Original Paper
Voltammetric
investigations
soda-lime-silica
glass
in
Na2S04-refined
melts
Thomas Kordon, Christian Rüssel and Eberhard Freude^)
Institut für Werkstoffwissenschaften, Lehrstuhl III: Glas und Keramik, Universität Erlangen-Nürnberg, Erlangen (FRG)
The redox behavior of sulfate-refined soda-hme-sihca glass melts was investigated by the aid of voltammetric methods. Here the
possibilities for a quantitative in-situ determination of sulfur and iron are predominatly treated. Measurements in technical glass melts show
that a determination of iron in melts with low sulfate and high iron concentration such as amber brown or green glass melts is easily possible.
In contrast, there are problems at glass melts with low iron and high sulfate concentrations, because the current-potential curves are
predominantly affected by sulfur Compounds.
Voltammetrische
U n t e r s u c h u n g e n
an
Na2S04-geläuterten
Kalk-Natronsilicatglasschmelzen
Das Redoxverhalten von sulfatgeläuterten Kalk-Natronsilicatglasschmelzen wurde mit Hilfe voltammetrischer Methoden untersucht.
Hierbei standen die Möghchkeiten zur quantitativen in-situ-Bestimmung von Schwefel und Eisen im Vordergrund. Untersuchungen an
technisch erschmolzenen Gläsern zeigen, daß die Erfassung von Eisen in sulfatarmen und eisenreichen Schmelzen wie beispielsweise
Kohlegelb- oder Grünglasschmelzen ohne weiteres möglich ist. Im Gegensatz hierzu bereitet die Eisenbestimmung in sulfatreichen und
eisenarmen Weißglasschmelzen Probleme, da die erhaltenen Strom-Spannungskurven von Schwefelverbindungen dominiert werden.
1.
2.
Introduction
Many physical properties of glass products are
i n f l u e n c e d
by
the
o x i d a t i o n
S t a t e
of
the
g l a s s
m e l t .
The oxidation State is d e t e r m i n e d by the type and
a m o u n t of the polyvalent ions and the oxygen activity
of the glass melt. The oxygen activity can be
measured by sensors, based on oxygen ion-con­
ducting solid electrolytes (mostly yttria-stabilized
zirconia) [1 and 2]. During the last few years, some
other electrochemical methods hke Cyclic Voltam­
metry (CV) [3 to 5], chronopotentiometry [6] and
S q u a r e - W a v e
V o h a m m e t r y
( S W V )
[7
to 9 ] were
s u c c e s s f u U y applied to glass melts. T h e S W V proved
to have the highest sensitivity and resolution. In
principle the concentration of polyvalent ions can be
determined quantitatively by this method [10]. Up to
now, most voltammetric investigations were made in
glass melts, containing only one type of polyvalent
element [3 to 9 ] . Technical glass melts, however,
usually contain refining agents, which are also
polyvalent elements. The refining agent most com­
monly used is N a 2 S 0 4 . In this paper, the electro­
chemical behavior of N a 2 S 0 4 - r e f i n e d soda-lime-sil­
ica model glass melts and technical glass melts are
investigated. The aim of this paper is to provide a
method for the quantitative in-situ determination of
iron in N a 2 S 0 4 - r e f i n e d glass melts.
Received 28 November 1989.
1) Now with: Schumacher GmbH & Co. KG, Crailsheim
(FRG).
Theory
The redox behavior of polyvalent elements is usually
described by the equilibrium constant of the redox
reaction:
Κ{Τ)
^(n-z)+
+ ^
O2
(1)
with ζ = number of electrons transferred, O2 =
physically dissolved oxygen,
and
= poly­
valent elements in the reduced and the oxidized
State, respectively.
The equilibrium constant K{T) usually is meas­
ured by equilibrating the glass melt with a gas
atmosphere of weU-defined oxygen partial pressure.
Afterwards the sample is rapidly cooled and physi­
cally or chemically analyzed. In presence of two types
of polyvalent elements this procedure is not appli­
cable, because the equilibrium according to equation
(2) will normally shift during coohng, and the redox
ratio both in the glass melt and in the cooled glass wiU
not be the same:
_|_
Y
ßM+
^
J^IN-Z)+
_μ Y
ß{M+ZLY)+
^
(2)
Using voltammetric methods, the normal potential EQ
of a redox reaction according to equation (3), can be
directly measured against a zirconia/02 electrode:
£0
(3)
Table 1. Composition (in wt%) of the technically melted glasses
investigated
-600
-400
£ in mV
Figure 1. Square-Wave Vohammogram (SWV) of a glass melt
doped with 0.2mol%
FCiOg at
1100 °C
(r=10ms,
A£=100mV).
The equihbrium constant of the redox reaction,
according to equation (1), then can be calculated by
the aid of equation (4):
A G % T )
= -z
FEOIT)
= -RTln
K{T)
(4)
with A G \ T ) = Standard free enthalpy, F = Faraday
constant and R = gas constant. In the case of the
SWV, the apphed potential is a staircase ramp,
superimposed by a rectangular wave of comparably
high frequency ( / = 5 to 500 s~^) and amplitude (at
1000 ° C : A E = 50 to 250 mV). The current is meas­
ured at the end of every half-wave and then
differentiated [11 and 12]. In the presence of
polyvalent elements, characteristic peaks appear in
the SWV. The half-width of a peak LE^/2) decreases,
if more than one electron is transferred:
E,n=TBIz
(5)
where 5 = 0.30 mV/K, ζ = number of electrons
transferred.
The peak current /p depends on the surface area Α
of the working electrode, the bulk concentration of
the polyvalent ion Q , the diffusion coefficient D and
the number of electrons transferred:
C n · D^'^ ' z^ const
(6)
for smaU AE: const = 0.31 · F^ AEIT^'^R
T with
τ = pulse time.
In the case of CV the potential is scanned from
an initial potential E^ with a constant rate to a
switching potential Εχ and then scanned back again
with the same rate [7 and 9]. In the case of a
completely reversible electrode reaction, the
current-potential curve has a cathodic peak, at
EQ - 1.1 R T/Z F and an anodic peak at
EQ+1.1RT/ZF.
glass no. 1
glass no. 2
amber brown green
glass no. 3
white
glass no. 4
white
Si02
AI2O3
Ti02
MgO
CaO
BaO
Na20
K2O
70.8
0.95
0.024
3.55
10.60
73.4
1.08
0.039
2.62
7.3
0.46
13.8
1.09
70.8
0.75
0.019
4.15
9.65
€Γ2θ3
-
-
-
Fe203
sulfur
-
13.70
0.25
0.138
0.00612)
2) calculated as S^";
3.
71.32
1.85
-
2.65
11.06
0.05
11.60
0.83
0.211
0.35
0.073)
0.025
0.253)
-
13.90
0.30
0.097
0.253)
3) calculated as SO3.
Experimental
All experiments were carried out in a resistanceheated (SiC) furnace with a vertical alumina muffle
tube and water-cooled flanges at the top and at the
bottom. Approximately in the middle of this tube a
platinum crucible with the molten glass was located.
Three electrodes were inserted from the top flange
and dipped into the glass melt. The working electrode
is a platinum wire (diameter : 0.5 mm), the counter
electrode is a platinum plate with the size of about
2cm^ and the reference electrode consists of a
zirconia probe, described in principle in [1, 2, and 8].
The inner part of the reference electrode is flushed
with air, aU mentioned potentials in this paper are
referenced to the zirconia/air electrode. To achieve a
reproducible working electrode area, the crucible was
adjusted in that manner that the conductivity be­
tween working and counter electrode was 140 mS.
Electronics are seifconstructed; the main part, the
potentiostat, controlhng the potentials between the
electrodes, is connected to a microcomputer via a
digital/analog and an analog/digital Converter. In that
way, any potential-time dependence can be given.
The microcomputer also records the current and
passes the values to printer, plotter and floppy disk.
Some experiments were carried out in a model glass
melt with the basic composition (in mol%) of
74 Si02, 16 Na20, 10 CaO. This glass was powdered
and mixed together with Na2S04 and in a part of the
experiments with another model glass of the com­
position (in mol%): 72 Si02, 16Na20, 10 CaO and
2 Fe203.
In Order to minimize the sulfur volatilization, a
crucible with the powdered glass was brought into a
furnace which already was preheated to a tempera­
ture of 1200 °C.
Another series of experiments was carried out
in technical soda-lime-silica glass melts, the
chemical composition of which is summarized in
table 1.
>
e
c
1300
Figure 2. SWVs of a glass melt refined by 0.4 mol% Na2S04 at
1200 °C with increasing pulse time r as parameter (AE = 100 mV).
Curve 1: τ = 1 ms, curve 2: τ = 2 ms, curve 3: τ = 5 ms, curve 4:
τ = 10 ms.
4.
4.1.
Figure 3. Peak potential E^ of the first cathodic peak from figure 2
as a function of the temperature in a glass melt refined by 0.4 mol%
Na2S04.
Results and discussion
Voltammetry in model glass melts
Figure 1 shows an experimental SWV of a glass melt,
doped with 0.2 mol% Fe203, recorded at 1100 °C.
One broad potential peak is visible at about
-550 mV. It is related to the reduction of the Fe^"^ to
the Fe^"^ ion, according to equation (7):
<
Ε
-1000
Fe3+ + e -
Fe,2+
The shape of the experimental curve in figure 1 is
approximately equal to the theoretical curve [7].
Figure 2 shows SWVs, recorded at 1200 °C from a
glass melt refined by 0.4 mol% Na2S04, containing
no other polyvalent element. Two potential peaks can
be observed: one peak at about -380 and another
one at about -590 mV. The intensity of these peaks
strongly depends on the pulse time. For a short pulse
time (τ = 1 ms), the first peak intensity (Ii) is higher
than the second one {I2), but for increasing pulse
times, the ratio I1/I2 decreases. This behavior is in
contrast to equation (6), but the same behavior was
already observed in glass melts doped with arsenic or
antimony oxide. Although up to now this is not yet
understood, for analytical apphcations it might be
useful to suppress a certain peak by varying the pulse
time. The chemical interpretations of the obtained
voltammograms are much more difficult than in the
case of glass melts doped with iron. In principle,
sulfur might be present in the oxidation states with
the following valencies: +6, +4, 0, and - 2 and the
electron transfer reactions (equations (8 to 10)) can
be suggested as:
S O ^ + 2 e - <:± SO2 + 2 O^-
(8)
SO2 + 4 e" ; ^ S^ 4- 2 O^" ,
(9)
+ 2 e - ; ^ S^- .
-600
(10)
The highest oxidation State (SO4 ion) is stable in a
glass melt at comparably low temperatures and
-400
-200
0
£ in mV
Figure 4. Comparison of an experimental SWV with theoretical
ones.
: experimental SWV of a glass melt refined by
0.4 mol% Na2S04 at 1200 °C (τ = 1 ms, AE = 100 mV);
:
theoretical SWV of a two-electron step at 1200 °C;
:
theoretical SWV of a four-electron step at 1200 °C.
-600
-400
£ in mV
Figure 5. Cychc voltammogram of a glass melt refined by 0.4 mol%
Na2S04 at 1200 °C with t; = 5 V/s.
decomposes during the refining process. Then the
reaction, according to equation (11) is shifted to the
right:
SO^
S«
-800
(7)
τ±
S02 + 1/2 θ2 + Ο,2-
(11)
SmaU amounts of SO2 remain physically dissolved in
the glass melt. Figure 3 shows the temperature
dependence of the first peak potential (-380 mV at
1100 °C, see figure 2). Α steep decrease of the normal
potential with increasing temperature can be ob-
served. But even if it is extrapolated to 1400 °C, the
normal potential wih stih be as low as -200 mV.
Therefore, the redox reaction, related to this peak,
cannot be the reduction of SO3 or the S04~ ion to SO2
<
ε
because the refining process takes place at far lower
temperatures. At these refining temperatures, the
normal potentials, related to equation (8) must
already possess a positive value and therefore, cannot
be observed by SWV [7 and 9]. Figure 4 shows
theoretically calculated SWVs for a four- and a
-1000
two-electron step. In comparison with a SWV of a
Na2S04-refined glass melt, it is obvious that the
Figure 6. SWVs of a glass melt refined by 0.4mol% Na2S04
observed first reduction step is a four- and not a
at 1200 °C (τ = 5 ms, AE = 100 mV) and doped with (amounts in
two-electron step. The right side of the experimental
mol%): curve 1 = 0.04 Fe203; curve 2 - 0.1 Fe203;
curve 3 = 0.2 Fe203.
SWV almost completely matches the theoretical
SWV of a four-electron step. Therefore, it can be
concluded that the observed reduction step is related
to equation (9), the reduction of SO2 to S^.
Consequently, the second reduction peak (see fig­
ure 2) should be related to the reduction of S^ to the
S^~ ion according to equation (10). Possibly, the
dependence of the peak currents on the pulse time
can be explained by the formation of S2-containing
gas bubbles during the first reduction step. Then at
high pulse times, more S^ is formed and the
electrodes could be partially blocked by the gas
bubbles.
Figure 5 shows a CV of a Na2S04-refined glass
meh. The potential was scanned with a constant rate
υ (5 V/s) from 0 to -800 mV (cathodic scan) and then
immediately scanned back again with the same rate
(anodic scan). At about -280 to -380 mV the
cathodic current increases sharply, due to the reduc­
tion of SO2 to S^. Another increase is observed at
more negative potentials, due to the reduction of S^
to the S^" ion. At the anodic scan, the S^~ ion is
oxidized again to S^ and at higher potentials to SO2.
The Sharp decrease of the anodic current at potentials
higher than -300 mV is another hint that gaseous S2
is formed and located at the electrode surface. If S2 or
other species were physically dissolved in the glass
meh, the decrease of the current would be controUed
by diffusion and follow a 1/V^ rate law.
Figure 6 shows SWVs of a glass melt, refined by
0.4mol% Na2S04 and doped by three different
amounts of iron oxide. The Upper curve was recorded
in a glass melt containing 0.2mol% Fe203. At a
potential of -550 mV, a broad peak related to Fe203
was observed and at around -380 mV a Shoulder
related to the reduction of the S"^"^ ion was visible. At
an iron concentration of 0.1 mol% Fe203, the iron
-600
-400
-1000
-800
peak is remarkably lower, and at the reduction
£ in mV —
Figures 7a to d. SWVs of technical glass melts recorded at 1200 °C, potential of the S"^"^ ion, a weU-pronounced peak can
a) amber brown glass (glass no. 1 from table 1) with r = 20 ms and be observed. The shape of the Fe-^"^ reduction peak in
AE = 100 mV; b) green glass (glass no. 2 from table 1) with
both curves seemed to be not drastically influenced
Γ = 20 ms and Δ£ = 100 mV; c) white glass (glass no. 3 from
by the current, related to the reduction of S^. At
table 1) with τ = 20 ms and AE = 100 mV; d) white glass (glass
further decreasing iron concentration (0.04 mol%
no. 4 from table 1) with pulse time as parameter and
^^2^3), only a very broad peak is visible in the
AE = 100 mV, curve 1: r = 1 ms, curve 2: τ = 2 ms, curve 3:
potential region from -500 to -650 mV and in
τ = 5 ms, curve 4: r = 10 ms, curve 5: τ = 20 ms.
comparison with the two upper curves, the peak
seems to be shifted to lower potentials. The peak,
related to the reduction of the Fe^"^ ion is not
weh-separated from the peak related to the reduction
to the
ion. It should be remarked, that a Fe203
concentration of 0.04 mol% is obviously too low to be
detected in a sulfate-refined glass melt by SWV at
1200 °C, but it should be possible to detect such a low
iron concentration near the surface of a technical
glass melt, because of the lower sulfur concentration,
due to the volatilization of the sulfur Compounds.
from a glass without iron doping (figure 2). At pulse
times of 10 and 20 ms a slight broadening of the
second reduction peak can be observed. Especially at
a pulse time of 20 ms, the currents at potentials lower
than -500 mV are remarkably increased. In compa­
rison with the SWVs of a glass melt doped with iron
(figure 6), this effect should be due to the small iron
concentration of glass no. 4 (see table 1). But it seems
to be rather difficult to utilize this small broadening
for a quantitative determination of iron.
5.
4.2.
Voltammetry in technically melted glasses
Several technically melted glasses were investigated
by SWV. The glass compositions are summarized in
table 1. Figure 7a shows the SWV of an amber brown
glass melt (glass no. 1, see table 1) with a comparably
high concentration of iron oxide (0.138 wt% Fe203)
and very low sulfur concentration (0.0061 wt% S^").
The shape of the voltammogram completely matches
the current-potential curve, recorded from a model
glass, solely doped with iron. Obviously the curve is
not influenced by the small sulfur concentration, but
it should be remarked that the peak is shifted to
higher potentials in comparison with figure 1. It is
assumed that this effect is due to the different
compositions of the glass matrices.
Figure 7b shows a SWV of a green glass melt
(glass no. 2, see table 1) with a very high iron
concentration (0.35 wt% Fe203) and a sulfur con­
centration of 0.07 wt% SO3. The broad potential
peak at -450 mV is related to the reduction of the
Fe-^"^ ion. But also a smah potential Shoulder at
-340 mV can be seen. In comparison with figure 2, it
can be c o n c l u d e d t h a t this S h o u l d e r is r e l a t e d to
sulfur and is due to the reduction of SO2.
Figure 7c shows a voltammogram of a white glass
melt (glass no. 3, see table 1) with a low iron
concentration (0.025 wt% Fe203) and a sulfur con­
centration of 0.25 wt% SO3. Two weh-pronounced
potential peaks at -350 and -510 mV can be seen.
The peaks are comparably sharp and the shape of the
whole voltammogram is very similar to those shown
in figure 2. The shape of the curve seems to be not
influenced by the presence of Fe203 in the glass melt,
since even no broadening of the second reduction
peak at -510 mV could be observed.
Figure 7d shows SWVs of a technically melted
white glass (glass no. 4, see table 1) with a compa­
rably high iron concentration (0.097 wt% Fe203) and
a sulfur concentration of 0.25 wt% SO3. The curves
are quite similar to those in the figures 2 and 7a. The
SWVs in figure 7d were recorded at different pulse
times to investigate whether the influence of iron can
be observed at lower pulse times. At pulse times of 1,
2 and 5 ms no remarkable changes in the shape of the
curves can be seen in comparison with those recorded
Conclusions
Sulfur in soda-lime-silica glass melts can easily be
detected by SWV. Two characteristic potential peaks
are visible. If the glass melt contains both, sulfur and
iron, a quantitative determination of iron is much
more difficult than in solely iron-containing glass
melts, because the second reduction peak, related to
sulfur and the Fe^"^ reduction peak appear at nearly
the same potential and, therefore, they are superim­
posed by each other. Α quantitative determination of
iron is possible in amber brown and in green glass
melts or generally in every model glass melt or
technical glass melt if the concentration of iron is high
in comparison with the sulfur concentration. In glass
mehs with high sulfur and low iron concentrations
such as the white glass melts investigated, a quanti­
tative determination seems to be nearly impossible at
the moment. But possibly, future improvement of
voltammetric techniques could enable a quantitative
determination of iron oxide in these glass melts,
too.
These investigations were conducted with the kind support of
the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen
(AIF), Köln, by agency of the Hüttentechnische Vereinigung der
Deutschen Glasindustrie (HVG), Frankfurt am Main, through the
resources of the Bundesministerium für Wirtschaft.
6.
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90R0842