Voltammetric Studies of Antimony Ions in Soda-lime-silica Glass Melts up to 1873 K

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1 ANALYTICAL SCIENCES JANUARY 2001, VOL The Japan Society for Analytical Chemistry 45 Voltammetric Studies of Antimony Ions in Soda-lime-silica Glass Melts up to 1873 K Hiroshi YAMASHITA, Shigeru YAMAGUCHI, Ryuichi NISHIMURA, and Takashi MAEKAWA Department of Applied Chemistry, Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime , Japan The half wave potential of reduction of Sb 5+ in 16Na 2O 10CaO 74SiO 2 glass melts was examined by differential pulse voltammetry up to 1873 K. The half wave potential shifted to the positive direction with an increase in temperature. The results indicate that the equilibrium of Sb 5+ /Sb 3+ shifted to negative direction with an increase in temperature. The half wave potential shifted to positive direction (48 mv at 1473 K) when the atmosphere over the melts changed from pure oxygen gas to air, in agreement with the theoretical prediction. The reversibility of Pt:O 2 reference electrode is confirmed. (Received June 20, 2000; Accepted September 29, 2000) The speciation of antimony ions in the high temperature glass melts is a subject of growing interest. One of the reasons is the elucidation of the fining process, i.e., the removal of bubbles originating from raw materials as well as from included air. Fining is believed to be achieved by evolution of oxygen gases (bubbles) released during the reduction of fining ions such as As 5+, Sb 5+ or SO 2 4 (SO 3). 1 6 Because of the highly toxic nature of arsenic, the latter two are used mainly now. The qualitative explanation of fining by antimony ions is shown in Fig. 1. At the first step of melting of the glasses, foam is present in glass melts, because of the decomposition of starting glass-materials and the contamination of vapor-phase gas. The big bubbles rise freely in the glass melts and move out to the vapor-phase. On the contrary, the small bubbles remain in the glass melts. At a high temperature, redox equilibria of fining ions such as antimony ions in glass melts shifts to the reduced direction and evolution of oxygen gas supports the rise of the small bubbles. When the temperature of the melts was lowered at the glassforming process, the redox equilibria shifts to the oxidized direction. Any oxygen gas which remained becomes incorporated into the glass network. Now it becomes important to deduce why antimony ions have been used as effective refining ions. The temperature dependence of the redox equilibria of antimony ions in glass melts must be throughly examined. Generally, the equilibrium concentration of Sb 5+ and Sb 3+ was measured by chemical analysis of the quenched glass. 7,8 However, possible deviation of redox equilibria may occur during cooling the melts. Thus, in situ determination of the redox equilibrium in high temperature melts has been required. Electrochemical methods such as voltammetry have been successfully applied to the study of the redox equilibria of multivalent ions in glass melts The half wave potential, E 1/2, can be related to the equilibrium concentration ratio between redox pair ions through Nernst s equation. E 1/2 values of reduction of Sb 5+ in silicate and borate melts were examined by a differential pulse voltammetry (DPV). 15 E 1/2 shifted to the Fig. 1 Qualitative explanation of fining by antimony ions (see text). To whom correspondence should be addressed.

2 46 ANALYTICAL SCIENCES JANUARY 2001, VOL. 17 Fig. 2 Schematic cell assembly for voltammetry. negative direction with an increase in content of sodium oxide and shifted to the positive direction with an increase in temperature; thus Sb 3+ /Sb 5+ equilibrium shifted to the oxidized direction with an increase in the basicity or with a decrease in temperature. The substitution of Na 2O by CaO shifted E 1/2 to the positive direction. Soda-lime-silica glass is one of the fundamental glasses in commercial uses. It was argued that CaO is a weak basic oxide compared to Na 2O. 15 A square wave voltammetry (SWV) study of reduction of antimony ions has also been published. 11 However, in the previous studies including our results, the data in high temperature region beyond 1500 K were not presented. In order to see the fining phenomena deeply, it is necessary to obtain more precise data about E 1/2. In this research, E 1/2 values of the melts of sodalime-silica system, i.e., 16Na 2O 10CaO 74SiO 2 in mol ratio, were determined by DPV. The usefulness of antimony ions in fining regents will also be seen. Experimental Electrode reaction The DPV uses three electrodes: i.e., a reference electrode, a working one and a counter one, in which current(i) potential(e) curves are recorded during the electrolysis. 15 Potential is measured with respect to that of a reference electrode, which corresponds to the following reaction: (1/2)O 2 + 2e = O 2. (1) On the working electrode, the reductions of solute metal ion take place: Ox + ne = Red. (2) In the case of the reduction of Sb 5+, Eq. (2) is replaced by Sb e = Sb 3+. (3) Thus, the overall reaction can be written by or Sb 5+ + O 2 = Sb 3+ + (1/2)O 2 (4) Fig. 3 DPV voltammogram of 16Na 2O 10CaO 74SiO 2 0.5Sb 2O 3 melt for various pulse heights ( E) at 1473 K and p(o 2) = Pa. 1/2(Sb 2O 5) in glass = 1/2(Sb 2O 3) in glass + 1/2O 2. (4) The voltage, E, the difference between working and reference electrode potentials, is related to a standard potential, E 0, or a formal potential, E 0 : E = E 0 + (RT/2F) ln a(sb 5+ )/a(sb 3+ ) + (RT/nF) ln a(o 2 ) (RT/4F) ln p(o 2) = E 0 + (RT/nF) ln([sb 5+ ]/[Sb 3+ ]). (5) Here, a(sb 5+ ), a(sb 3+ ) and a(o 2 ) refer to activities of Sb 5+, Sb 3+, and O 2, respectively. The bracket, [i] means concentration of ion i. The formal potential E 0 includes all the terms other than the concentration ratio of the redox ion pair and depends on the basicities of the solvent melts, a(o 2 ), and the oxygen pressure over the melts. The relationship of E 1/2 and E 0 is the following: E 1/2 = E 0 + (RT/2F) ln[d(sb 3+ )/D(Sb 5+ )] 1/2. (6) D(Sb 3+ ) and D(Sb 5+ ) are diffusion coefficients of Sb 3+ and Sb 5+, respectively. If the diffusion coefficient of Sb 5+ is assumed to be the same as that of Sb 3+, E 1/2 determined by DPV gives the formal potential directly. Apparatus and reagents The desired mol numbers of sodium carbonate, calcium carbonate and SiO 2 and 0.5 mol % per solvent glass of Sb 2O 3 were melted at 1273 K in a platinum crucible. Care must be paid in order to set this crucible into the high temperature reaction tube, because many bubbles are evolved within the melt and the melt would drop away from the crucible by a rapid heating. All experiments were conducted in a furnace under air or Pa oxygen gas. The flow rate of both the gases was ml/min. The cell assembly was the same as that shown in a previous paper. 15 Figure 2 shows a schematic view of a reaction cell. Three electrodes were inserted into a liquid in a platinum crucible. A working electrode made of a coiled platinum wire, 0.4 mm in diameter, was well immersed into the liquid. A reference electrode of fold platinum wire was half

3 ANALYTICAL SCIENCES JANUARY 2001, VOL Table 1 Relation between the peak potential and the halfwave potential E/mV E p/mv ( E/2)/mV (E p E/2)/mV Fig. 4 Typical voltammogram of 16Na 2O 10CaO 74SiO 2 0.5Sb 2O 3 melt measured by the difference of sweep direction at 1430 K and p(o 2) = Pa. immersed on the surface of the liquid; thus, a reversible O 2/O 2 reaction could be expected. The platinum crucible was used as a counter electrode. This cell assembly with three electrodes was fixed to alumina rods and hung on the middle part of a reaction tube. Oxygen gas or air flowed from the bottom of the reaction tube. The three electrodes were connected to a pulse polarographic analyzer (Yanaco P-1100). After measurement of DPV, several selected glasses were dissolved in HF aqueous solution and subjected to an inducting coupled plasma (ICP) (Perkin Elmer, Optima 3000) analysis operated at nm. The peak potentials of DPV depend on the pulse height ( E). In DPV, the peak potential, E p, is related to the half wave potential as: E p = E 1/2 + E/2, (7) where E is the pulse height (see below). For infinitely small pulses, the peak potential occurs at the polarographic half wave potential. 20,21 The influence of the pulse height on the voltammogram was examined first. Figure 3 represents the DPV voltammogram for various pulse heights. As E increases, the peak potential moved to the anodic direction (more positive direction) for a cathodic sweep. Table 1 shows the relation between the peak potential and the half wave potential. One sees a good relation between them. In order to avoid the disturbance around the working electrode, the pulse height was set to 10 mv. The duration of the pulse, that of the scan rate and the time between pulses were set to 50 ms, 10 mv/s and 100 ms, respectively. In usual voltammetry, the potential sweep is initiated from zero volt. However, in the present case, the peak potential is located around 0 V, so that Fig. 5 Typical voltammogram of 16Na 2O 10CaO 74SiO 2 melt at 1473 K and p(o 2) = Pa. (a): solid line, containing 0.5 mol% Sb 2O 3; dotted line, Sb 2O 3-free. (b): solid line, after subtraction of Sb 2O 3-free melt from containing Sb 2O 3 one; dotted line, theoretical curve. the initial potential must be set to a positive value. The difference of voltammograms between the cathodic sweep from 0.3 V and the anodic sweep from 0 V was examined. Figure 4 represents the two curves. No difference could be found by two different sweeps. Thus, in the present experiment, the voltammograms were measured only by the cathodic sweep. Results and Discussion Reaction on the working electrode Figure 5(a) represents a typical voltammogram of the melt containing 0.5 mol per cent Sb 2O 3 at 1473 K. By cathodic sweep, two reduction peaks are observed. Peak 1 is somewhat ambiguous due to overlap of the current in the solvent melt. By subtracting the current due to the solvent (dotted line), the peak comes to be seen more clear. Figure 5(b) shows the theoretical curves for a two-electron step and a three-electron step, respectively. 22 Thus, the first step reduction on the working

4 48 ANALYTICAL SCIENCES JANUARY 2001, VOL. 17 Fig. 7 Variation of the potential of peak 1 with the oxygen pressure over the melt. Fig. 6 Temperature dependence of the voltammogram of 16Na 2O 10CaO 74SiO 2 0.5Sb 2O 3 melt under Pa of O 2 gas. Table 2 ICP results of the total antimony content remaining in selected glasses after the measurements of DPV Melting temperature/k Melting time/ h Antimony content, mol% Remaining percentage electrode can be written by Eq. (3), that is n equals 2, whereas the second step reduction expressed by n = 3 corresponds to the following reaction: Sb e = Sb 0 (second step). (8) Above about 1673 K, the peak 2 split into two as shown in Fig. 6. The additional peak was seen when the oxidized or reduced ions adsorbed on the electrode. At 1873 K, the peak intensity of the additional peak decreased with time and appreciable amounts of noise were seen in i E curves. This phenomenon may be the evaporation of metallic antimony deposited on the electrode. Unfortunately, at present, a more detailed discussion can not be made. Possible formation of platinum + antimony alloy should also be considered. Table 2 shows the ICP results of the total residual antimony content in selected quenched glasses after the DPV measurements of the corresponding melts were carried out. The remaining total antimony contents decrease with an increase in melting temperature. However, the half wave potential did not change with the antimony contents. Pt:O 2/electrode Figure 7 represents the variation of the potential of peak 1 with the oxygen pressure over the melt. By changing the oxygen pressure from pure oxygen to air, the peak potential shifts gradually to the positive direction and finally reaches a constant value. When the atmosphere changes again to oxygen gas, the peak potential then shifts to the negative direction. When the oxygen pressure was changed, the reference electrode potential is changed. On the other hand, the working electrode potential when a(sb 5+ ) = a(sb 3+ ) does not change, so that the half wave potential under any oxygen pressure shifts to the positive direction according to the following equation: E 1/2* = E 1/2[p(O 2) = Pa] (RT/4F) ln p(o 2). (9) E 1/2* is the half wave potential of Eq. (4) under any oxygen pressure. The peak potential of the peak 1 shifts to positive direction with a decrease in the oxygen pressure over the melts. The difference of voltages between under pure oxygen and air is calculated to be 49 mv at 1473 K. The calculated value coincides with that expected from Eq. (9). The experimental value was 48 mv. The response of the peak potential of peak 2 is also seen. However, the difference is somewhat small (36 mv) compared to that of peak 1. As cited above, this may be due to an irreversible reaction on the working electrode, that is, deposition of metallic antimony. For peak 1, it is possible to calculate E 1/2 of any oxygen pressure, if E 1/2 under Pa of oxygen pressure is known. The reversibility of the oxygen reference electrode in glass melts was discussed. 23 The Pt:O 2/electrode, the present ones, took longer times to reach equilibrium than the Pt:ZrO 2/electrode, although the former had the good point of being stable in high temperature oxide melts. It is argued that the Pt:O 2/electrode functioned as an oxygen gas reference electrode in the DPV used here. Half wave potential and fining The temperature dependence of the voltammogram is shown in Fig. 6 under Pa of O 2 gas. Here, the background currents due to solvents are subtracted from the measured ones. In Table 3, the E 1/2 values of the peaks 1 and 2 are listed, such values are determined by a least-squares method with a second order equation. The number of the bracket means the measured times. The error was expressed by t-distribution of 95% certainty. In Fig. 8, the relation between E 1/2 and the

5 ANALYTICAL SCIENCES JANUARY 2001, VOL Table 3 Relation between the temperature and the E 1/2 E 1/2/mV (peak 1) E 1/2/mV (peak 2) T/K O 2 (p(o 2) = Pa) Air (p(o 2) = Pa) O 2 (p(o 2) = Pa) Air (p(o 2) = Pa) ±52[5] 356±47[5] ±18[6] 345±20[6] ±29[5] 104[1] 331±20[5] 306[1] ±15[10] 128±12[4] 325±10[10] 297±14[4] ±28[6] 314±14[6] ±9[5] 308±25[5] ±8[6] 205[2] 305±10[6] 264[2] ±12[5] 252[1] 267±11[5] 220[1] ±22[5] 220±15[5] The figures in the brackets are the number of measurements. Fig. 8 Relation between E 1/2 and the temperature. Fig. 9 Relation between [Sb 3+ ]/[Sb 5+ ] concentration ratios and the temperature. temperature is plotted. The solid lines are fitted by the leastsquares method. E 1/2 increases linearly with an increase in temperature as E 1/2/V = T 0.60 (under air) E 1/2/V = T 0.79 (under Pa oxygen). The dotted lines are estimated from Eq. (9) based on the line of E 1/2 of Pa oxygen pressure. [Sb 5+ ]/[Sb 3+ ] concentration ratios derived from Eq. (6) are shown in Fig. 9. The Sb 5+ /Sb 3+ equilibrium shifts toward more reduced state with an increase in temperature. When the temperature is raised, Sb 5+ converted to Sb 3+ with release of oxygen gas as shown in Fig. 1 and Eq. (4). If the evaporation of the total antimony ions (0.5 mol %) in the melts is neglected in the process, the change of evolved and disappeared gas quantities is maximum at the temperature where [Sb 5+ ] = [Sb 3+ ] (i.e. around 1200 K), as shown in Fig. 1. Imagawa et al. measured the quantities of the oxygen gas by a mass spectroscopic analysis. It increases with an increase in temperature and the evolution finished in the vicinity of 1673 K. 24 The oxygen gas is evolved markedly at around 1573 K. The evolution increased with an increase of Sb 2O 3 content. They concluded that the oxygen evolution is conducted by the refining reaction: Sb 2O 5 Sb 2O 3 + O 2 which occurred during the heating period. Effective evolution of oxygen bubbles supports the rise of bubbles originated from starting materials as shown in Fig.1. In glass melts, antimony ions are present as complex ions accompanying a certain number of oxide ions, because the [Sb 5+ ]/[Sb 3+ ] ratio increases with an increase of basicity, i.e. [O 2 ]. Thus, the notations of SbO (2n 5) n and SbO (2m 3) m are preferable to free Sb 5+ and Sb In order for the above ratio to increase with an increase in the basicity, n>m must be followed. When the temperature is lowered, the equilibrium shifted to the oxidized direction, so that the remaining oxygen (small bubbles) reacted with Sb 5+ to form Sb 5+ -complex and dissolved stable in melts. The bubbles may be eliminated from the melts and fining can be established. Conclusion The half wave potential of reduction of Sb 5+ in 16Na 2O 10CaO 74SiO 2 glass melts was examined by differential pulse voltammetry at various temperatures. The

6 50 ANALYTICAL SCIENCES JANUARY 2001, VOL. 17 half wave potential shifts to the positive direction with an increase in temperature, which corresponds to the increase of Sb 3+. Antimony ions may become an active refining agent like arsenic ions. The half wave potential moves to the positive side (48 mv at 1473 K) when the atmosphere changes from pure oxygen gas to air, in agreement with the theoretical prediction. Acknowledgements This work has been made under the Research and Development Program for International Standards for Supporting New Industries funded by New Energy and Industrial Technology Development Organization (NEDO). The work has also in part been supported by a Grant-in-Aid for Scientific Research (No ) from the Ministry of Education, Science, Sports and Culture, Japan. References 1. G. E. Rindone, Glass Industry, 1957, 38, 489, 516, 526, G. E. Rindone, Glass Industry, 1957, 38, 561, M. Cable, Glass Tech., 1961, 2, M. Cable and M. A. Haroon, Glass Tech., 1970, 11, R. Pyare, S. P. Singh, A. Singh, and P. Nath, Phys. Chem. Glasses, 1982, 23, M. C. Weinberg, J. Non-Cryst. Solids, 1986, 87, W. D. Johnston, J. Am. Cer. Soc., 1965, 48, D. M. Krol and P. J. Rommers, Glass Tech., 1984, 25, K. Takahashi and Y. Miura, J. Non-Cryst. Solids, 1986, 80, T. Yokokawa, K. Kawamura, and S. Denzumi, Tr. Electrochem., 1992, 1, C. Rüssel and E. Freude, Phys. Chem. Glasses, 1989, 30, C. Rüssel, J. Non-Cryst. Solids, 1990, 119, C. Rüssel and G. Sprachmann, J. Non-Cryst. Solids, 1991, 127, O. Clauβen and C. Rüssel, Glastech. Ber. Glass Sci. Technol., 1997, 70, M. Yokozeki, T. Moriyasu, H. Yamashita, and T. Maekawa, J. Non-Cryst. Solids, 1996, 202, M. Nakashima, H. Yamashita, and T. Maekawa, J. Non- Cryst. Solids, 1998, 223, H. Yamashita, S. Shimaoka, S. Yamaguchi, and T. Maekawa, J. Cer. Soc. Jpn., 1999, 107, H. Yamashita, S. Yamaguchi, M. Yokozeki, M. Nakashima, and T. Maekawa, J. Cer. Soc. Jpn., 1999, 107, S. Gerlach, O. Clauβen, and C. Rüssel, J. Non-Cryst. Solids, 1998, 226, E. P. Parry and R. A. Osteryoung, Anal. Chem., 1965, 37, J. H. Christie, J. Osteryoung, and R. A. Osteryoung, Anal. Chem., 1973, 45, J. Osteryoung and J. J. O Dea, in Electrochemical Chemistry, ed. A. J. Bard, 1986, Dekker, New York, 14, T. Tran and M. P. Brungs, Phys. Chem. Glasses, 1980, 21, H. Imagawa, M. Aoyagi, K. Saitoh, and S. Uchiyama, Glastech. Ber. Glass Sci. Tech., 1995, 68C2, H. Hirashima, T. Yoshida, and R. Brückner, Glasstech. Ber., 1988, 61, 283.