprachmann, J. Non-Cryst. Solids 127 (1991) 197. [16] O. Claussen, C. R ussel, Glastech. Ber. Glass Sci. Technol. 69 (1996) 95. [17] H. Hirashima, T. Yoshida, R. Br uckner, Glastech. Ber. 61 (1988) 283. [18] K.D. Sienerth, E.M. Hondrogiannis, G. Mamantov, J. Electrochem. Soc. 141 (1994) 1763. [19] K. Takahashi, Y. Miura, J. Non-Cryst. Solids 80 (1986) 11. [20] K. Takahashi, Y. Miura, J. Non-Cryst. Solids 38&39 (1980) 527. [21] R.O. Colson, L.A. Haskin, D. Crane, Geochim. Cosmochim. Acta 54 (1990) 3353. [22] C. R ussel, Phys. Chem. Glasses 32 (1991) 138. [23] O. Claussen, C. R ussel, Ber. Bunsenges. Phys. Chem. 100 (1996) 1475. [24] J.C. Simpson, T.V. Palmiter, L.D. Pye, Ceram. Trans. 61 (1995) 159.
Electrochemical determination of reduction potentials in glass-forming melts Michael W. Medlin, Karl D. Sienerth, Henry D. Schreiber
*
Department of Chemistry, Virginia Military Institute, Lexington, VA 24450, USA Received 6 November 1997; received in revised form 18 February 1998
Abstract Square-wave voltammetry (SWV) has been used to determine the reduction potentials of several multivalent elements in situ in an alkali borosilicate melt. Such reduction potentials measured at melt temperature were found to be proportional to the relative reduction potentials obtained from analyses of the quenched glasses. Accordingly, the redox state of the glass typically re¯ects the redox state of the melt. Although SWV is a sensitive tool for monitoring the total concentration of speci®c multivalent elements in the melt, the method does not directly monitor the redox state of the melt. Ó 1998 Elsevier Science B.V. All rights reserved.
1. Introduction 1.1. Redox state of glass-forming systems Many elements included in glass batches can exist in more than one redox state. As one example, iron is present as a mixture of Fe2 and Fe3 ions in most glass-forming systems; as another example, chromium can be stabilized as Cr6 3 2 (CrO2ÿ ions in glasses. The 4 ), Cr , and/or Cr concentrations of speci®c redox ions of a multivalent element in a glass-forming system are controlled principally by composition, temperature, atmosphere (oxygen partial pressure), and/or addition of oxidizing or reducing agents [1]. Multivalent elements in the glass batch play key roles in the processing of the melt as well as in controlling the properties of the resulting glass.
* Corresponding author. Tel.: +1-540 464 7416; fax: +1-540 464 7261; e-mail: [email protected].
For example, redox components impose constraints on ®ning, thermal conductivity, refractory corrosion, and volatile loss during processing. The optical, electrical, and magnetic properties of the glass product are also related to the nature and the amounts of redox ions contained therein. Thus, a knowledge of the redox states of the multivalent elements and how to control their concentrations in the melt and/or glass is necessary to manufacture glass with desired target characteristics [2]. Because iron is present, either as a batch additive or impurity, in most glass-forming systems, the Fe2 /Fe3 ratio is commonly used as a measure of the redox state of the glass. This iron redox ratio is typically measured by analyzing the glass by chemical or spectroscopic methods [1]. Identi®cations and concentrations of speci®c redox ions of other multivalent elements are similarly obtained through analyses of the glass [3]. However, analyzing the glass for its redox state is not timely, especially if the redox state of the melt needs to be re-adjusted through batch additives in
0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 6 9 7 - 8
194
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
real time in order to obtain the target redox state. It would be advantageous to monitor the redox state on-line, that is, in situ in the melt during processing; but most analytical methods have limitations that do not allow their application to high temperatures. Electrochemical methods have shown promise for such on-line measurement and, thus, control of the redox state of the melt. In particular, square-wave voltammetry (SWV) has been proposed as a potentially useful in situ analytical tool to determine the system's redox state during the processing of commercial glasses [4,5] as well as for waste vitri®cation [6]. 1.2. Redox chemistry of glass-forming systems Electrochemical methods can also supply information that will enhance the fundamental understanding of the redox chemistry of melts. Much has been reported on the redox chemistry of multivalent elements when measured in glass, but electrochemical methods allow the redox chemistry to be investigated in situ in the melt as the precursor to glass. Of particular interest is the relationship between the redox state of the melt versus that of the glass. Because quench eects on the redox state have been identi®ed during cooling from melt to glass [7], it is uncertain how accurately the redox state of the glass re¯ects the actual redox state of the melt. In situ electrochemistry provides a means for determining the reduction potentials of individual multivalent elements in the glass-forming melt. The concept of reduction potentials is central to understanding and organizing redox reactions in any solvent, whether aqueous or glass-forming melt [8,9]. Reduction potentials of redox couples are eectively used to predict reaction spontaneity in that solvent. The reduction and oxidation potentials of the two reacting components can simply be added; and if the net potential is positive, then a redox reaction is expected from a thermodynamic perspective. Thus, de®ning which reactions do or do not occur in the glass-forming system can identify what oxidizing or reducing agents might be added to the batch to achieve a desired product. A comprehensive electromotive force series, or table of reduction potentials, has been previously
established for multivalent elements in an alkali borosilicate glass [8±11]. This series was based on analyses of glasses produced by equilibrating glass melts containing individual multivalent elements at 1150°C at dierent imposed oxygen fugacities. The x-intercept of a resulting plot of ÿlog fO2 as a function of log([reduced ion]/[oxidized ion]) was then related to the relative reduction potential, E0 , of that couple at that temperature and composition [11]. Two limitations of this series were that (1) it provided only a relative scale of reduction potentials, and (2) experimental measurements were done on glasses which were assumed to re¯ect melt conditions. 2. Objectives One goal of this study is to use SWV to determine the in situ reduction potentials for several multivalent elements in a borosilicate melt. The position and the intensity of the electrochemical signal are correspondingly tested as monitors for the identity and concentration of individual redox components. The second goal is to correlate such in situ reduction potentials with relative reduction potentials characteristic of the same multivalent elements in the quenched glass. This then shows whether the redox state of the glass is indeed representative of the redox state of the melt during its processing at much higher temperature. The ®nal goal is to evaluate the applicability of SWV as a monitoring sensor for redox in glass processing. In particular, the results should show whether SWV can be routinely used for on-line analysis and control of the redox state. 3. Experimental procedures 3.1. Base composition The base composition used in this study has been previously identi®ed as SRL-131, an alkali borosilicate composition supplied by the Savannah River Site as a homogeneous glass frit. Weight percentages of its components are 57.9% SiO2 , 1.0% TiO2 , 0.5% ZrO2 , 14.7% B2 O3 , 0.5% La2 O3 ,
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
2.0% MgO, 17.7% Na2 O, and 5.7% Li2 O [10]. This composition has not only been used as a model glass composition for redox studies for nuclear waste immobilization but also for commercial glasses [10]. It is a well-characterized glass composition for which a comprehensive series of relative reduction potentials of multivalent elements has already been prepared from glasses quenched from 1150°C [8±11]. Glass frits were prepared containing a single multivalent element at a concentration of 1 wt% in the SRL-131 base composition. Dopants used were cerium, chromium, copper, europium, iron, manganese, nickel, tin, uranium, and vanadium; and each was added to the SRL-131 base glass as a high-purity oxide. After being mechanically mixed, 30 g batches were melted in platinum crucibles at air and 1150°C for 24 h. The melts were rapidly quenched to glasses and then powdered for use in the electrochemical experiments. The reference concentration, which is not a standard condition in the thermodynamic sense, was de®ned as 1 wt% of the redox component dissolved in the glassforming melt, simply because this is the dopant level used in prior redox studies of this system. Additionally, glass frits were prepared without any dopant as well as with 5 and 10 wt% iron. Although SRL-131 contained the multivalent element titanium as a component in its base composition, the presence of titanium did not interfere with the SWV analyses of other multivalent elements in the melt. Extremely reducing conditions are needed to stabilize Ti3 in this system [11]. In addition, titanium's presence in this alkali borosilicate glass did not hinder prior redox studies in this reference composition [8±11]. 3.2. SWV analyses of melts All SWV trials were conducted in a DelTech (model DT31-VTOSD) furnace dedicated for electrochemical experimentation. Furnace temperature was controlled by a programmable microcomputer unit that allowed the furnace to be heated to temperature on a ramp and to be maintained at that temperature for the desired length of time. Glass frits were melted in the furnace in a 40 ml platinum crucible positioned on a
195
pedestal and loaded from the bottom into the furnace. The electrode assemblage was lowered into the melt from the top of the furnace. The fundamental design of the electrode assemblage has been described previously by R ussel [12,13]. At the center was an oxide±oxygen reference electrode. An alumina tube was tipped with a yttria-stabilized zirconia plug; the interior of the tube contains a platinum thermocouple junction contacting the zirconia plug. Air was employed as the reference gas in the probe. The working electrode was made from 0.5 mm diameter platinum wire which was ¯ame polished to produce a spherical tip. This spherical bead protruded from the bottom of the assemblage by about 1 cm. The design aorded a reproducible operational surface area of the working electrode from melt to melt as well as a durable assembly. The counter electrode was a heavy gauge platinum post which made contact with the crucible bottom, resulting in the entire platinum crucible serving as part of the counter electrode and ensuring a reproducible depth in lowering the electrode assemblage. The amount of glass frit and the counter electrode's post length were adjusted so that (1) the reference electrode would be just wetted by the melt and (2) the spherical tip of the working electrode would be fully engulfed by the melt. The electrodes were allowed to soak in the molten system for about 30 min. The exact time was unique for this particular sample con®guration, and was needed in order for the oxide±oxygen equilibrium to be established across the zirconia junction of the reference electrode. Without this lag time for wetting, the resulting voltammogram exhibited an unacceptable amount of noise. Each electrode in the assembly was connected via platinum wire to an EG&G Princeton Applied Research (PAR) electrochemical unit which was interfaced to a personal computer for collection, storage, and manipulation of the experimental data using the PAR M270 software package. The PAR was used in the SWV mode with a pulse height of 50 mV, a frequency of 60±540 Hz, a scan increment of 2.00 mV, and a potential range limited from +1.00 to )1.00 V. While the pulse height and scan increment were the same for each melt,
196
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
the frequency and potential range were varied and optimized for each species under study. The theoretical basis for SWV as applied to silicate melts has been previously discussed [12±15]. Once a SWV scan was obtained on a given melt, a 5 min rest period was allowed. Without this rest period, the reduction potential for the signal tended to shift. After an optimized signal for a particular melt was obtained, the melt was tested several times with the same parameters (pulse height, frequency, scan increment, and potential range) to ensure reproducibility of the results. The output data were plotted as the cell current versus the applied potential; prominent peaks in each of the scans were smoothed by the sliding average method. One limitation of the electrode assemblage was that melts doped with 5 and 10 wt% iron were extremely corrosive to the reference electrode. Although distinct signals were achieved for the iron reduction process, the zirconia tip of the reference electrode was rapidly destroyed upon minimal exposure to the melts. Thus, the reference electrode would only be useful on a long-term basis for the analysis of melts with relatively low iron concentrations (1 wt% or less). This would present a signi®cant problem for on-line monitoring of nuclear waste processing by SWV, because some high-level nuclear waste vitri®cation streams may contain upwards of 10±15 wt% iron.
Fig. 1. SWV for the background of reference alkali borosilicate melt at 1150°C in air.
electrode and/or residual amounts of ®ning agents (for example, sulfur) left in the frit from its production. The lower and upper limits of the potential range were constrained by both melt and electrode properties.
4. Results 4.1. Reduction potentials The SWV scan (current versus potential) without any intentional dopant included in the SRL131 melt at 1150°C is shown in Fig. 1. It features a residual current of about 10 mA over its entire potential range, but over any particular region has a somewhat sloping background. The characteristics of the background scan with weak current signals evident between +0.5 and )0.5 V are unique to the SRL-131 composition and have not been observed in other less-complex compositions. Possible explanations for such features in the background may be oxide±oxygen reactions at the
Fig. 2. SWV of the reference alkali borosilicate melt containing 1 wt% Cr at 1150°C in air.
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
197
Fig. 3. SWV of the reference alkali borosilicate melt containing 1 wt% Eu at 1150°C in air.
Fig. 5. SWV of the reference alkali borosilicate melt containing 1 wt% Mn at 1150°C in air.
Figs. 2±6 show representative SWV scans for SRL-131 melts containing 1 wt% of a sole multivalent element; the signal peaks on the scans are assigned to speci®c reductions of each additive. Scans in these ®gures are shown without back-
ground subtraction. However, background corrections to yield a net SWV scan were performed to obtain the values reported in Tables 1 and 2, which relied on the quantitative determination of the potential and the current at the identi®ed
Fig. 4. SWV of the reference alkali borosilicate melt containing 1 wt% Fe at 1150°C in air.
Fig. 6. SWV of the reference alkali borosilicate melt containing 1 wt% U at 1150°C in air.
198
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
Table 1 Compilation of reduction potentials determined for redox couples in situ by SWV, E, (this study) and in the quenched glasses by chemical analyses, E0 [8±12] Reduction 3
2
Ni ® Ni Mn3 ® Mn2 Ce4 ® Ce3 Fe3 ® Fe2 U6 ® U5 Cr6 ® Cr3 Cu2 ® Cu1 V5 ® V4 U5 ® U4 Cu1 ® Cu0 V4 ® V3 Cr3 ® Cr2 Eu3 ® Eu2 Ni2 ® Ni0 Sn4 ® Sn2 Fe2 ® Fe0
Emelt (V)
0 Eglass V
+0.59 +0.16 +0.16 )0.10 )0.11 )0.15 )0.20 )0.38 )0.59 )0.60 )0.72 )0.76 )0.80 )0.83 )0.84 )0.90
+1.7 +0.8 )0.1 )1.7 )1.5 )0.3 )0.8 )1.9 )2.2 )3.3 )4.0 )3.4 )4.3 )5.3 )5.5 )6.3
System: alkali borosilicate (SRL-131) at 1150°C. Reference oxygen fugacity for E is air, while that for E0 is pure oxygen (both at 1 atm pressure). Error in measuring Emelt is 0.01 V, 0 is 0.2 V. while that in determining Eglass
peaks. Depending on the multivalent additive, the intensity of the peak current for a reduction ranges from a signal just barely above the background to a signal where the background correction is negligible. Even though certain features, for example the Fe3 ® Fe2 reduction in Fig. 4, in the scans Table 2 Diusion coecients in alkali borosilicate (SRL-131) melt at 1150°C Diusing ion 4
Ce Cu1 U5 Eu3 Mn3 Cu2 Cr3 U6 Sn4 V4 Ni2 V5 Ni3 Fe3
Diusion coecient (cm2 /s) 8.7 1.8 1.5 1.2 9.8 1.2 1.2 8.2 4.5 3.2 4.8 2.8 2.2 1.8
´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´ ´
10ÿ5 10ÿ5 10ÿ5 10ÿ5 10ÿ6 10ÿ6 10ÿ6 10ÿ7 10ÿ7 10ÿ7 10ÿ8 10ÿ8 10ÿ8 10ÿ8
Error in determining each diusion coecient is estimated at an order of magnitude
are less intense than the background features identi®ed in Fig. 1, such signals are real and reproducible, as has been reported in prior studies [12±15]. Further, the intensities of such signals are proportional to the concentration of the dopant multivalent element. The peak potential unique for that multivalent element was read directly from the scan (after background subtraction) via the tracing cursor, and was found to be reproducible to about 0.01 V. For ion-to-metal reductions, the peak potentials were further clari®ed by the stripping potentials, because in such cases the reductions were obscured by the background in the normal SWV scans. The SWV scan in Fig. 4 was obtained at a frequency of 60 cps. At higher frequencies, the Fe3 ® Fe2 signal tended to separate into two distinct signals, indicative that faster reductions involving intermediates were operational. As has been explained by R ussel [5], the iron ions cluster when present at 1 wt% total iron, with the separated signals indicating the stepwise reduction of a ferric iron cluster to a mixed ferric±ferrous iron cluster followed by the reduction to a ferrous iron cluster. The position of its reduction potential was unique for each multivalent element under the conditions of the analyses (SRL-131, 1150°C, air in the reference electrode). This reduction potential or peak potential was de®ned as E for that reduction; it was not E0 for this melt and temperature because measurements were not done under standard conditions. This E value is independent of the relative concentrations of the redox ions of the multivalent element. That is, the measured reduction potential by SWV is the same no matter whether the melt contains 20% Fe2 and 80% Fe3 or contains 20% Fe3 and 80% Fe2 at equilibrium in the melt. The SWV procedure experimentally oxidizes all the iron (or other multivalent element) within the diusion sphere of the electrode to Fe3 before the SWV scan is initiated. Thus, the current intensity as well as potential of the reduction signal is attributed to the reduction of the total concentration of the multivalent element and not to the reduction of a speci®c redox ion's concentration at equilibrium [16]. The SWV scans for chromium (Fig. 2), uranium (Fig. 6), and vanadium contained multiple peaks.
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
These could readily be attributed to the multistep reductions of Cr6 ® Cr3 and Cr3 ® Cr2 , of U6 ® U5 and U5 ® U4 , and of V5 ® V4 and V4 ® V3 . However, several additional SWV peaks were observed in the chromium and vanadium SWV scans, as illustrated in Fig. 2. These two elements in particular are notorious for existing in numerous oxidation states and/or oxyanionic species in solvents. Thus, one reasonable explanation for the extra peaks in these SWV scans is to attribute them to the formation of intermediate oxo-complexes by the multivalent chromium and vanadium species [17]. These complexes, even though they are short-lived, register a reduction peak when they undergo reductions to lower redox states. This behavior has also been observed for refractory and transition elements in molten salt electrochemistry [18]. Table 1 compiles the in situ reduction potentials, E, for the multivalent elements studied in SRL-131 at 1150°C. The redox couples are arranged in order of their electromotive force, with the diculty of the reduction increasing as the potential decreases or becomes more negative. As such, the listing de®nes the ease of reduction of the redox couples in the SRL-131 melt with respect to one another. The values (and relative order) of the measured reduction potentials are generally consistent with those reported in situ by R ussel [12± 15] and others [19] for silicate and borate melts. 4.2. Diusion coecients Diusion coecients of the oxidized ions through the melt have been calculated from information gleaned from SWV scans [20±22]. The peak height of the current's signal, DIpeak , is governed by the equation 1=2
DIpeak nFAD0 C f 1=2 DWpeak = p1=2 ;
1
where n is the number of electrons transferred in the reduction, F Faraday's constant, A the surface area of the working electrode, D0 the diusion coecient of the ionic species (in this case the oxidized ion), C the concentration of the species, f the frequency of the scan, and DWpeak a function of the scan increment, pulse height and temperature.
199
Because D0 is the only unknown in the equation, the diusion coecient of the oxidized ion of the multivalent element through molten SRL-131 at 1150°C can be calculated. However, only a determination to within about an order of magnitude is possible, because parameters such as the surface area are only approximately the same from sample to sample. The diusion coecients for the doped SRL-131 series are compiled in Table 2; values are compatible with those determined in other studies [20±22].
5. Discussion 5.1. Melt±glass correlations SWV provides the reduction potentials, E, for redox couples in the SRL-131 melt at the temperature of analysis (1150°C). Previous studies of the quenched SRL-131 glass have provided a comprehensive compilation of relative reduction potentials, E0 , for the same redox couples in the quenched glass. Such values of E0 are listed in Table 1 and were determined from samples quenched from melts prepared at varying oxygen fugacities at 1150°C. Fig. 7 shows the correlation between these two reduction potential scales. Indeed, the correlation shown in Fig. 7 also provides con®dence in the identi®cation of the reduction potentials for the vanadium and chromium systems, both of which contained multiple peaks in their original SWV scans. It is comforting that the strong correlation exists between E and E0 ; that is, the redox state of the glass is representative of the redox state of the melt. Correlations are also present among the reduction potentials in the SRL-131 glass versus those in other glasses as well as in other solvents (including water) [8]. The over-riding factor determining the reduction potential of a particular redox couple is its inherent ability to gain an electron. Solvation factors seem to be approximately equal for all redox couples, albeit with several exceptions. This property can further be extended to imply that solvations of the redox couples in the melt are similar to those in the glass.
200
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
Fig. 7. Correlation of reduction potentials in the reference alkali borosilicate system at 1150°C. E0 , the relative reduction potential from analyzing the glass, versus E, the in situ reduction potential from analyzing the melt via SWV. Error bars, based on uncertainties stated in Table 1, are within the size of the symbol drawn.
However, the series of reduction potentials from this study does not completely agree with the recent compilation of reduction potentials in several glass melt compositions by Claussen and R ussel [23]. Their general trend in reduction potentials for molten glass is not consistent with the ordering in aqueous solutions or in SRL-131 melt (despite the similarity in their borosilicate composition). One source for this discrepancy may be that the positions of the peaks in the SWV scans determined in this study did not rely on deconvoluting peaks to ascertain reduction potentials, unlike the study of Claussen and R ussel [23] which used larger voltage steps. 5.2. Diusion of oxidized ions In a sense, the diusion coecient of the oxidized ion of a redox couple controls its peak intensity (current) of the SWV signal. SWV scans are `analogous' to optical spectra in which some redox ions have strong while others have weak absorptivities. Unfortunately, one of the more important
redox components, Fe3 , has a very weak SWV signal due its low diusion coecient as shown in Table 2. The SWV analysis for total iron can nevertheless be enhanced by computer modeling [16]. Oddly, the ions with the highest diusion coef®cients through the SRL-131 melt at 1150°C are larger ions such as Ce4 and Eu3 . The slowest diusion coecients are possessed by smaller ions such as Fe3 and V5 . Whereas the ions with higher diusion rates through the melt are relatively unencumbered by direct bondings to oxygen species, the oxidized iron and vanadium species form oxyanions such as the ferrite (FeOÿ 2 ) and 4 and U6 vanadate (VOÿ 3 ) ions. Ions such as V with intermediate diusion coecients have a tendency to form oxycations such as VO2 and UO2 2 . The diusion of the oxyanions is impeded by their apparently larger size as well as charge repulsions with the borosilicate network. 5.3. Limitations of SWV for on-line control of redox state of iron The ideal technique for redox state measurement and control on-line is one that analyzes selectively for one of the two (or both) iron redox states in situ. Less important is for the procedure to measure the total iron concentration, because that can usually be obtained reliably by analyses of the glass. Knowledge of either Fe2 or Fe3 concentration along with the total iron concentration would then de®ne the redox ratio, or the redox state, of the melt. Alternatively, speci®c redox ions of another multivalent element, such as chromium or manganese, could be analyzed to provide an indicator of the redox state of the melt. A crucial limitation for the application of SWV as an eective in situ monitor of a melt's redox state is its inability to determine the equilibrium concentrations of speci®c redox ions. With SWV, sampling of the melt occurs only in the region directly surrounding the working electrode. At the start of a SWV experiment, all ions near the working electrode are oxidized at the initial potential; these diusion sphere ions are then reduced during the subsequent sweep to a lower potential, giving rise to the familiar current-potential peak.
M.W. Medlin et al. / Journal of Non-Crystalline Solids 240 (1998) 193±201
Because SWV trials are relatively short in duration, there is not sucient time for other ions to migrate into the sampling region. Thus, SWV is limited to determining the total concentration of a particular multivalent element and not its redox ratio [16]. In the case of iron, the SWV signal is proportional to the total iron concentration, not equilibrium concentrations of Fe3 or Fe2 . SWV must therefore be used in conjunction with another method, for example an oxygen electrode, for it to be employed as a monitor of the redox state of the melt [16,24]. This results in an indirect measurement of the redox state because it relies on theoretical relations between the oxide ion activity of the melt and speci®c redox ion concentrations.
6. Conclusions SWV can be used to analyze a wide scope of multivalent elements in a glass-forming melt. The position of a signal in terms of applied potential can be used to identify the element, while the peak's current can be related to the element's concentration. The reduction potential of the redox couple in the melt correlates with reduction potentials previously measured in the glass, meaning that the redox state of the glass is representative of redox processes occurring in the melt. The limitation of SWV for on-line processing is that it monitors the concentration of speci®c multivalent elements (for example, total iron concentration) and not speci®c redox ion concentrations (for example, Fe3 and Fe2 ion concentrations, or their redox ratio).
Acknowledgements This research was supported by the NSF Industry-University Center for Glass Research
201
located at the New York State College of Ceramics at Alfred University. The electrochemical analyzer and electrodes were furnished through Dennis Bickford of the Westinghouse-Savannah River Site. References [1] I. Joseph, Glass Res. 3 (1993) 13. [2] H.D. Schreiber, J. Non-Cryst. Solids 84 (1986) 129. [3] H.D. Schreiber, in: D.R. Rossington, R.A. Condrate, R.L. Snyder (Eds.), Advances in Materials Characterization, Plenum, 1983, p. 647. [4] M. Zink, C. R ussel, H. M uller-Simon, K.W. Mergler, Glastech. Ber. 62 (1992) 25. [5] C. R ussel, Ceram. Trans. 29 (1993) 259. [6] D.F. Bickford, Ceram. Trans. 23 (1991) 607. [7] C. R ussel, Glastech. Ber. 62 (1989) 199. [8] H.D. Schreiber, M.T. Coolbaugh, J. Non-Cryst. Solids 181 (1995) 225. [9] H.D. Schreiber, Glass Res. 3 (1993) 6. [10] H.D. Schreiber, G.B. Balazs, B.E. Carpenter, J.E. Kirkley, L.M. Minnix, P.L. Jamison, J. Am. Ceram. Soc. 70 (1984) C106. [11] H.D. Schreiber, J. Geophys. Res. 70 (1987) 9225. [12] C. Montel, C. R ussel, E. Freude, Glastech. Ber. 61 (1988) 59. [13] C. R ussel, E. Freude, Phys. Chem. Glasses 30 (1989) 62. [14] C. R ussel, J. Non-Cryst. Solids 119 (1990) 303. [15] C. R ussel, G. Sprachmann, J. Non-Cryst. Solids 127 (1991) 197. [16] O. Claussen, C. R ussel, Glastech. Ber. Glass Sci. Technol. 69 (1996) 95. [17] H. Hirashima, T. Yoshida, R. Br uckner, Glastech. Ber. 61 (1988) 283. [18] K.D. Sienerth, E.M. Hondrogiannis, G. Mamantov, J. Electrochem. Soc. 141 (1994) 1763. [19] K. Takahashi, Y. Miura, J. Non-Cryst. Solids 80 (1986) 11. [20] K. Takahashi, Y. Miura, J. Non-Cryst. Solids 38&39 (1980) 527. [21] R.O. Colson, L.A. Haskin, D. Crane, Geochim. Cosmochim. Acta 54 (1990) 3353. [22] C. R ussel, Phys. Chem. Glasses 32 (1991) 138. [23] O. Claussen, C. R ussel, Ber. Bunsenges. Phys. Chem. 100 (1996) 1475. [24] J.C. Simpson, T.V. Palmiter, L.D. Pye, Ceram. Trans. 61 (1995) 159.