Solid-State Detectors for the Potentiometric Determination
of Gaseous Oxides
I. Measurement in Air
M . Gauthier* and A. Chamberland
Hydro-Quebec Research Institute, Varennes, Quebec JOL 2PO, Canada
ABSTRACT
This paper suggests that the potentiometric detection of sulfur oxides i n air
is feasible with solid-sulfate electrolytes, a n d presents e x p e r i m e n t a l results on
the electrode Pt, SO2, O2/SO4 =. Several e x p e r i m e n t a l parameters, namely,
Ps02, temperature, and flow raSe, are studied with concentration cells using
potassium sulfate, as the electrolyte. A solid reference electrode based on an
A g / A g + electrochemical couple is also studied as a replacement for the gas
reference electrode. The autaors conclude that rapid conversion of several
gaseous sulfur compounds, namely, H2S, CHsSH, etc., into oxides occurs u n d e r
the cell operating conditions, and that rapid e q u i l i b r i u m is reached b e t w e e n
SO2, SOs, and 02 at the electrode. P r e l i m i n a r y results of tests using solid electrolytes to detect other gaseous oxides in air are also given for the Pt, COs,
O ~ I C O s =, and Pt, NO2, O2/NO3- systems.
Success with solid-state oxygen detectors based on
stabilized zirconia i n the p p m concentration r a n g e has
led the authors to u n d e r t a k e research into other types
of detectors for the m e a s u r e m e n t of a m b i e n t and i n dustrial pollutants such as s u l f u r dioxide, carbon
monoxide, carbon dioxide, and the different nitrogen
oxides. Originally, potentiometric r a t h e r t h a n polarographic methods of analysis were selected to avoid
relying on high ionic conductivity electrolytes, which
are scarce. F u r t h e r m o r e , this choice dispenses with
the complex process of d e t e r m i n i n g the anionic-tocationic n a t u r e of the conductivity, and all that remains to be done is to ensure there is no electronic
conductivity at all. I n the selection of, suitable electrolytes, attention can then be focused on the possible
electrode reactions.
Several studies of electrode reactions b e t w e e n
m o l t e n oxyanions and oxides, such as n i t r a t e and sulfate electrodes, m a y be found i n the l i t e r a t u r e (1, 2).
The mechanisms involved are still disputed, moreover,
because of their complexity and dependence upon several e x p e r i m e n t a l parameters.
The initial step in this research was to try to adapt
previous studies on fused-salt systems to solid-electrolyte techniques using sulfate and sulfur oxides. A systematic study was made of a solid electrolyte of potassium sulfate with sulfur dioxide as the gaseous oxide.
The tests were based on the following reaction mechanism as described by Salzano for the SOs, O2, P t /
SO4 -2 electrode (2).
89 O3 4- 2 e - ~
0 -2 (electrochemical reaction) [1]
SOs 4- O -3 ~---SO4 -2 (chemical reaction)
Since sulfur dioxide is the m a j o r pollutant, and is
easier to h a n d l e i n a laboratory, it can be used to replace S Q i n the experiments. I n this case, either the
equilibrium
SO~ ~ SO2 +
K =
O2
[7]
(Pso2) (Po2) 1/3
[7']
(Psos)
must be reached rapidly or the SO2 to SOa conversion
must be small and occur at constant r a t e for a given
gas flow, whatever the initial SO2 concentration, in
accordance with Salzano's i n t e r p r e t a t i o n of his own
laboratory observations (2). Taking this into account,
the following expression for SO3 is obtained
E ----
RT
Po2
RT
Po2'
2F
'in ...... +
2F
Pso2
In ~
[8]
Psog.'
which, in air, simplifies to
E --
RT
Pso2
In ~
2F
[9]
Pso2'
Experimental
Different models of oxide concentration cells have
been devised and studied e x p e r i m e n t a l l y ; a general
device is illustrated schematically in Fig. 1. The arr a n g e m e n t comprises two a l u m i n a tubes b e t w e e n
which the potassium sulfate pellet is pinned. Two small
single bore a l u m i n a tubes allow the gas to circulate
[2]
These two reactions can be combined to yield the overall equation
S03 4- 8902 4- 2e- ~ S(~4-2
[3]
89
with
s
eJectrolyfe
Gold seal
Sobd
electrolyfe
s ve
Silver
on~'~.....
ons
AI
no
uml
ereetrode
Pf wire
seal
The resulting Nernst equation for the following cell
SOs, Pt/K2SO4/Pt, SOs'
0~.
0~'
Type A
Type B
[4]
is
E =
RT
2F
(Po2) 1/~ Psos
In
[5]
(Po2') 1/~ Pso~'
or
RT
E
=
4F
Po2
In
,,, 4-
P03'
RT
2F
I
Pso8
In
, ,
~-
_
I
i
Teflon stopper
Gas ouffet
[6]
Pso3'
9 Electrochemical Society Active Member.
Key words: solid electrolytes, emf measurements, SOG-SO~-O2
platinum electrode, sulfates, sulfur dioxide analysis.
Fig. 1. Experimental apparatus used to study the SO2, 02, Pt/
SO4-- system. A, two-compartment cell; B, cell with a silver/
silver ion reference electrode.
1579
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J. Electrochem. Sac.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y
1580
n e a r the m e a s u r i n g electrode. P l a t i n u m w i r e and p l a t i n u m p o w d e r obtained b y v a c u u m deposition ensure
electrical contact. In the hot zone, the seal is m a d e b y
pressing gold rings b e t w e e n the a l u m i n a tubes and
the electrolyte while conventional O-rings are used
for sealing outside the furnace.
A n optimized d e v i c e d e r i v e d from the first one (Fig.
1A) was used, in p a r t i c u l a r , for response time and
flow rate studies. As i l l u s t r a t e d in Fig. 2, the two comp a r t m e n t s h e r e are located on the same side of the
electrolyte. The seal is m a d e b y a p p l y i n g a p p r o p r i a t e
pressure b e t w e e n the a l u m i n a tube a n d the electrolyte
at high t e m p e r a t u r e before t h e first utilization. The
resulting slight d e f o r m a t i o n of the electrolyte ensures
gas tightness b e t w e e n the two c o m p a r t m e n t s and be=
t w e e n each c o m p a r t m e n t and the outside. The dead
v o l u m e of the complete gas circuit inside the sensor is
a r o u n d 0.8 cm ~ including t h e f o u r - h o l e a l u m i n a rod
(998 McDannel, 0.250 in. OD, 0.067 irL ID) and the 316
stainless steel gas feedt'hrough, 1/16 in. OD. P o w d e r e d potassium sulfate 100-mesh ( B a k e r analyzed,
r e a g e n t grade) was used for the electrolyte because
this nonhygroscopic salt becomes a r e l a t i v e l y good
conductor in the h e x a g o n a l form above 579~
(3),
and can be easily sintered. The p o w d e r was subjected
to unidirectional p r e s s u r e of 2 t o n / c m 2, then sintered
at 940~ for at least 2 days. The pellets thus obtained
can hold 760 m m Hg pressure difference for hours
without a p p a r e n t l e a k a g e both at room t e m p e r a t u r e
and, once fitted in the sensor, at operating t e m p e r a t u r e .
The gas m i x t u r e s are p r e p a r e d b y injecting p u r e oxides into 1O and 100 l i t e r M y l a r bags a l r e a d y filled
with a m b i e n t or synthetic air; the precision for this
technique is a p p r o x i m a t e l y 3%. The SO~ m i x t u r e s a r e
then stable for the d u r a t i o n of the test. The t e m p e r a t u r e is m a i n t a i n e d at _+ 2~ b y a Thermoelectric p r o portional control, Model 100, w i t h C h r o m e l - A l u m e l
thermocouples. A Tacussel Aries 10,000 high i m p e d a n c e
( ~ 101~) m i l l i v o l t m e t e r is used to m e a s u r e the electromotive force ( e m f ) , and signals are r e c o r d e d on a
H e w l e t t - P a c k a r d strip c h a r t recorder, No. 7100-B.
Results and Discussion
Different tests w e r e m a d e using the a p p a r a t u s i l l u s t r a t e d in Fig. 1A and 2 w i t h sulfur dioxide and
potassium sulfate, at t e m p e r a t u r e s r a n g i n g b e t w e e n
600 ~ and 925~ Unless otherwise specified, the r e f e r ence electrode is a m i x t u r e of 100 p p m SO2 in air, and
all gas flow rates a r e fixed at 100 cmS/min.
Emf variation with S02 partial pressure.--The emf
time plot w i t h different values of the SO2 p a r t i a l p r e s sure is given by w a y of e x a m p l e in Fig. 3. The concentrations given in this and the following figures correspond to the initial p a r t i a l pressures obtained b y
injecting SO2 at room t e m p e r a t u r e , (Pso2)im The r e sults p r e s e n t e d in this figure w e r e obtained w i t h the
a p p a r a t u s i l l u s t r a t e d in Fig. 2.
G e n e r a l l y speaking, the response times v a r y from a
Iew seconds to several minutes, m a i n l y according to
Gaz inlets and
I
+300 t
t
I
I
I
I
I
I
-
.,~------~0000 ppm
.,o%o I\
~O000ppm
L~ppm
.200~
.~
/
;,~176
F
~
lOOppm
-200
,/ Ippm
-300 -
I
2O
40
I
Time (min.)
I
I
60
I
I
80
t
100
Fig. 3. Typical response time to S02 variations obtained with the
experimental device of Fig. 2. Temperature, 820~ reference electrode, 100 ppm S02 in air; gas flow rates, 100 cm~/min.
the flow rate, ~he concentration range u n d e r study,
and the g e o m e t r y of the apparatus. The chemical
properties of the sulfur oxides m a k e it e x t r e m e l y
complex to p e r f o r m a d e t a i l e d analysis of the p r o c esses d e t e r m i n i n g the rise and fall of the e m f signal.
A d s o r p t i o n definitely has a significant influence, especially in view of the wide v a r i a t i o n of the concentration u n d e r study (over 4 decades).
Typical e x p e r i m e n t a l results w i t h the SO2 detector
a r e p r e s e n t e d as emf vs. log ( P s o 2 ) i n . in Fig. 4. The
e x p e r i m e n t a l slope coincides w i t h a t w o - e l e c t r o n r e action (n = 2.12) in accordance w i t h Eq. [3]. E x pressed in parts p e r miliion (I0-6 a r m ) , t h e concentration r a n g e extends from 0.5 to 40,000 ppm. The
e x p e r i m e n t a l signal obtained at lower concentrations
coincides w i t h the l i m i t of g a s - m i x t u r e p r e p a r a t i o n
by microinjection techniques, which means that it is
impossible to d e t e r m i n e the lower operating limit of
the SO2, SO8, 02, Pt/SO4 -2 electrode in air using this
p a r t i c u l a r m e t h o d of p r e p a r a t i o n .
Era] dependence on temperature.--Experimental and
calculated values of emf for a concentration cell filled
with SO2 in air, n a m e l y 10 p p m on one side and 1O0
p p m on the other, at different t e m p e r a t u r e s b e t w e e n
600 ~ and 925~ are r e p o r t e d in Fig. 5. Below 650~
e x p e r i m e n t a l emf values a r e e r r a t i c and decrease
r a p i d l y c o m p a r e d w i t h t h e c a l c u l a t e d values. H o w I
]
I
I
]
/)"
z///
200
iO(]
~
/ oo..,~ \
I
October 1977
,/~/'/'/'/'Z/
9
o
w
Ste'nesssleetubes~ \ \\
~Set screw
~
// // I I
////
//
Ploti
.... lectmdes
\ ~,-~lid electroh'topellet
Experiment
-'100
/~///~'~" Calculated
,,
J~!
~:-r.~ ;/
~
- 200
-7
LCoil sprlng ~ ' T ...... ,"
\, 'I
- ~.hol:: , . . ~
A1urnin,umgu,de!' ~-Slld,ng:';(:'::ef:: e
/ Q. . . . . . t
LAI....... pport
I:ig. :~. Simplified experimental device with a reduced dead volume and both gas circulating compartments on the same side of
the electrolyte.
J /
"//~//
//
/I
-6
10ppm
~
~
-5
100ppm
v~
l
I
-4
-5
log4o ( Pso2)in. (at m.)
I
-2
-~
Fig. 4. Plot of emf vs. logarithm of the SO~ partial pressure
expressed in atmospheres. A concentration of 100 ppm SO2 in
air was circulated in the reference compartment. Temperature,
820~ flow rate, 100 cm3/min in both compartments. The dashed
carve represents the calculated slope 2.3 RT/2F.
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Wol. 124, No. 10
-120
I
[
l
I
I
I
1
ditions, t h e r m o d y n a m i c e q u i l i b r i u m b e t w e e n SO2 and
SOs is reached rapidly, at least in the vicinity of the
p l a t i n u m electrode. This result differs from ~hose obtained by Salzano with m o l t e n sulfates i n which he
observed a 55 mV dependence for each variation of 1
decade i n the flow rate. It therefore seems justified to
use Eq. [7] and [7'] without a n y need to consider a
fixed SO2-SOs conversion rate as Salzano did (2).
U n d e r e q u i l i b r i u m conditions, therefore, the detector
validates the equation
[
"1"10
Calculated
E=R'F In I00
2F
10
/
Experimental
(Pso2)~-. = (Pso2) + ( P s o s )
~00
E
u_
l.U
9(?
80
i
70
(9
I
I
400
1581
SOLID-STATE DETECTORS
I
600
I
I
800
I
I
t000
I
T(~
Fig. 5. Plot of emf vs. temperature (~
Measuring compartment, 100 ppm S02 in air; reference compartment, 10 ppm S02.
in air; flaw rate, 100 cm3/min in bath compartments.
ever, above this temperature, the emf values agree
w i t h i n _~ 1 mV. Special care was t a k e n i n this test
to control and measure the t e m p e r a t u r e of the electrolyte; nevertheless, a bias potential of 1.2 _ 0.2 mV
was found b e t w e e n the two electrodes as a single p r e p aration of 10 ppm SO2 i n air was circulated on both
sides of the cell, a value which corresponds to the difference b e t w e e n the e x p e r i m e n t a l and calculated emf
found i n Fig. 5. Application o f the emf technique (4)
to the results of Fig. 5 shows that the ionic transport
n u m b e r , t~, in potassium sulfate is one w i t h i n the p r e cision of the present experiments.
Emy dependence on flow rate.--Various tests to det e r m i n e the influence of the flow rate are reported in
Fig. 6, which shows the effect of the concentration, the
temperature, and the presence of a catalyst w h e n the
flow rate is varied from a few cm3/min to 500 cmS/
min. It m a y be noted that a n operating range exists
in which the emf almost ceases to vary with the flow
rate. F r o m this, it was concluded that u n d e r such con-
[10]
where (Pso2)m. is the gas partial pressure initially
introduced, and (pso2) and (Pso3) are the partial pressures in e q u i l i b r i u m at the electrode. Thus, the detector signal is equivalent to an over-all m e a s u r e m e n t of
the q u a n t i t y of sulfur oxide regardless of w h e t h e r the
gas is introduced as SO2 or SOs.
The range of flow rates i n which the sulfur-oxide
detector operates at e q u i l i b r i u m depends on several
parameters; its geometry, the types of m a t e r i a l used,
operating conditions, etc. At high flow rates (Fig. 6)
the r a n g e is limited, p a r t i c u l a r l y with strong SO2
concentrations or w h e n the t e m p e r a t u r e drops to
around 700~ This m a y be explained by the fact that
the conversion of SO2 into SO3 no longer reaches equil i b r i u m value u n d e r these conditions. Moreover, this
seems to confirm the catalytic n a t u r e of the p l a t i n u m
wire of the electrode w h e n placed at the inlet rather
than at the outlet of the test gas. Meanwhile, at low
flow rates, the limit could be a result of convection
competing with the forced circulation of the gas,
which could result in condensation (SOs or HzSOd)
in the cold zones of the detector. This would n a t u r a l l y
modify theSO2-SO3 mixture. Only devices with zero
or u n d e r 10 mV emf dependence on the flow rate were
used for the studies described here.
Solid reference electrode.--For reasons of convenience, one of the two gas electrodes of the cell [4]
was replaced by a solid-state reference electrode such
as A g V A g + i n which the silver ion was present as a
solid solution. The following e x p e r i m e n t a l cells were
studied
Ag~
in K2SOd//K28Od/SO2, 02, P t
[Ii]
and
Ag~
in K2804/SO2, O2, P t
[ll']
whose over-all cell equation is
Ag ~ + SO2 + O2 ~ Ag2SO4 (as a solution i n K2804)
[12]
The emf of cells [11] and [11'] can be expressed as
RT
RT
RT
E = const. -- - - F i n SAg+ -~- - ~ - In Pso2 -t- ~
i n Po2
[13]
I
I
t"O O O- p p- -m _ _ _ i
+ iO0
E
I
Pt wire into gas inlet
T
Pt wire into gas outlet
100ppm
S
0
(8i66C)
(S~6~
o
-I00
t
where const, is a constant that includes the E ~ of pure
solid silver sulfate. This expression of the emf is
valid as long as there is a solid solution of AgzS04 in
K2804. I n air and at constant silver-ion activity, aAg+,
the emf depends only on the SO2 partial pressure
(816~
RT
E -- const.' + ~
"10ppm
I
t00
I
I
200
300
Flow rate ( c c / m i n )
400
500
Fig. 6. Emf dependence on flaw rate at three different $02 parrio| press,res; 10, 100, 1000 ppm. Atso illustrated here is the
effect of the operating temperature and platinum catalytic activity on the emf dependence on flow rate.
In Psoa
[14]
A modification to the earlier e x p e r i m e n t a l a r r a n g e ment made it possible to use the s i l v e r / s i l v e r - i o n
reference electrode for the tests (Fig. 1B).
Emy dependence on (Psoz)in. with A g ~
+ as the
reference electrode.---Signals obtained at different SO~
partial pressures are presented in Fig. 7. The silversulfate concentration was prepared at a 1% mole fraction by melting k n o w n proportions of silver sulfate and
potassium sulfate, ball-milling them to 100 mesh, and
then sintering. The results are in good agreement with
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J. Electrochem. Sot.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y
1582
500
400
300
I
I
S02
HzS
~ CH3SH
o COS
~
I
[
600
I
[
x = Ag + 0,t0(0.2t)%
I
mol.fract
9
9
4-
~ 200 r
<[
82ooc
/~,/C~
+ : ,, 0 . 5 2 ( 0 . 7 5 ) %
,,
"
9= .
,,
.
A :
500
.//
/v
>
E
m
[
October 1977
O=
0.98(0.98)%
,, 4.00 ( 1 . 9 5 7 ) % , ,
" t5.64(1.20)%
"
/
/
"
"
//
/
•
400
>
E
+/+/N0 Z (481~
0 L-
uJ
/ / + / x/xJ .CO z (726 o C)
W
300
/4: */
+/++/~•
-100 -
/ /
-200 -
200
500
-7
/+///
/ I/~
/(//
u_
I
-6
I
-5
I
I
I
-4
-3
-2
Iog~o(P x )in (etm.)
/r
//0
I
-1
o//~/
0
Fig. 7. Dependence of the emf on the logarithm of the concentration of different anhydrides, For SO~, HzS, CH3SH, and COS,
a
K2S04 electrolyte is used white K2CO~ an4 Ba(N03)~. electrolytes ere used for C02 and NO2, respectively. In all cases, results were obtained with Ag/Ag + reference electrodes (see text).
relation [14]. Cells [11] and [11'] both present the
same behavior.
Emf dependence on aA~+.--In order to check the
validity of Eq. [13] for the Nernst dependence of the
signal on the silver-ion activity, an attempt was made
to study different concentrations of silver sulfate in
the electrolyte fixed during the preparation, and r a n g ing from 0.1 to 15.0% mole fractions. For each of the
5 silver-sulfate concentrations, a cell similar to [11']
was prepared and studied from the point of view of
the SO2 partial pressure (Fig. 8). Also included in ~his
figure are the results of an atomic-absorption analysis
of the silver-ion concentrations in the electrolyte after
each test, compared to the values fixed d u r i n g p r e p aration. A large difference b e t w e e n prepared and observed concentrations is observed, p a r t i c u l a r l y for the
two higher prepared concentrations where metallic
silver was visible after sintering at 900~
This is
probably a result of partial decomposition of the silver
sulfate.
W h e n the emf values of Fig. 8 are plotted as a f u n c tion of the logarithm of the silver-ion mole fraction
for a given (Pso2)in. of 3 X 10 -4 arm in air, the results
appear to be in good agreement with the slope calculated from Eq. [13], Fig. 9. The silver-sulfate concentration used is those of the atomic-absorption analysis.
Consequently, even if the concentration range studied
is in fact limited, it may be reasonably concluded that
an authentic solid solution of silver sulfate exists in
potassium sulfate, at least around the 1% mole fraction.
The tests reported here do not eliminate the possibility that the s i l v e r / s i l v e r - i o n electrode reaction is
just one of several electrochemical reactions; indeed,
others such as the O2/O = couple m a y well exist at
these temperatures. Nevertheless, this solid reference
electrode greatly simplifies the e x p e r i m e n t a l procedure
and, as shown by the tests, provides satisfactory results.
Emf dependence on other sulfur compounds.--The
solid reference cell used with SO~ was also tested
u n d e r the same e x p e r i m e n t a l conditions with other
s u l f u r - c o n t a i n i n g compounds such as hydrogen sulfide,
carbonyl sulfide, and methyl mercaptan. The results,
100.
-6
I
-5
I
-4
(Pso2) in (arm)
I~
I
-3
-2
Fig. g, Dependence of the emf on the logarithm of the S02
concentration in air for different mole fractions of Ag2S04 in
K2S04. Given in parentheses are the concentration values obtained
by atomic absarption analysis after the test. Test temperature,
820~ fixed flow rates, 100 cm3/min.
I
I
I
I
I
I
I
500
450
\
4OO
E
u_ 350
\
\
\
\\\
/ 2.303 RT
\
Ld
300
\
\
%
\
\
\
o
250
\
\
\
\
200
-30
I
-s
I
I
I
I
-E.6
-2.4
-E,2
-2.0
Log4o Aq+{mole fract.)
l
-1.B
I
-1.6
Fig. 9. Dependence of the emf on the logarithm of the silver
ion concentration in K2S04 for a constant SO~ concentration in
air; 3 X 10 -4 arm. The silver-ion concentrations used here were
obtained by atomic-absorption analysis. The dashed curve shows
the calculated value of 2.3 RT/F in accordance with Eq. [13].
Temperature, 820~ flow rate, 100 cm3/min.
shown i n Fig. 7, are almost identical to those with
SO2, whatever gas enters the cell. In fact, even by
v a r y i n g the temperature, t'he concentration, or the flow
rate of these gases, it was impossible to distinguish between the signals they produced and the signal from
an equivalent concentration of SO~. It m a y therefore
be assumed that in air these gases are rapidly oxidized
and quantitatively converted into sulfur oxides at the
electrode. This conclusion is borne out by the existence
of excessive quantities of oxygen compared with the
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Vol. I24, No. I0
SOLID-STATE DETECTORS
test gases, by the high operating t e m p e r a t u r e of the
sensor, a n d also by the presence of p l a t i n u m at the
electrode.
Tests on other oxide-oxyanion systems.--The last
e x p e r i m e n t a l results to be presented are those of tests
conducted on other o x i d e - o x y a n i o n systems to see
how far the application of solid-state electrochemical
cells is feasible for potentiometric d e t e r m i n a t i o n of
different gaseous oxides. The e x p e r i m e n t a l procedure
and conditions were the same as those described for
the tests on sulfur oxides except that the operating
t e m p e r a t u r e s were limited by the m e l t i n g point of the
electrolytes used. The A g / A g + reference electrode
type illustrated in Fig. 1B was selected to make the
tests easier to perform. The performance of a carbon
dioxide detector was studied using potassium carbonate
as the electrolyte. This was prepared b y m e l t i n g a
m i x t u r e of a 1% mole fraction of silver sulfate i n potassium carbonate at 900~ and sintering at 759~ for
24 hr. No subsequent control of the composition was
performed.
The emf variation with logarithm of the CO2 partial
pressure i n air is presented in Fig. 7. The slope corresponds to a t w o - e l e c t r o n mechanism (n ---- 2.12), as
suggested b y the reaction
CO~ -k ~ 02 -k 2 e - ~ CO3 =
[15]
The possibility of building a n i t r o g e n oxide detector
was also investigated using a solid electrolyte obtained
by m e l t i n g a m i x t u r e of a 1% mole fraction of silver
chloride in b a r i u m n i t r a t e at 600~ a n d sintering at
540~ for 24 hr. The results obtained from the NO~
partial pressure variations i n air are given in Fig. 7.
The slope seems to correspond to a single electron
mechanism (n ---- 1.22), as expected from the electrode
reaction
NO~ -k u 02 -b l e - ~ NOa[16]
The response times observed for the nitrogen dioxide
and carbon dioxide detectors are of the same order of
m a g n i t u d e as those obtained with the sulfur oxide
detector.
Conclusion
This study has shown that electrochemical cells
b a s e d on solid electrolytes containing oxyanions can
be used for the potentiometric d e t e r m i n a t i o n of SO2,
SO8, CO2, and NO2 i n air, or i n a n y gas with a fixed
oxygen content. The sensors bear a strong similarity to
1583
oxygen-zirconia detectors even if the electrolytes are
poor-to-moderate cationic conductors, and despite the
relatively complex reactions of the oxides. The reasons
for this can be traced to ~he utilization of solid electrolytes and the high operating t e m p e r a t u r e s of the
solid-state detectors. Solid electrolytes enhance rapid
surface equilibria because of the total absence of convection; furthermore, the o x i d e - o x y a n i o n systems become simpler as the t e m p e r a t u r e rises.
F u r t h e r analysis of the SO3, O2, Pt/SO4 = electrode
reveals that u n d e r certain given conditions, equilibr i u m b e t w e e n sulfur dioxide, oxygen, and sulfur t r i oxide is reached rapidly, at least in the area closest
to the electrode. The ,study also shows that gaseous
s u l f u r - b e a r i n g components circulated i n the detector
produce a signal identical to the one produced by SO~.
This seems to be due to the rapid conversion of the
gases to oxides inside the device.
Finally, the results obtained are in agreement with
the over-all electrode reaction proposed by Salzano (2)
for molten sulfates, and seem to indicate that similar
electrode reactions m a y be extended to other oxidesolid oxyanion systems.
Acknowledgments
Thanks are due to H y d r o - Q u e b e c Research Institute
for the financial support of this work through
a research and development project. The authors also
wish to t h a n k Marc Poirier and Andr~ B~langer for
m a n y valuable discussions, and Roger Bellemare and
Denis Ricard for carrying out the e x p e r i m e n t a l part
of this work.
Manuscript submitted J u n e 16, 1976; revised m a n u script received May 25, 1977.
A n y discussion of this paper will appear in a Discussion Section to be published in the J u n e 1978 JOURNAL.
All discussions for the J u n e 1978 Discussion Section
should be s u b m i t t e d by Feb. 1, 1978.
Publication costs of this article were assisted by the
Hydro-Quebec Research Institute.
REFERENCES
1. A. F. Goeting and J. A. A. Ketelaar, Electrochim.
Acta, 19, 267 (1974).
2. F. J. Salzano and L. Newman, This Journat, 119,
1273 (1972).
3. M. Natara~an and E. A. Secco, Can. J. Che~z~., 53,
1542 (1975).
4. C. Wagner, Z. Phys. Chem., B21, 25 (1933).
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