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Studies on solid-state mixed sulfide active materials for ion-selective electrodes
Solid State Ionics 3/4 (1981) 197-201 North-Holland Publishing Company
STUDIES ON SOLID-STATE MIXED SULFIDE ACTIVE M A T E R I A L S FOR ION-SELECTIVE E L E C T R O D E S S. I K E D A , N. M A T S U D A ,
G. N A K A G A W A
a n d K. I T O
Department of Synthetic Chemistry, Nagoya Institute o[ Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan Electrical conductivities, transport numbers and crystal structures of mixed sulfides of Ag and Cu, Pb or Cd have been measured to be compared with the potential response of ion-selective electrodes (ISEs) using the mixed sulfide as the active material. In the case of Cu(II) ISEs, Jalpaite (AgI55Cu045S), formed in the simultaneously precipitated mixed sulfide of Ag and Cu, plays an important role in the performance of Cu ISEs. In the cases of Pb(ll) and Cd(lI) ISEs, solid solutions of PbS or CdS containing a small amount of Ag2S play important roles in the response properties. In the case of Ag ISEs prepared with stabilized a-Ag2Se, Ag+ ion activity in the active material affects the response of ISEs in spite of a low deviation of composition (Ar = 0.005).
1. Introduction I o n - s e l e c t i v e e l e c t r o d e s (ISEs) with m i x e d sulfide m e m b r a n e s h a v e b e e n d e v e l o p e d a n d widely u s e d for p o t e n t i o m e t r i c d e t e r m i n a t i o n of t h e h e a v y m e t a l i o n s such as A g ÷, Cu 2÷, Pb 2÷ a n d C d 2÷. T h e active m a t e r i a l s of the e l e c t r o d e s consist of m i x e d sulfides of such m e t a l s a n d silvers; h o w e v e r , t h e role of Ag2S which has b e e n well k n o w n as a m i x e d c o n d u c t o r has not b e e n clarified. A c c o r d i n g l y , c h a r a c t e r i s t i c prop e r t i e s of t h e active m a t e r i a l s h a v e b e e n m e a s u r e d to b e c o m p a r e d with t h e p o t e n t i a l r e s p o n s e of the I S E s p r e p a r e d with such m i x e d chalcogenides. The response and characteristic p r o p e r t i e s of C u ( I I ) I S E s with t h e C u S - A g 2 S s y s t e m a n d of P b ( I I ) I S E s with the P b S - A g 2 S system w e r e p r e v i o u s l y r e p o r t e d by t h e a u t h o r s [1, 2]. T h e r e f o r e , in this p a p e r , C d ( I I ) I S E s m a d e with t h e C d S - A g z S s y s t e m a n d A g I S E s m a d e with s t a b i l i z e d o~-Ag2Se, (Ag2Se)o.925(Ag3PO4)o.075, h a v e b e e n e x a m i n e d a n d d i s c u s s e d t o g e t h e r with t h e results previously o b t a i n e d with C u ( I I ) a n d P b ( I I ) ISEs.
2. Experimental procedures
2.1. Cd(II) ISEs with CdS--AgeS system T h e v a r i o u s m i x t u r e s of C d S a n d Ag2S w e r e h e a t e d at 4 2 0 - 4 3 0 ° C for 48 h u n d e r a sulfur
a t m o s p h e r e in s e a l e d Pyrex tubes. T h e n t h e y w e r e m o l d e d i n t o d i s k s by hot pressing at 310°C a n d 19.6 M P a . T h e disk s h a p e d active m e m b r a n e was d i r e c t l y c o n n e c t e d to a Cu wire with a silver c o n d u c t i v e paint, D u Pont No. 4817, a n d then was m o l d e d in e p o x y resin. P o t e n t i a l m e a s u r e m e n t s w e r e c a r r i e d o u t in test s o l u t i o n s of 10-1-10 -6 M C d 2+ at 25.0-+ 0.1°C using a p H m e t e r , H i t a c h i - H o r i b a m o d e l F7, as a p o t e n t i o m e t e r . T h e test s o l u t i o n s w e r e prep a r e d by dilution of 0.1 M C d 2+ stock s o l u t i o n m a d e f r o m 99.999% C d m e t a l a n d HNO3, a n d t h e ionic s t r e n g t h was m a d e 0.3 by t h e a d d i t i o n of KNO3. X - r a y diffraction p a t t e r n s of t h e prep a r e d m i x e d sulfides w e r e t a k e n by an X - r a y d i f f r a c t o m e t e r with Cu Kot r a d i a t i o n .
2.2. Ag(I) ISEs with a-AgeSe-type solid solution S t a b i l i z e d a - A g 2 S e , (Ag2Se)0.92_s(Ag3PO4)0.075, was p r e p a r e d by t h e m e t h o d r e p o r t e d by T a k a h a s h i et al. [3]. It was used as an active m a t e r i a l of A g ISEs, which w e r e also c o n s t r u c t e d as solid c o u l o m e t r i c t i t r a t i o n ( C ~ ) cells as shown in fig. 1. T h e activity of A g + ion in the active m a t e r i a l has b e e n c h a n g e d by C T with a c o n s t a n t c u r r e n t of 75 ~ A / c m : at 25°C. T h e voltage, AE, b e t w e e n t h e a n o d e , Pt(+), a n d the A g ( R ) e l e c t r o d e of the C T cell in fig. 1 was c h a n g e d f r o m 0 to 260 m V s t e p w i s e l y (static method) or continuously (dynamic method).
0167-2738/81/000--000/$02.50 O N o r t h - H o l l a n d Publishing C o m p a n y
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S. l k e d a et al. / M i x e d sulfides as active materials in I S E s
A?:
,@~
(-)
(k)
(%)
t t-~]-- A£cI tWO4 ']'i::l ,2L Iul i~t/ Fig. I. Schematic diagram of Ag ISE with stabilized a Ag2Sc controlled by coulometric titration.
The test solutions of 10-L10-6M AgNO3 were prepared by dilution of 0.1 M Ag ÷ stock solution made with G R grade AgNO3, and the ionic strength was made 0.1 by the addition of KNO3. Potential response properties of the Ag ISEs with varous activities of Ag + ion in the solid active membrane, i.e. at various values of AE, were measured.
3. Results and discussion
3. I. Cu(II) and Pb(II) ISEs In the previous papers [1] on the active materials for Cu(II) ISEs, the X-ray diffraction analysis of the simultaneously precipitated (SP) mixed sulfide of Ag and Cu, which was available for Cu ISEs, has indicated occurrence of Jalpaite, Ag155Cu0.45S, in addition to CuS and Ag:S. The occurrence of Jalpaite in such mixed sulfides has been independently found and already reported at the 2nd Symposium of SolidIonics (Japanese), Nagoya, in 1974 by the authors [4]. This fact was also recently reported by Heijne and van der Linden [5]. According to our paper [1], the total conductivity of synthesized Jalpaite was 1.6 x 10-4S cm -J at 25°C, and transport numbers of Ag +, Cu + and hole were 0.69, 0.30 and 0.016 at 25°C. Jalpaite formed in the SP mixed sulfide of Ag and Cu will play an important role in the performance of the Cu ISEs. In the previous paper [2] on the active materials for Pb(II) ISEs, Pb ISEs with PbS-
Ag2S system containing 0.5-75 mol% Ag2S gave a nernstian response to Pb 2+ ion and had a response time of 1-2 min, while those with pure PbS or AgzS could not show such a response. No new phase could be found in the system. Electrical conductivities of PbS containing Ag2S suddenly increased on addition of a small amount of Ag2S and reached a maximum value at 0.5 mol% Ag2S. The standard potentials of the prepared Pb ISEs gave nearly constant values of -52 to -56 mV versus SCE in the range 0.5-75 mol% Ag2S and became more noble outside of this range. These results suggest that a solid solution will be formed by dissolving Ag2S in the PbS lattice; its solid solubility limit will be ~0.5 mol% Ag2S, and it will play an important role in the performance of Pb ISEs.
3.2. Cd(II) ISEs The potential-ion-activity curves measured by the Cd(II) ISEs with various active materials are shown in fig. 2. The electrode with pure CdS or Ag2S did not respond to Cd 2+ ion, while those with active materials of 1.6-90 mol% Ag2S showed a nernstian response. The best response has been obtained by the electrode of 40 mol% Ag:S, whose slope was 29.6 mV/decade and the response time was within 1 min. The standard potentials of the prepared Cd ISEs gave nearly constant values of 70-76 mV versus SCE in the range 10-90 mol% AgzS except for 33 tool% (44 mV versus SCE) and 60 mol% ( - 6 0 mV versus SCE). According to SEM observations, sensitive surfaces of the electrodes containing 1.650 mol% Ag:S looked homogeneous, while those with pure CdS had large grains and those with more than 60 mol% Ag2S showed two phases with random distribution. The lattice spacing of CdS (103) was expanded by addition of AgzS and had a constant value in the range 20-80 tool% Ag2S except for 33 and 60 mol%. However, no new phase was found in the CdSAg2S system by X-ray diffraction. Hence, it may be concluded that the solid solution formed by dissolving Ag2S into the CdS
S. lkeda et al. / Mixed sulfides as active materials in ISEs i00
I
I
I
I
199
I
I
I L - -
500 ....90 %
10 -l M AgNO 3
50
o
1,6 --33
90 % 8O
10 .2 M
O3
aoo
(/3
-50
10 .3 M
10 4,0J//J//////z
/ p - 60 % Ag2S
-ioo
v
g 10 -4 M 3OO
.
-150
10 .5 M
-200
10 -6 M
-
- , % 1 ' < , ,,,
200 -250
~.... -300
I -6
I -5
I -4
I -3
I -2
0 % I -i
log aCd 2+
i00
,
Fig. 2. Potential-ion-activity curves of Cd(II) ISEs with CdS-Ag2S system.
lattice plays an important role in the response of Cd ISEs similar to that of the PbS-Ag2S system previously mentioned [2]. 3.3. A g ( I ) I S E s
The CT cell voltage, AE, corresponding to the Ag ÷ ion activity in the solid-state active material was saturated at 260 mV, which agreed well with the decomposition voltage of Ag2Se calculated from AG, where the deviation from the stoichiometric composition, Ar in a Ag2-a, Se, was ~0.005. The response properties of the ISEs measured by the dynamic method are shown in fig. 3. In 10-L10-3M AgNO3 solutions, the potential difference, EA,, between the Ag(R) electrode and the SCE was not changed by CT with AE of 0-260 mV. In concentrations lower than 10 -4 M, E A g - A E curves showed hysteresis for A E values above 200 mV. The potential-ion-activity curves measured
I
,
i
I00 200 Coul.Titrn, Cell Voltage, dE
L
300 ( mV )
Fig. 3. EAg-AE curves of Ag ISE when the activity of Ag + ion in the active m e m b r a n e was continuously changed by coulometric titration. EA~: potential difference between Ag(R) electrode and SCE in fig. l, AE: coulometric titration cell voltage.
by the static method are shown in fig. 4. When AE was less than 200 mV, the electrode showed a nernstian response to Ag + ion concentrations of 10-L10 -5 M. A b o v e 200 mV, however, it, became poor in response to Ag + ion concentrations lower than 10 -3 M, and showed less noble potentials. The standard potential, E °, was obtained by extrapolation of the potential-ion-activity curve measured at A E = 0 m V to a Ag ÷ ion activity of the solution of unity, and agreed well with the theoretical value of 799 m V versus N H E at 25°C. The C T cell voltage, AE, is represented by, AE =
R T In a ( A g + ' a )
F
= -(RT/F)
a (Ag +, M)
In a ( A g +, a),
(1)
200
S. l k e d a et al. / M i x e d sulfides as active materials in I S E s
Cou1,Titrn, 0
Cel] Voltoge, AE
80
the stoichiometric composition, Ar in ce a,Se, is as small as 0.005 at 260 mV, however, it is enough to affect the response of the Ag ISEs at lower concentrations of the solutions. Accordingly, it may be concluded that Ag + ion activity in the active material plays an important role in the performance of the ISEs.
( mV )
160
240
Ag2
]
z
>
4. conclusion
.2 09
nO
O
1
2
3
4
5
6
- log oA~+
Fig. 4. Potential-ion-activity curves of Ag ISlE measured at various values of AE.
where a (Ag +, a) is the Ag ÷ ion activity in the solid-state active material and a (Ag +, M) is that in the Ag metal electrode, which may be assumed to be unity. The potential of the Ag ISEs, E, is given by E = E°+
~_
a (Ag ÷, s) In a (Ag +, a)'
(2)
where a ( A g +, s) is the Ag ÷ ion activity in the test solution. When a ( A g +, s) is unity, we obtain, from eqs. (1) and (2), E' = E ° - (RT]F) ln a ( A g +, a ) = E ° + A E ,
(3)
where E ' is the potential obtained by extrapolation of the potential-ion-activity curves to an a ( A g +, s) value of unity. The linear relationship between E ' and AE in eq. (3) is confirmed by the result shown in fig. 4 as black triangles. These results indicate that the deviation from
The following results were obtained. (1) Jalpaite, Agl.55Cu0.45S, occurs in the simultaneously precipitated mixed sulfide of Cu and Ag, which is available as an active material for Cu(II) ISEs. Jalpaite plays an important role in the performance of the Cu ISEs. (2) No new phase occurs in the PbS-Ag2S system. Pb(II) ISEs with active materials of 0.575 mol% Ag2S exhibit a nernstian response. Addition of a small amount of Ag2S, i.e. 0.5 m o l % , to PbS causes a sudden increase of the electrical conductivity. A solid solution is formed by dissolving AgzS in the PbS lattice, and it plays an important role in the performance of Pb ISEs. (3) No new phase forms in the CdS--Ag2S system. Cd(II) ISEs with active materials of 1.6-90 mol% Ag2S show a nernstian response. The best response is obtained at 40 mol% Ag2S. The lattice spacing of CdS (103) is expandeed by addition of Ag2S and has a constant value at 20-80 m o l % Ag2S except at 33 and 60 tool%. The solid solution of CdS-Ag2S is important in the response of Cd ISEs similar to Pb ISEs. (4) Stabilized a-Ag2Se, (Ag2Se)0.925(Ag3PO4)0.075, has been used as an active material for Ag ISEs. When Ag + ion activity in the active material has been changed by CT, the ISE shows a nernstian response at higher concentrations than 10 -3 M , while it becomes p o o r in response by increasing the CT cell voltage, AE, over 200 inV. Although the deviation of the composition, Ar in ot-Ag2 arSe, is as small as 0.005 at 260 mV, it is enough to affect the response of ISEs at lower concentrations. It may be concluded that Ag ÷ ion
S. Ikeda et al. / Mixed sulfides as active materials in ISEs
activity in the active materials plays an important role in the performance of Ag ISEs. According to the results obtained, it may be concluded that the activities of metal ions to be determined and also of Ag + ion in the active materials for ISEs would affect the response properties of the ISEs.
Acknowledgement The present work was partially supported by a Grant-in-Aid for Scientific Research (No. 255313) and a Grant-in-Aid for Encouragement of Young Scientists (No. 475613) from the Ministry of Education, Japan.
201
References [1] S. Ikeda, N. Matsuda, G. Nakagawa and K. lto, Denki Kagaku 47 (1979) 281; 48 (1980) 16, 199. [2] K. Ito, N. Matsuda, T. Maeda, S. Ikeda, T. lida and G. Nakagawa, Denki Kagaku 47 (1979) 22[). [3] T. Takahashi and O. Y a m a m o t o , J. Electrochem. Soc. 119 (1972) 1735. [4] K. Ito, S. lkeda, N. Matsuda and G. Nakagawa, Extended Abstracts of the 2nd Symposium on Solid-Ionics, Nagoya (1974) p. 251. [5] G.J.M. Heijne and W.E. van der Linden, Anal. Chim. Acta 93 (1977) 99.
STUDIES ON SOLID-STATE MIXED SULFIDE ACTIVE M A T E R I A L S FOR ION-SELECTIVE E L E C T R O D E S S. I K E D A , N. M A T S U D A ,
G. N A K A G A W A
a n d K. I T O
Department of Synthetic Chemistry, Nagoya Institute o[ Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan Electrical conductivities, transport numbers and crystal structures of mixed sulfides of Ag and Cu, Pb or Cd have been measured to be compared with the potential response of ion-selective electrodes (ISEs) using the mixed sulfide as the active material. In the case of Cu(II) ISEs, Jalpaite (AgI55Cu045S), formed in the simultaneously precipitated mixed sulfide of Ag and Cu, plays an important role in the performance of Cu ISEs. In the cases of Pb(ll) and Cd(lI) ISEs, solid solutions of PbS or CdS containing a small amount of Ag2S play important roles in the response properties. In the case of Ag ISEs prepared with stabilized a-Ag2Se, Ag+ ion activity in the active material affects the response of ISEs in spite of a low deviation of composition (Ar = 0.005).
1. Introduction I o n - s e l e c t i v e e l e c t r o d e s (ISEs) with m i x e d sulfide m e m b r a n e s h a v e b e e n d e v e l o p e d a n d widely u s e d for p o t e n t i o m e t r i c d e t e r m i n a t i o n of t h e h e a v y m e t a l i o n s such as A g ÷, Cu 2÷, Pb 2÷ a n d C d 2÷. T h e active m a t e r i a l s of the e l e c t r o d e s consist of m i x e d sulfides of such m e t a l s a n d silvers; h o w e v e r , t h e role of Ag2S which has b e e n well k n o w n as a m i x e d c o n d u c t o r has not b e e n clarified. A c c o r d i n g l y , c h a r a c t e r i s t i c prop e r t i e s of t h e active m a t e r i a l s h a v e b e e n m e a s u r e d to b e c o m p a r e d with t h e p o t e n t i a l r e s p o n s e of the I S E s p r e p a r e d with such m i x e d chalcogenides. The response and characteristic p r o p e r t i e s of C u ( I I ) I S E s with t h e C u S - A g 2 S s y s t e m a n d of P b ( I I ) I S E s with the P b S - A g 2 S system w e r e p r e v i o u s l y r e p o r t e d by t h e a u t h o r s [1, 2]. T h e r e f o r e , in this p a p e r , C d ( I I ) I S E s m a d e with t h e C d S - A g z S s y s t e m a n d A g I S E s m a d e with s t a b i l i z e d o~-Ag2Se, (Ag2Se)o.925(Ag3PO4)o.075, h a v e b e e n e x a m i n e d a n d d i s c u s s e d t o g e t h e r with t h e results previously o b t a i n e d with C u ( I I ) a n d P b ( I I ) ISEs.
2. Experimental procedures
2.1. Cd(II) ISEs with CdS--AgeS system T h e v a r i o u s m i x t u r e s of C d S a n d Ag2S w e r e h e a t e d at 4 2 0 - 4 3 0 ° C for 48 h u n d e r a sulfur
a t m o s p h e r e in s e a l e d Pyrex tubes. T h e n t h e y w e r e m o l d e d i n t o d i s k s by hot pressing at 310°C a n d 19.6 M P a . T h e disk s h a p e d active m e m b r a n e was d i r e c t l y c o n n e c t e d to a Cu wire with a silver c o n d u c t i v e paint, D u Pont No. 4817, a n d then was m o l d e d in e p o x y resin. P o t e n t i a l m e a s u r e m e n t s w e r e c a r r i e d o u t in test s o l u t i o n s of 10-1-10 -6 M C d 2+ at 25.0-+ 0.1°C using a p H m e t e r , H i t a c h i - H o r i b a m o d e l F7, as a p o t e n t i o m e t e r . T h e test s o l u t i o n s w e r e prep a r e d by dilution of 0.1 M C d 2+ stock s o l u t i o n m a d e f r o m 99.999% C d m e t a l a n d HNO3, a n d t h e ionic s t r e n g t h was m a d e 0.3 by t h e a d d i t i o n of KNO3. X - r a y diffraction p a t t e r n s of t h e prep a r e d m i x e d sulfides w e r e t a k e n by an X - r a y d i f f r a c t o m e t e r with Cu Kot r a d i a t i o n .
2.2. Ag(I) ISEs with a-AgeSe-type solid solution S t a b i l i z e d a - A g 2 S e , (Ag2Se)0.92_s(Ag3PO4)0.075, was p r e p a r e d by t h e m e t h o d r e p o r t e d by T a k a h a s h i et al. [3]. It was used as an active m a t e r i a l of A g ISEs, which w e r e also c o n s t r u c t e d as solid c o u l o m e t r i c t i t r a t i o n ( C ~ ) cells as shown in fig. 1. T h e activity of A g + ion in the active m a t e r i a l has b e e n c h a n g e d by C T with a c o n s t a n t c u r r e n t of 75 ~ A / c m : at 25°C. T h e voltage, AE, b e t w e e n t h e a n o d e , Pt(+), a n d the A g ( R ) e l e c t r o d e of the C T cell in fig. 1 was c h a n g e d f r o m 0 to 260 m V s t e p w i s e l y (static method) or continuously (dynamic method).
0167-2738/81/000--000/$02.50 O N o r t h - H o l l a n d Publishing C o m p a n y
198
S. l k e d a et al. / M i x e d sulfides as active materials in I S E s
A?:
,@~
(-)
(k)
(%)
t t-~]-- A£cI tWO4 ']'i::l ,2L Iul i~t/ Fig. I. Schematic diagram of Ag ISE with stabilized a Ag2Sc controlled by coulometric titration.
The test solutions of 10-L10-6M AgNO3 were prepared by dilution of 0.1 M Ag ÷ stock solution made with G R grade AgNO3, and the ionic strength was made 0.1 by the addition of KNO3. Potential response properties of the Ag ISEs with varous activities of Ag + ion in the solid active membrane, i.e. at various values of AE, were measured.
3. Results and discussion
3. I. Cu(II) and Pb(II) ISEs In the previous papers [1] on the active materials for Cu(II) ISEs, the X-ray diffraction analysis of the simultaneously precipitated (SP) mixed sulfide of Ag and Cu, which was available for Cu ISEs, has indicated occurrence of Jalpaite, Ag155Cu0.45S, in addition to CuS and Ag:S. The occurrence of Jalpaite in such mixed sulfides has been independently found and already reported at the 2nd Symposium of SolidIonics (Japanese), Nagoya, in 1974 by the authors [4]. This fact was also recently reported by Heijne and van der Linden [5]. According to our paper [1], the total conductivity of synthesized Jalpaite was 1.6 x 10-4S cm -J at 25°C, and transport numbers of Ag +, Cu + and hole were 0.69, 0.30 and 0.016 at 25°C. Jalpaite formed in the SP mixed sulfide of Ag and Cu will play an important role in the performance of the Cu ISEs. In the previous paper [2] on the active materials for Pb(II) ISEs, Pb ISEs with PbS-
Ag2S system containing 0.5-75 mol% Ag2S gave a nernstian response to Pb 2+ ion and had a response time of 1-2 min, while those with pure PbS or AgzS could not show such a response. No new phase could be found in the system. Electrical conductivities of PbS containing Ag2S suddenly increased on addition of a small amount of Ag2S and reached a maximum value at 0.5 mol% Ag2S. The standard potentials of the prepared Pb ISEs gave nearly constant values of -52 to -56 mV versus SCE in the range 0.5-75 mol% Ag2S and became more noble outside of this range. These results suggest that a solid solution will be formed by dissolving Ag2S in the PbS lattice; its solid solubility limit will be ~0.5 mol% Ag2S, and it will play an important role in the performance of Pb ISEs.
3.2. Cd(II) ISEs The potential-ion-activity curves measured by the Cd(II) ISEs with various active materials are shown in fig. 2. The electrode with pure CdS or Ag2S did not respond to Cd 2+ ion, while those with active materials of 1.6-90 mol% Ag2S showed a nernstian response. The best response has been obtained by the electrode of 40 mol% Ag:S, whose slope was 29.6 mV/decade and the response time was within 1 min. The standard potentials of the prepared Cd ISEs gave nearly constant values of 70-76 mV versus SCE in the range 10-90 mol% AgzS except for 33 tool% (44 mV versus SCE) and 60 mol% ( - 6 0 mV versus SCE). According to SEM observations, sensitive surfaces of the electrodes containing 1.650 mol% Ag:S looked homogeneous, while those with pure CdS had large grains and those with more than 60 mol% Ag2S showed two phases with random distribution. The lattice spacing of CdS (103) was expanded by addition of AgzS and had a constant value in the range 20-80 tool% Ag2S except for 33 and 60 mol%. However, no new phase was found in the CdSAg2S system by X-ray diffraction. Hence, it may be concluded that the solid solution formed by dissolving Ag2S into the CdS
S. lkeda et al. / Mixed sulfides as active materials in ISEs i00
I
I
I
I
199
I
I
I L - -
500 ....90 %
10 -l M AgNO 3
50
o
1,6 --33
90 % 8O
10 .2 M
O3
aoo
(/3
-50
10 .3 M
10 4,0J//J//////z
/ p - 60 % Ag2S
-ioo
v
g 10 -4 M 3OO
.
-150
10 .5 M
-200
10 -6 M
-
- , % 1 ' < , ,,,
200 -250
~.... -300
I -6
I -5
I -4
I -3
I -2
0 % I -i
log aCd 2+
i00
,
Fig. 2. Potential-ion-activity curves of Cd(II) ISEs with CdS-Ag2S system.
lattice plays an important role in the response of Cd ISEs similar to that of the PbS-Ag2S system previously mentioned [2]. 3.3. A g ( I ) I S E s
The CT cell voltage, AE, corresponding to the Ag ÷ ion activity in the solid-state active material was saturated at 260 mV, which agreed well with the decomposition voltage of Ag2Se calculated from AG, where the deviation from the stoichiometric composition, Ar in a Ag2-a, Se, was ~0.005. The response properties of the ISEs measured by the dynamic method are shown in fig. 3. In 10-L10-3M AgNO3 solutions, the potential difference, EA,, between the Ag(R) electrode and the SCE was not changed by CT with AE of 0-260 mV. In concentrations lower than 10 -4 M, E A g - A E curves showed hysteresis for A E values above 200 mV. The potential-ion-activity curves measured
I
,
i
I00 200 Coul.Titrn, Cell Voltage, dE
L
300 ( mV )
Fig. 3. EAg-AE curves of Ag ISE when the activity of Ag + ion in the active m e m b r a n e was continuously changed by coulometric titration. EA~: potential difference between Ag(R) electrode and SCE in fig. l, AE: coulometric titration cell voltage.
by the static method are shown in fig. 4. When AE was less than 200 mV, the electrode showed a nernstian response to Ag + ion concentrations of 10-L10 -5 M. A b o v e 200 mV, however, it, became poor in response to Ag + ion concentrations lower than 10 -3 M, and showed less noble potentials. The standard potential, E °, was obtained by extrapolation of the potential-ion-activity curve measured at A E = 0 m V to a Ag ÷ ion activity of the solution of unity, and agreed well with the theoretical value of 799 m V versus N H E at 25°C. The C T cell voltage, AE, is represented by, AE =
R T In a ( A g + ' a )
F
= -(RT/F)
a (Ag +, M)
In a ( A g +, a),
(1)
200
S. l k e d a et al. / M i x e d sulfides as active materials in I S E s
Cou1,Titrn, 0
Cel] Voltoge, AE
80
the stoichiometric composition, Ar in ce a,Se, is as small as 0.005 at 260 mV, however, it is enough to affect the response of the Ag ISEs at lower concentrations of the solutions. Accordingly, it may be concluded that Ag + ion activity in the active material plays an important role in the performance of the ISEs.
( mV )
160
240
Ag2
]
z
>
4. conclusion
.2 09
nO
O
1
2
3
4
5
6
- log oA~+
Fig. 4. Potential-ion-activity curves of Ag ISlE measured at various values of AE.
where a (Ag +, a) is the Ag ÷ ion activity in the solid-state active material and a (Ag +, M) is that in the Ag metal electrode, which may be assumed to be unity. The potential of the Ag ISEs, E, is given by E = E°+
~_
a (Ag ÷, s) In a (Ag +, a)'
(2)
where a ( A g +, s) is the Ag ÷ ion activity in the test solution. When a ( A g +, s) is unity, we obtain, from eqs. (1) and (2), E' = E ° - (RT]F) ln a ( A g +, a ) = E ° + A E ,
(3)
where E ' is the potential obtained by extrapolation of the potential-ion-activity curves to an a ( A g +, s) value of unity. The linear relationship between E ' and AE in eq. (3) is confirmed by the result shown in fig. 4 as black triangles. These results indicate that the deviation from
The following results were obtained. (1) Jalpaite, Agl.55Cu0.45S, occurs in the simultaneously precipitated mixed sulfide of Cu and Ag, which is available as an active material for Cu(II) ISEs. Jalpaite plays an important role in the performance of the Cu ISEs. (2) No new phase occurs in the PbS-Ag2S system. Pb(II) ISEs with active materials of 0.575 mol% Ag2S exhibit a nernstian response. Addition of a small amount of Ag2S, i.e. 0.5 m o l % , to PbS causes a sudden increase of the electrical conductivity. A solid solution is formed by dissolving AgzS in the PbS lattice, and it plays an important role in the performance of Pb ISEs. (3) No new phase forms in the CdS--Ag2S system. Cd(II) ISEs with active materials of 1.6-90 mol% Ag2S show a nernstian response. The best response is obtained at 40 mol% Ag2S. The lattice spacing of CdS (103) is expandeed by addition of Ag2S and has a constant value at 20-80 m o l % Ag2S except at 33 and 60 tool%. The solid solution of CdS-Ag2S is important in the response of Cd ISEs similar to Pb ISEs. (4) Stabilized a-Ag2Se, (Ag2Se)0.925(Ag3PO4)0.075, has been used as an active material for Ag ISEs. When Ag + ion activity in the active material has been changed by CT, the ISE shows a nernstian response at higher concentrations than 10 -3 M , while it becomes p o o r in response by increasing the CT cell voltage, AE, over 200 inV. Although the deviation of the composition, Ar in ot-Ag2 arSe, is as small as 0.005 at 260 mV, it is enough to affect the response of ISEs at lower concentrations. It may be concluded that Ag ÷ ion
S. Ikeda et al. / Mixed sulfides as active materials in ISEs
activity in the active materials plays an important role in the performance of Ag ISEs. According to the results obtained, it may be concluded that the activities of metal ions to be determined and also of Ag + ion in the active materials for ISEs would affect the response properties of the ISEs.
Acknowledgement The present work was partially supported by a Grant-in-Aid for Scientific Research (No. 255313) and a Grant-in-Aid for Encouragement of Young Scientists (No. 475613) from the Ministry of Education, Japan.
201
References [1] S. Ikeda, N. Matsuda, G. Nakagawa and K. lto, Denki Kagaku 47 (1979) 281; 48 (1980) 16, 199. [2] K. Ito, N. Matsuda, T. Maeda, S. Ikeda, T. lida and G. Nakagawa, Denki Kagaku 47 (1979) 22[). [3] T. Takahashi and O. Y a m a m o t o , J. Electrochem. Soc. 119 (1972) 1735. [4] K. Ito, S. lkeda, N. Matsuda and G. Nakagawa, Extended Abstracts of the 2nd Symposium on Solid-Ionics, Nagoya (1974) p. 251. [5] G.J.M. Heijne and W.E. van der Linden, Anal. Chim. Acta 93 (1977) 99.