J. Phys. Chem. 1995,99, 9189-9193
9189
Redox Equilibria and Corrosion in Molten Silicates: Relationship with Electrode Potentials in Aqueous Solution J. A. Duffy* Chemistry Department, University of Aberdeen, Scotland, U.K.
F. G. K. Baucket Schott Glaswerke, Mainz, Germany Received: January 3, 1995; In Final Form: March 21, 1995@
Previously obtained data for redox equilibria of metal ion couples in molten silicates indicate dependence on melt basicity. Treating these data in the context of the optical basicity model allows the prediction of redox equilibria for ion couples simply from standard electrode potentials, E", in aqueous solution. The method is extended to obtain electrode potentials for 25 metaumetal ion couples in the melts and to show the effect of basicity changes (through compositional adjustments, for example). Experimental data, from voltametric studies in silicate melts, are available for nine of the metaumetal ion couples, and they show reasonably good agreement with the electrode potentials predicted from the optical basicity values of the melts. In the basicity region of interest to the glass maker, the results show an electrochemical series which is somewhat different from that in aqueous solution. Increasing melt basicity causes a decrease in the difference between the electrode potentials, but little change in the series. The difference almost disappears for basicities where the silicate network is undergoing large-scale disruption to individual SiQ4- units. This basicity region corresponds to the composition of metallurgical slags used in steel making.
Introduction There are many situations where molten silicates are in contact with metals. On the technological scale, for example, this contact occurs during processes associated with extraction metallurgy and also in the preparation of certain optical glasses, while on a smaller scale, contact is made with metal electrodes when molten silicates are subjected to electrochemical measurement. Under oxidizing conditions, e.g. air atmosphere, molten silicates are extremely corrosive with the occurrence of reactions which can be represented by the following half-equations:
+
M = M ~ + ze-
(1)
and
The silicate melt provides an ideal coordination sphere for dissolution of the metal ions, while the 02-ions enter the network si04 tetrahedra by reacting with bridging Si-0-Si groups. Little is known about the corrosion of most metals in molten silicates. At such high temperatures, these equilibria (e.g. eqs 1 and 2) are difficult to study, for example, by electrochemical measurement. Investigations are confined to a small number of metals, and the range of melt composition is so limited that the effects of melt basicity or acidity are largely unknown. In order to provide a rough guide for assessing metal corrosion in molten salts, one must sometimes resort to electrode potential data (E" values) in aqueous solution. However, this procedure has never received any scientific justification.
* To whom correspondence +
@
should be addressed. Honorary Visiting Research Fellow in the University of Aberdeen. Abstract published in Advance ACS Abstracts, May 1, 1995.
Here it is shown that certain empirical relationships exist which do indeed make it possible to use E" values in aqueous alkaline and acidic solution for predicting reduction potentials in silicate melts. Furthermore, it is possible to predict how the resulting values are affected by the basicity of the melt.
Redox Ion Couples in Melts Owing to their importance in glass making, a number of redox ion couples have been studied in molten alkali or alkaline earth silicates, for example, the equilibrium: 4Fe3+
+ 2 0 2 - = 4Fe2+ + 0,
(3)
Although there is great need to known how changes in melt composition affect such equilibria, available data are restricted to the ion couples Fe2+/Fe3+,Cr3+/Cfi+,Ce3+/Ce4+,Sn2+/Sn4+, As3+/As5+,and Cu+/Cu2+. All measurements are for 1400 "C and air atmosphere.'-4 They indicate that, except for the Cu+/ Cu2+couple, increasing melt basicity favors the higher oxidation state. The effect of oxide ion activity is more than outweighed by the stabilizing environment provided by the electron density at the sites for the metal ions. The exceptional behavior for copper has been discussed elsewhere5 and will not be featured in this discussion. The magnitude of the electron density at the metal ion sites depends on how willing the oxygen atoms are to donate negative charge, that is, the Lewis basicity. The Lewis basicity of an oxidic medium, such as a silicate or a single oxide, can be expressed numerically as the "optical ba~icity".~-~ p i s is best regarded as the electron donor power of the oxygen atoms or ions of the medium compared with that for crystalline calcium oxide, for which the optical basicity is assigned a value of unity. The optical basicity model has been applied with much success to oxidic network systems, for example, molten silicates used as slags in extraction metallurgyg-I6and for glass making.l'-*l
0022-3654/95/2099-9189$09.00/0 0 1995 American Chemical Society
9190 J. Phys. Chem., Vol. 99, No. 22, 1995
Duffy and Baucke
TABLE 1: Basicity Moderating Parameters, y, of Some Elements and Optical Basicity Values, A, of Some Oxides element Y oxide A0 potassium(1) sodium(1) lithium calcium(I1) iron(I1) magnesium(I1) aluminum(II1) silicon(1V) boron(II1) hydrogen(1) phosphorus(V)
Kz0 Na2O Liz0 CaO FeO MgO A1203 Si02
0.73 0.87 1.o 1.oo 1.o 1.3 1.65 2.1 2.35 2.5 3.0
B203
H20 pzos
shown in ref 6, Figure 3). It has been f ~ u n d ' that ~ . ~the ~ logarithm of the equilibrium redox ratio, R ( R = [Fe2+/Fe3+] for eq 3, for example), has an almost linear relationship with A; expressions obtained for the other ion couples, mentioned above, are in Table 2.
1.4 1.15 1.o 1.oo 1.o 0.78
Redox Equilibria in Aqueous Solution Values of E" are available for many ion couples in aqueous solution under acidic and alkaline conditions. We shall use these data in conjunction with the equilibrium redox ratio, R, relevant to the reaction
0.60 0.48 0.42 0.40 0.33
Values of A are expressed to the nearest 0.05 for oxides of formula M20 and MO (except CaO and MgO).
M(Z+n)++ ne- = Mz+ ( R = [Mz+]/[M(z+n'+]) (7)
a
Recently, its application to thermodynamic aspects of mineralogical and ceramic systems has been highlighted.22 Originally, optical basicity was obtained by observing the orbital expansion (nephelauxeticZ3) effects experienced by certain metal ions which were added for probing the sites available in the oxidic medium. The most satisfactory ions were found to be T1+, Pb2+, and Bi3+ whose 'SO 3P1 transition, measured as the ultraviolet absorption frequency maximum, v , underwent massive red shifts compared with the free, gaseous ion value, vf. The optical basicity, A, is given by
-
A = (Vf - W ( V f - VCaO)
(4)
where vca0 is the frequency of the probe ion in CaO. Subsequently,it was shown that the optical basicity of a medium could be calculated from its composition by assigning "basicity moderating parameters", y , to elements to express their attenuating effect on the oxide ions (or oxygen atoms) with which they interact (Table l).5-7 Optical basicity is then calculated from eq 5: xA
A =-
YA
+ - + ... xB
YB
where XA,XB, ... are equivalent fractions, i.e. the proportions of negative charge (arising by regarding all oxygen in the formal state of -2) neutralized by the elements A, B, ... in their formal oxidation states. Equation 5 can be rewritten as
A = X,A (AO,,)
+ XBA(BO,,,) + ...
(6)
since the optical basicities A (AOd2), A (B@/2), ... of the single oxides AO,,, Bob/,, ... are simply l / y ~ l/ye, , .... Use of eq 6 is exemplified in deriving A for a sodium silicate melt with Na2O:SiOz mole ratio of 1:2; X Nis~ 1/5 and Xs, is 4/5, and using Table 1, A is (1.15 0.48 x 4)/5, Le. 0.614. One chief advantage of the optical basicity model is that it allows comparisons between different oxidic systems. For example, it is possible to calculate the composition of a potassium silicate melt having the same basicity as the 1:2 Na2O-SiO2 melt above. If x denotes the percentage of K20, then ( 1 . 4 ~ 0.48 x 2 (100 - x))/(200 - x ) equals 0.614 and hence x = 25; similarly, a Li2O-SiO2 melt having this basicity would contgin 41% Li20. It was mentioned earlier that increasing melt basicity usually favors the upper oxidation state for an ion couple. Experimentally, separate trends are observed for systems of different compositions, for example, the Li,O-SiO,, Na20-Si02, and K20-Si02 glass systems, but when plotted against optical basicity, separate trends are unified into a single line (this is
+
+
Standard electrode potentials obtained under acidic conditions refer to the metal ion existing as the aqua ion, for which eq 7 is more properly expressed as, for example
(Pacid)
+
[ F e ( H ~ 0 ) 6 ] ~ +e- = [Fe(H20)6]2f
(8)
When the metal ion exists as the hydroxo species as, for example, in the half-reaction Fe(OH),
+ e- = Fe(OH), + OH-
(9)
the appropriate standard electrode potential is that obtained under alkaline conditions (Eoalk). The metal ions are in environments provided by oxygen atoms of water molecules (eq 8) and of hydroxide ions (eq 9), and the optical basicity values for these two conditions are 0.40 and 0.70, re~pectively.~~ The optical basicity model implies that different media with the same optical basicitiy value provide (on average) sites of equivalent electron density for hosted metal ions. For example, the electron density of sites in a hydroxo species is equivalent to those in a sodium silicate melt having A = 0.70 (Le. by eq 6, containing 49% NazO). Thus, even though there is an enormous temperature difference between aqueous conditions and the silicate melts (1400 "C), it might be expected that there is a straightforward relationship existing between the standard electrode potential in alkaline solution and the redox ratio, R, obtained by substituting the value of 0.70 for A in the appropriate expression for the melt (Table 2). This has been shown to be the case26by plotting nPdk against log R (Figure la). The points lie close to the expression log R = 0.42nEoa,, - 1.1
(10)
Similarly (Figure lb), for acidic conditions (with A = 0.40)
log R = 2.5nEoa,, - 1.5
(1 1)
(Figure 1 omits the single point for Cr6+/Cr3+in (b) which fits only by setting A = 0.10, which is the condition for an environment provided by H30+ ions; the reason for this inconsistency is not presently understood.) In effect, eqs 10 and 11 provide an empirical connection for redox equilibria under two extremely different conditions: (i) at 1400 "C in molten silicates and (ii) in aqueous solution at 25 "C. The possibility of making such a connection was suggested 20 years ago by Wong and Angel1 in a discussion of optical basicity.27
Redox Equilibria in Melts It was pointed out earlier that there are data for very few ion couples in molten silicates. However, it is possible to exploit eqs 10 and 11 to generate empirical relationships between log
Redox Equilibria in Molten Silicates
J. Phys. Chem., Vol. 99,No. 22, 1995 9191
TABLE 2: Redox Ratio Relationship# for Silicate Melts electrode potential, V ion couple
--
Fe3+ Clb+ Ce4+ Sn4+ Ass+ Mn3+ Sb5+
E"dk
Fez+ Cr3+ Ce3+ Sn2+ As3+ Mn2+ Sb3+
-0.56 -0.12 1.71 -0.96 -0.08 -0.40 -0.59 1.00 1.93 1.46 1.25 1.84
----
----
vs+
v4+
Ni4+ Pb4+
Ni2+ Pb2+ ~ 1 3 + TI+ co3+ co2+
pacid
equilibrium
redox relationshipb
-0.036 1.10 1.45 0.15
4Fe3+ 202- = 4Fe2+ 0 2 4CP+ 602- = 4Cr3+ 302 W e 4 + 202- = 4Ce3+ 0 2 2sn4+ 202- = 2Sn2+ 0 2 2 ~ s 5 + 202- = 2 ~ s 3 + o2 4Mn3+ 202- = 4Mn2+ 0 2 2sb5+ 202- = 2sb3+ o2 44v5+ 202- = v4+ 02 2Ni4+ 202- = 2Ni2+ 0 2 2Pb4+ 202- = 2Pb2+ 0 2 2T13+ 202- = 2T1+ O2 4CO3+ f 20'- = 4CO2+ 0 2
+ + + + +
0.58
+
+ + + + + + +
+
1.51 0.64 -0.74 0.49 0.28 -0.05 0.20
log [Fe2+]/[Fe3+]= 3.2 - 6.5A log [Cr3+]/[Clb+]= 8.2 - 13.7A log [Ce3+]/[Ce4+]= 5.4 - 8.3A log [Sn2+]/[Sn4+]= 0.6 - 3.6A log [As~+]/[As~+] = 5.2 - 8.9A log [Mn2+]/[Mn3+]= 7 - 12A log [Sb3+]/[Sb5+]= 6 - 11A log [V4+]/[V5+]= 4 - SA log [Ni2+]/[Ni4+]= 17 - 2512 log [Pb2+]/[Pb4+]= 15 - 22A log [T1+]/[T13+]= 13 - 20A log [CO~+]/[CO~+] = 9 - 14A
+ +
+
+ + + +
+
+
For silicate melts at 1400 "C in air atmosphere. The ratios of concentrations are for equilibrium conditions.
TABLE 3: Electrode Potential Dataa ~~
(25 "C)
(25 "C)
E,,,,,: relationship (1400 "C)
0.342 0.16 0.0984 -0.224 -0.3445 -0.361 -0.46 -0.578 -0.66 -0.66 -0.71 -0.73 -0.79 -0.815 -0.877 -1.2 -1.216 - 1.22 - 1.47 -2.28 -2.33 -2.35 -2.60 -2.64 -2.67 0.401
0.800 1.2 0.854 0.3402 -0.336 0.522 0.32 -0.126 0.2 12 -0.250 0.234 -0.28 -0.136 -0.402 -0.440 -0.74 -0.762 -0.52 - 1.05 - 1.70 -1.43 - 1.706 - 1.68 -1.80 -2.375 1.229
-1.61A 0.81 -3.02A 1.95 -2.09A 1.30 -0.82A 0.36 1.2111 - 1.25 -1.17A 0.40 0.48 -0.95A 0.30A - 0.47 -0.74A 0.3 1 0.61A - 0.70 0.36 -0.83A 0.66A - 0.74 0.23A - 0.45 0.95A - 0.96 1.03A - 1.02 1.64A - 1.43 1.76A - 1.58 1.02A - 1.00 2.44A - 2.09 3.8612 - 3.19 2.99A - 2.50 3.7712 - 3.08 3.55A - 2.93 3.86A - 3.15 5.55A - 4.42 -2.99A 1.96
E"dk
couple
---Pt cu+ -----------Ag+
nE'
Ag
Pt2+
I
1
I
1
-2
-1
0
ut 1
-2
1
I
I
I
-1
0
1
2
log R
Figure 1. Relationship between standard electrode potential in aqueous solution and equilibrium redox ratio, R, in molten silicates at 1400 "C, illustrated by plotting nE" for designated ion couples against log R. E" values are for (a) alkaline and (b) acidic solution, and R is calculated (Table 2) by setting A = 0.70 and 0.40, respectively.
R and A for any ion couple using electrode potentials for aqueous solution. Thus, eq 10 will provide the equilibrium redox ratio, R, for A = 0.70 by substituting the value of E" for alkaline solution; similarly, R is obtained for A = 0.40 by substituting E" for acidic conditions in eq 11. These two values of R then yield a linear equation relating R with A. For example, the reaction
+
+
~ S V + 202- = 2 ~ b 3 + 0,
(12)
corresponds to E" values in aqueous solution of -0.59 and 0.64 V under alkaline and acidic conditions, respectively, and eqs 10 and 11 (with Table 2) yield values of -1.6 and 1.7 for log R. These correspond to A = 0.70 and 0.40 and therefore to the relationship log R = 6 - 11A. Expressions for several other ion couples are obtained similarly (Table 2) and have been discussed in the context of molten glass chemistry.26 The behavior predicted from these expressions is in good agreement with the known chemistry of the ion couples in these melts.
Oxidation of Metallic Elements The method is now applied to equilibria where the metal is oxidized to the metal ion (equations continue to be expressed with the reduced species on the right-hand side): 4M"'
+ 2n02- = 4M + no2
Our objective is to obtain an empirical expression for each metal which indicates how the electrode potential, designated Emel,,
Hg2+ Hg cu2+ c u Tl+ T1 cu Bi3+ Bi Pb2+ Pb Sb3+ Sb Ni2+ Ni As3+ As co2+ c o Sn2+ Sn Cd2+ Cd Fez+ Fe Cr3+ Cr ZnZ+ Zn Ga3+ Ga Mn2+ Mn Be2+ Be Zr Zr'+ ~ 1 3 + AI Hf4+ Hf Th4+ Th Mg2+ Mg 0 2
02-
"Values of bFrom eq 18.
Eoalk
and
pacid
E'acid
+ + + + + + + +
+
in aqueous solution are from ref 32.
varies with the composition (optical basicity) of the melt. It must be remembered that the conditions are for 1400 "C, and for this temperature the expression 2.303RTlnF is 0.332ln (compared with 0.0591n for 25 "C). The required standard electrode potentials of the elements in alkaline and acidic solution are in Table 3. These values of P a r e used in conjunction with eqs 10 and 11 for obtaining values of log R for alkaline conditions, where A = 0.70, and acidic conditions, where A = 0.40. In the case of arsenic, for example, the half-reactions
+ 2H,O + 3e- = As + 40HAs20, + 6H+ + 6e- = 2As + 3 H 2 0
As0,-
(14) (15)
have E" values of -0.71 and 0.234 V, respectively, yielding values of -1.995 (eq 10) and 0.255 (eq 11) for log R. Since the electrode potential for the melt at 1400 "C, Emelt,is related to the equilibrium redox ratio by (see above)
Emelt = (0.332h) log R
9192 J. Phys. Chem., Vol. 99, No. 22, 1995
0
Duffy and Baucke
TABLE 4: Experimental and Predicted Electrode Potentials (volts) for Metals in Molten Silicates
c
electrode reaction
-------
As3+
As
Sb3+ Pb2+ NiZt co2+ Cd2+ Zn2+ Mn2+
Sb Pb Ni co Cd Zn Mn
NaZO-CaO-Si02 (15:13:72) (A =0.578)
Na20-CaO-Si02 (16:10:74) (A = 0.571)
experimental predicted (1400 “C)O (1400 ‘C)*
experimental predicted (1400 oC)c (1400 ‘C)*
cu+ cu
-0.34 -0.46 -0.50 -0.58 -0.74
-0.58 -0.59 -0.64 -0.80 -0.91
-0.24 -0.36 -0.20 -0.58 -0.40
-0.37 -0.52 -0.37 -0.55 -0.60
-0.52
-0.83
From ref 28. bPredicted E” values are for 1400 “C and are calculated from relationships in Table 3 (right-hand column) with the values of A above. Extrapolated values from ref 29.
04
05
06
07
08
-08
Optical Basicity
Figure 2. Plot of corrected electrode potential (E’,,,,) for couples in molten silicate at 1400 “C versus optical basicity of the melt. Adjustment has been made so that the trends are relative to the 0 2 - / 0 2 couple. 0
these values of log R yield Emell= -0.22 and 0.03 V corresponding to the conditions for A = 0.70 and 0.40, respectively (in the melt). The linear equation fulfilling these values is Emell= 0.36 - 0.83A
iPbJ
(17)
-0.3
In effect, this equation is obtained by making substitutions for Eoalk, Eoacid, and n in the general empirical equation
+
+
Emelt= (0.46E0,,, - 2.76E0,,, 0.44/n) A (1.93Eo,,, -0.185E0,,, - 0.67/n) (18) which is derived from eqs 10, 11, and 16. Equation 18 is used to derive expressions showing the dependence of Emelton A (analogous to eq 17) for all the equilibria in Table 3 (see righthand column).
Discussion It is possible to use the expressions in Table 3 to obtain values of Emeltfor any silicate composition simply by substituting the A value of the melt (calculated from eq 6). Voltammetric investigation^^^,^^ in molten silicates for a small number of metals provide the opportunity to make comparisons with experimental electrode potentials. Since these were obtained relative to the oxygen/oxide electrode, it is necessary to adjust the expressions for Emeltin Table 3 so that they are relative to the half-reaction 0,
+ 4e- = 20*-
(19)
This entails subtracting the Emeltexpression for eq 19 from all of the other expressions in Table 3. Thus, the “corrected” electrode potential (designated F m e l t ) for Agf Ag, for example, is 1.38A - 1.15. Figure 2 shows the plot of E’me~t for each couple versus optical basicity. (In effect, Emellfor eq 19 coincides with the abscissa.) The available experimental data2x.29are for two sodiumcalcium-silicate melts of slightly different compositions (Table 4) for which A = 0.578 and 0.571. These values are substituted
-
,
I
-0.2
I
I
I
-0.6
-0.7
I
-0.3 -0.4 -0.5 Experimental
Figure 3. Plot of corrected electrode potentials, (E’,,lt, predicted from optical basicity) against experimental values from voltametry for Na20CaO-Si02 melts with mole ratio compositions 15:13:72 ( 0 )and 16: 10:74 (0)(data in volts, from Table 4). The best-fitting straight line ignores the one ill-fitting point (Pb2+ Pb).
-
in the expressions in Table 3 to obtain Emellfor each composition. The values of Emeltfor the 0 2 - / 0 2 couple (obtained similarly) are subtracted in order to obtain the corrected electrode potentials, For example, for the Na2O-CaOSi02 melt with A = 0.614 (Table 4), substituting this value of A into the expressions for Ni2+ Ni and 0 2 02-(Table 3) yields Emeltvalues of -0.347 and +0.232 V, respectively; the corrected electrode potential, E’melt, for Ni2+ Ni is therefore -0.58 V. The results are in Table 4. Considering the bold steps taken in this treatment, the agreement between the predicted and experimental values is reasonably good. The data in Table 4 are plotted in Figure 3, and the straight line, drawn through all the points except the least best-fitting (for lead), is such that the error is less than 17%. Furthermore, the electrochemical series derived from the predicted values is the same as from the experimental values except for just one couple, namely Pb2+ Pb. The comparison therefore provides support for using Figure 2 at least as an approximate guide. Emeltfor the 0 2 - / 0 2 couple (eq 19) deserves comment. It is obtained from the Eo values for aqueous solution (Table 3) where the half-reactions are
-
-
-
0, and
+ 2 H 2 0 + 4e- = 40H-
(20)
Redox Equilibria in Molten Silicates
0,
J. Phys. Chem., Vol. 99, No. 22, 1995 9193
+ 4H' + 4e- = 4H,O
(21)
Since 0,- ions do not appear in these equations, it might be argued that the expression for Emelt,based on the Eo data, does not correspond to the half-reaction, eq 19, for the melt (where oxide ions are involved). However, it must be borne in mind that eq 19 is a schematic depiction and that there are virtually no "raw" oxide ions present in the melt. The 02-ions undergo an effective degree of neutralization by being incorporated into the silicate network as nonbridging oxygen atoms (see earlier). The situation can be regarded in terms of severe polarization of the 0,- ion: by Si4+ cations in the silicate melt and by Hf ions in the H20 molecule or OH- ion. This is the viewpoint of the optical basicity model. Its application provides not only the facility for comparing different oxidic media but also the avoidance of such difficulties as those associated with handling oxide ion activities. Silicate compositions corresponding to soda-lime-silica glass have optical basicities in the range approximately 0.550.65. For A = 0.60, Figure 3 indicates (a shortened) electrochemical series: pt, Hg, Sb = Cu, Ag,
Sn, Pb, Co, Ni, Cd, Fe, T1, Zn
These metals are of interest to glass scientists from the point of view not only of corrosion, e.g., when used as container or electrode materials, but also with respect to the tendency for some of them to precipitate out from the glassy phase (see below). It is an important question whether the above series would have been expected on the basis of E" values in aqueous solution.30 Since glass melts are regarded as basic media, the choice is for E" values in alkaline solution (Table 3) which corresponds to the sequence Ag, Pt, Hg, T1, Cu, Pb, Ni = Sb, Co, Sn, Cd, Fe, Zn This is somewhat different from the sequence based on Emelt values given above. The proximity of the Cu2+/Cu,Agf/Ag, and R2+Ptcouples to the abscissa in Figure 2 (that is, the Emeltline for 02- 0 2 ) is consistent with the tendency, well-known in glass making, or copper, silver, and platinum to precipitate and to redissolve in the melt only under highly oxidizing conditions. These occurrences can be exploited to advantage for some applications, for example, in "striking" a copper ruby glass. However, they can be troublesome, as in the formation of liquid silver and lead and also the precipitation of platinum in phosphate glasses intended for laser focusing. Remarkable features of the relationships in Figure 2 are the rise in Emeltfor most couples and the trend toward convergence with increasing optical basicity. This is even more apparent if the singly charged metal ions Ag', T1+, and Cu+ are excluded. Although there is no obvious explanation for this, it might be significant that the melt basicity where the trends coincide is for a A value of approximately 0.74. This optical basicity value is between that for Na2Si03 and N a s i 0 4 (A values of 0.70 and 0.82, respectively), corresponding to the compositional range where the silicate network is fragmenting into single Si044units. The metal ions under these conditions are coordinated more and more by these units instead of being incorporated into the silicate network. It is worth noting that steel-making slags very often have optical basicities in this r e g i ~ n . ~ - l ~ Figure 2 is for silicate melts at 1400 "C because the relationships are based on experimental data for ion couples (Table 2 ) at this temperature. Since the equilibria involving these ion couples are temperature-dependent, it follows that the relationships for Eomeltwill differ from those in Table 3 for
-
different temperatures. The effect of temperature on the basicity of oxidic melts has received only limited study.31
Conclusions It is possible to link the redox behavior of metal ions in water and their behavior in a silicate melts at 1400 "C by regarding the two situations in terms of the optical basicity model. In both media, the metal ions are stabilized by having an immediate environment of oxygen atoms which provide a suitable electron density. A measure of this electron density is the optical basicity value, and this can be assigned regardless of whether the oxygen atoms exist in water molecules (at 25 "C) or in a molten silicate network (at 1400 "C). This facility provides the link between electrode potentials of ion couples in aqueous solution and their redox behavior in molten silicates. Once this link is established, it is possible to use it for predicting the electrode potential in a silicate melt (at 1400 "C) for any metdmetal ion couple whose standard electrode potentials in alkaline and acidic aqueous solution are available. The results indicate that the electrode potential depends on the melt composition in terms of its basicity and are supported by the limited experimental data available. They should provide a guide for the glass chemist dealing with problems of not only corrosion by glass melts but also the precipitation of elementary metals when their ions are present in the melt.
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