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A new double-jet cell for fast ion-sensitive electrodes
Sensors and ActuatorsB, 15-16 (1993) 173-178
173
A new double-jet cell for fast ion-sensitive electrodes M. Attari and P. Fabry LIESG (Laboratoire d’lonique et d’Electrochimie du Solide de Grenoble), ENSEEGIINPG. BP 75-38402, Saint Martin #H&es Cedex (France)
H. Mall% and G. Quezel Laboratoire d’lnstrummtation, Micro-Informaiique ef Electroniquede Grenoble, UJF-G, BP 53 X, Grenoble CJdex (Frunce)
Abstract A double-jet system has been chosen to study the transient response of Na+ fast ion-sensitive membranes such as NASICON (Na, +XZr,Si,P,_, O,,). The Nat solutions are thrown by a neutral gas onto the membrane. To avoid the residual drop at the LYE/reference electrode contact, two reference electrodes are used, one in each solution. The system is driven by a microcomputer. Comparisons between an Na+-glass membrane and NASICON have shown a difference of one order of magnitude on the response time values. A potential overshoot phenomenon is observed on the glass membrane during the K+ interference experiment which does not appear significantly with NASICON. This point may be important in biological applications where there is competition between K+ and Na+ ions.
Introduction The ion-selective electrodes (ISE) are very interesting tools because they are active sensors easy to use, for instance in process control or analytical applications. In such examples the use of fast sensors is interesting, it is therefore necessary to know their dynamic properties. The study of response times can also be useful to identify the physicochemical phenomena which take place during the response of the membrane. We have previously suggested the use of NASICON as an Na+-sensitive mem(Na, +xZr,Si,P,_X0,2) brane [ 1,2]. This compound was first synthesized by Hong [3]. It is a crystallized material which is an Na+ fast ionic conductor (a is about 10e3 S cm-’ at room temperature). Its preparation processes and its steadystate characteristics as ion-sensitive material are described elsewhere [2,4]. A high ionic exchange current has been observed at the membrane/solution interface [Sj: the relaxation time of this phenomenon is about 0.1 to 1 ms. In such a case, it becomes necessary to have a special cell to measure the speed of response. As described in the review papers [6,7], several methods have been proposed for studying the transient responses of ISE: electrical methods (complex impedance, voltage step, polarization and exchange current studies. . .) activity step methods l
l
09254005/93/$6.00
In the present work, an activity step method was chosen because it corresponds to a real situation for the sensors. In such methods, a change of the ion concentration is given at the ISE membrane surface. The voltage is then recorded as a function of time. The response time t, is defined as the time necessary to reach the ratio G(of the Nernstian variation (for instance 0.99, 0.9 or 0.5). The shape of the concentration step which is given by the system is then of the highest importance and must be accurately defined, the more so, the faster is the sensor. As mentioned by several authors [6,7], one can distinguish two types of cell to measure the response times: 9 the dipping methods, so called immersion methods: the ISE is conditioned in an initial solution (activity a,) and then, after a fast washing and/or shaking of the sensor, the ISE is immersed into a solution of activity u2. With this method, the activity is not well defined during the transfer. A system with a flowthrough technique is then preferable, for instance a double-jet system with a fast commutation, each jet with a solution of the corresponding activity, a, or a,. the injection methods: the sensor is conditioned in a carrier solution of activity u1 and a small volume of a concentrated solution is quickly introduced into the carrier solution, with a strong stirring action. Several experimental cells are described in the review papers i&71. l
@ 1993
-
ElsevierSequoia.All rights
reserved
174
Experimental A dipping system with pulsed jets was chosen because this type of apparatus is quasi-universal; any ISE device can be tested. The set-up is relatively easy to construct. The apparatus is similar to the one developed by Toth and Pungor [ 81.Both solutions are contained in separate vessels. The jets are thrown out by a neutral gas, for example argon. The commutation of the solution is ensured by a fast shift of the jets (Fig. l(a)). Each jet is thrown perpendicularly to the ISE surface. The distance to the ISE membrane is between one and two ccntimeters Another possibility consists of shifting a mask system in front of both immobilized jets. With such a system, the membrane is not in contact with a solution for too long a time. For this reason, a system with the jets attached to the disk was preferred. Circular shifting
Jat 2
(a)
r’
Reference electmde
(‘-9
Residual dmp
/A
Sensor
” H
(4 Fig. 1. Double-jet cell: (a) double-jet translation system; (b) drop formation; (c)general view: ( 1) inert gas, (2) and (3) manometers, (4) vessel of the C, solution, (5) reference electrode 1, (6) reference electrode 2, (7) vessel of the C, solution, (8) taps, (9) pipes in TygonR, (IO) double-jet driver.
The jet-carrier is very light. It is an openworked disk (Plexiglassa) with a low moment of inertia. The disk is fixed to the axis of a stepper motor. A rubbing system (viscous grease) prevents any oscillation. The connections with both vessels are made by fine and supple pipes (Tygon@). The shift movement is controlled by a microcomputer (Apple IIC? in the first apparatus) with an interface card (Adalab”). In the cells described for instance by Toth and coworkers [7,8], the reference electrode is located near the ISE membrane. A thin film solution ensures an ionic contact between both sensors. The major disadvantage of such cells is the residual drop which remains between the ISE surface and the reference one, especially if the ISE has a spherical shape, as shown in Fig. l(b). Because the drop must be drained, the measured response time is increased, especially for fast sensors. Lindner et al. [9] have improved the set-up by introducing a reference electrode in the part which supports the ISE, the liquid junction is then ensured by a capillary up to the ISE membrane proximity. Such a system is well adapted to plane membranes. Generally, it is assumed that there is no junction potential at the reference electrode. If such a potential occurs, for instance due to a retrodiffusion from the solution to the reference electrode (acid medium), it can result in a delayed response time. To overcome this inconvenience, we have included two reference electrodes, one in each vessel. Both reference electrodes are connected to the electrical ground. A general description is given in Fig. l(c) [lo]. In our device, if a Henderson potential occurs, it takes a steady-state value for each electrode, before the measurement. At the worst, a constant voltage lag is observed (the voltage step does not obey the Nernst law). The movement of the jet support is ensured by an angular change corresponding to one integer step of the motor. The electrical signal is generated by a VIA component (6522 Rockwellfi) which drives a control circuit (L297, SGSG). An amplifier (L298N, SGS@) supplies the power necessary to the motor. A high input impedance differential amplifier with a large common mode rejection ratio (CMRR) adapts the ISE signal. Noise immunity is achieved through shielding. The instrumentation amplifier is configured with two OPA 111 (Burr Brown@) as impedance adapters which are connected to both inputs of a differential amplifier. Frequency compensation is ensured by two 0P07 amplifiers in feedback with capacitors. If the ISE impedance is too high, one of the differential amplifiers (B on Fig. 2) is connected to a similar ISE immersed in a third solution. The third reference electrode is also connected to the ground. With such a system, the common mode voltage is reduced. If the ISE impedance is low, the B-input can
175
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Pressure
(kPa)
Fig. 3. Response time vs. over-pressure (NASICON sensor)
GffwtandCMRR
Ground
Fig. 2. Instrumentation
amplifier diagram.
be directly connected to the ground (i.e. to the common reference electrodes). The analog to digital conversion is made by a 12 bit successive-approximation ADC (AD 674 A Burr Brown@, minimum conversion time: 15 us). The input voltage is firstly conditioned by a Butterworth filter (f, about 1 kHz) [ 1l] and by a sample and hold circuit (SHC 298 A, Burr Brown@).The sampling rate is set by the converter status line. The ADC time-base is connected to the microprocessor by a special line. Because the Apple Basic language is too slow, assembly subroutines have been written to drive the data acquisition procedure. A detailed description of the apparatus is given in Attari’s thesis [lo].
over-pressure seems to be 8 kPa in our set-up. This pressure was used throughout the studies. Beyond this value, the response time increases. It can be due to a turbulent flow which creates variations in the diffusion layer thickness, the average value being higher. Reference electrodes
The choice of the reference electrodes is very important because their impedance characteristics can cause some difficulties. For instance, saturated calomel electrodes have too high impedances and, in spite of drastic precautions, a 50 Hz noise disturbs significantly the sensor response, as shown in Fig. 4. Such difficulties were not observed with Ag/AgCl, KC1 3 M reference electrodes which are better from this point of view. As discussed above, if the reference electrode is located near the ISE membrane, the effect of the residual drop on the transient response becomes apparent, especially moving from high activity to low activity. For
100
00
Results Hydrodynamic optimization
As the response phenomena can be limited by diffusion in the solution, it is necessary to optimize the jet propulsion to minimize the delay. According to the model of Markovic or Morf [ 12, 131,for a given activity step, it is necessary to reduce the diffusion layer thickness to its minimum value. For NASICON membranes which are very fast, the experimental conditions were optimized by changing the over-pressure inside the vessels. The response times measured in this manner are given in Fig. 3. The best
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Time (ms) Fig. 4. Noise effect observed with calomel reference electrodes (NASICON sensor).
176
instance with NASICON membrane, we have measured 350 ms (t& with such a device and only 15 ms with our device (two reference electrode system) for 1 to 0.1 M NaCl solution. Tests to Na+ response
The responses of the Na+-glass membrane (PNAV from Tacussel@) and the NASICON membrane are plotted in Fig. 5(a) and (b), respectively, from 0.1 to 1 M NaCl concentrations and vice versa. A ratio of one order of magnitude is obtained between both ISE. The response curves can be relatively well fitted according to the relation established by Rechnitz and Hameka [ 141or Johansson and Norberg [ 151for glass ISE E(t) = & + RT/Fexp( -t/r) ln(C,/C2)
(1)
where E, is the final value of voltage, C, /C, is the ratio of both concentrations, z is a time constant, R, T and F having their usual meanings. In this model, r depends on the double capacitance and other electrical characteristics of the system (RC equivalent circuits). The t
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Time (ms)
(a) 130
,
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values are about 150 to 200 ms and 10 to 15 ms for the glass membrane and the NASICON one, respectively, with a correlation factor close to 0.99. The response times (to,& are 340-440 ms and 25-37 ms, respectively. With the model proposed by Markovic and Osbum [12] for a diffusion process in the solution film, the response obeys the following relation E(t) = E, + RT/F ln[( 1 - C, /C,) exp( -t/r)]
(2)
where r is a function of the diffusion layer thickness. In this model, the response times are approximately 700 ms and 25-40 ms for the glass membrane and the NASICON one, respectively, with a correlation factor of 0.93 and 0.99. Infuence of the internal reference system
The preceding sensors were built with the internal reference system Ag/AgCI, NaCl 0.1 M, because, as mentioned elsewhere (51, the Na+ ionic exchange between NASICON and aqueous solutions is very fast. For comparison, an ISE built with a solid-state internal reference was also tested, for instance polyethylene oxide (PEO) doped with NaI and AgI [ 161to ensure a thermodynamical reversibility between NASICON and silver used as an electrical connection. The response is then distinctly slower: the response time t,,, is about 500-700ms and the value of 7 is a few 1OOms. The observed difference is too high to be attributed to the geometric area of this new ISE (the diameter is only slightly larger than the preceding one). In such an internal reference system, up to now, we know only the conductivity value and the oxido-reduction kinetics (an ionic exchange study is in progress in our laboratory). The time constants are about 10m4and 1 s, respectively, for both phenomena [ 16, 171.The order of magnitude of the reaction kinetics at the PEO/silver electrode is relatively similar to the response one. This is probably the major limiting phenomenon in the device. On one hand, such a solid-state ISE is advantageous for practical reasons (free position, possibility of high temperature treatment, miniaturization. . .), however, on the other hand, the response time does not seem outstanding. Transient K+ interference
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For particular applications, for instance in biological monitoring, it may be necessary to know the transient phenomena in the presence of interfering ions, for example KC with our Na+ sensors. For this purpose, transient responses of an Na •t -glass ISE were compared with a NASICON membrane. Up to now, the response was only studied as a function of two separate solutions:
50
Time (ms)
Fig. 5. Transient response to Na+: (a) glass membrane; NASICON membrane.
solution A: aNa= 0.1, aK = 0 (b)
solution B: aNa= 0, aK = 0.1
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165 160 155 150 0
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60 F 5 8 4o g 3 20
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(W
the NASICON one. On the other hand, no transient junction phenomenon can interfere with the response, because our reference electrodes are in steady-state conditions, as discussed above. From our results, we can only attribute this effect to a phenomenon related to the diffusion into the membrane. The NASICON conductivity is very high and the sites are very well calibrated to Na+ ions [5], contrary to the glass membranes, and we think that these phenomena are determining in the selectivity effect by diffusion or by interfacial exchange. Several models are for instance discussed in ref. 7. More experimental results will be necessary to choose an adapted model.
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Fig. 6. Transient responseto K+ interferingion (separate solution method, ai = 0.1): (a) glass membrane; (b) NASICON membrane.
Our double-jet cell is interesting for fast ISE responses. A similarity is observed between the response time and the characteristic time of ionic exchange at the NASICON/solution interface, showing a correlation between both phenomena. Nevertheless, the response time is still slightly too high and the transient response would be considered as partially limited by a diffusion phenomenon. From our results, NASICON seems to be an advantageous membrane when compared to glass materials. One of the essential points is the fast response. Another point is the absence of a potential overshoot in transient response when NASICON is used as selective membrane under interfering conditions. From a theoretical point of view, this phenomenon is not very clear, nevertheless it could be advantageous for biological studies where K+ and Na+ are competitive ions.
The steady-state response is, according to the Nickolskii relation: E = E” + RTIF ln(a,, + KNa,KaK)
where KNa,K is the selectivity coefficient of the membrane. KNa,K being lower than unity, the voltage in solution B is lower, for both ISE (see for instance refs. 2 and 4 for KNa,K values in these ISE examples). Nevertheless, with the glass electrode, one can observe a potential overshoot (Fig. 6(a)) which is positive in the transfer from solution A to B and negative in the >pposite direction. This phenomenon is very similar to p.he potential overshoots mentioned for instance on AgX ISE [7]. No noticeable overshoot appears on NASICON responses in the scale of our observations, as shown in Fig. 6(b). The overshoot effect cannot be attributed to the diffusion phenomena of ions in the solution, because the used solutions and their propulsion conditions were identical in both experiments. Only the geometries are different: spherical in the glass ISE device and plane in
References
Fabry, J. P. Gros and M. Kleitz, Solid state ionics for ISFET,Symp. Elewochemical Sensors, Rome, Italy, June 12-
1 P.
14, 1984. 2 P. Fabry, J. P. Gros, J. F. Million-Brodaz and M. Kleitz, NASICON an ionic conductor for solid state Na+ selective electrode, Sensors and Actuators, 15 (1988) 33-49. 3 H. Y. P. Hong, Crystal structures and crystal chemistry in the system Na, +.Zr,Si,P,_,O,,, Mater. Rex Bull., 11 (1976) 173-182. 4 A. Caneiro, P. Fabry, H. Khireddine and E. Siebert, Performance characteristics of sodium super ionic conductor prepared by sol-gel route for sodium ion sensors, Anal. Chem., 63 (1991) 2550-2557. 5 P. Fabry and E. Siebert, in S. Yamauchi (ed.), Chemical Sensors Technology, Vol. 4, Kodansha, Tokyo, 1992, pp. I1 l124. 6 J. Koryta and K. Stulik, Ion-Selective Elecrrodes, Cambridge University Press, Cambridge, 2nd edn., 1983. 7 E. Lindner, K. Toth and E. Pungor, Dynamic Characteristics of Ion-Selective Electrodes, CRC, Boca Raton, FL, 1988.
178 8 K. Toth and E. Pungor, Recent results on the dynamic response of precipitate based ion-selective electrodes, Anal. Chem., 64 (1973) 417-421. 9 E. Lindner, K. Toth, J. Jeney, E. Pungor, R. P. Buck and P. Kemeny, in E. Pungor (ed.), Ion-Se/e&e Elecfrodes, Vol. 5, Pergamon, Oxford, 1989, pp. 459-479. M. Attari, Pilotage automatique de tests de capteurs $ ions, Thesis, Grenoble, 1991. J. D. Chatelain and R. Dessoulavy, Electronique, Presses Polytechniques Romandes, Lausanne, 1982. P. L. Markovic and J. 0. Osbum, Dynamic response of some ion selective electrodes, AIChe J., 19 (1973) 504-510. W. E. Morf, E. Lindner and W. Simon, Theoretical treatment of the dynamic response of ion-selective membrane electrodes, Anal. Chem.. 4711975) 1596-1601. 14 G. A. Re&itz ‘and H. F. Hameka, The theory of glass electrode response, Z. Anal. Chem., 214 (1965) 252-257. 15 G. Johansson and K. Norberg, Dynamic response of the glass electrode. J. Electroanal. Chem.. 18 (1968) 239-250. C. Montero-OcamG anh M.’ Armand, Polymer 16 P. Fabj, electrolyte as internal ionic bridge for ion solid-state sensors, Sensors and Actuators, 15 (1988) l-9. 17 P. Fabry and M. Armand, Mise au point dBlectrodes sp& cifiques tout-solide, Jourties d’Etudes Electrochimie et Environnement, Gifsur Yvette, Apr. 17-18, 1991, Proc. SEE, Paris, 1991, pp. 85-92.
Bibliographies Mokhtar Attari was born on August 14, 1962 in Algiers (Algeria) and was educated at the ‘Ecole Nationale Polytechnique’ in Algiers. He received his Doctor degree in 1991 from the University of Grenoble on instrumentation and measurement. He is now ‘Maitre Assistant’ at the University of Science and Technology of Algiers. He specializes in electrochemical instrumen-
tation (ISE conditioners, data acquisition and signal processing, dynamic analysis of ISE) and, more recently, in biomedical instrumentation and electrochemical noise generated by ISE. Pierre Fabry was born in Saint Chamond (France) in 1943. He received the degree of Doctor in Physics in 1976 from the Scientific and Medical University (Joseph Fourier) of Grenoble. He is presently assistant professor in this University (Department of Chemistry) and he is head of a Sensor Group in the ‘Laboratoire d’Ionique et d’Electrochimie du Solide de Grenoble’ (ENSEEG-INPG). His research interest is in solid-state electrochemistry and more specially in the use of solid conductors for ion analysis in biomedical or environmental applications. Hubert Mall2 was born in Omans (France) in 1934. He is assistant professor in the Scientific and Medical University of Grenoble (Department of Physics). He has worked in the LIME since its foundation and his research activity is in the field of electronic and microcomputing. Georges Quezel was born in ChambCry (France) in 1932. He graduated from the Scientific and Medical University of Grenoble with a specialization in solidstate physics. In 1984, he founded the ‘Laboratoire d’Instrumentation, Micro-Informatique et Electronique de Grenoble’ (LIME-UJF-G) which deals with industrial and biomedical instrumentation. From that time, his activity has been principally devoted to sensor research.
173
A new double-jet cell for fast ion-sensitive electrodes M. Attari and P. Fabry LIESG (Laboratoire d’lonique et d’Electrochimie du Solide de Grenoble), ENSEEGIINPG. BP 75-38402, Saint Martin #H&es Cedex (France)
H. Mall% and G. Quezel Laboratoire d’lnstrummtation, Micro-Informaiique ef Electroniquede Grenoble, UJF-G, BP 53 X, Grenoble CJdex (Frunce)
Abstract A double-jet system has been chosen to study the transient response of Na+ fast ion-sensitive membranes such as NASICON (Na, +XZr,Si,P,_, O,,). The Nat solutions are thrown by a neutral gas onto the membrane. To avoid the residual drop at the LYE/reference electrode contact, two reference electrodes are used, one in each solution. The system is driven by a microcomputer. Comparisons between an Na+-glass membrane and NASICON have shown a difference of one order of magnitude on the response time values. A potential overshoot phenomenon is observed on the glass membrane during the K+ interference experiment which does not appear significantly with NASICON. This point may be important in biological applications where there is competition between K+ and Na+ ions.
Introduction The ion-selective electrodes (ISE) are very interesting tools because they are active sensors easy to use, for instance in process control or analytical applications. In such examples the use of fast sensors is interesting, it is therefore necessary to know their dynamic properties. The study of response times can also be useful to identify the physicochemical phenomena which take place during the response of the membrane. We have previously suggested the use of NASICON as an Na+-sensitive mem(Na, +xZr,Si,P,_X0,2) brane [ 1,2]. This compound was first synthesized by Hong [3]. It is a crystallized material which is an Na+ fast ionic conductor (a is about 10e3 S cm-’ at room temperature). Its preparation processes and its steadystate characteristics as ion-sensitive material are described elsewhere [2,4]. A high ionic exchange current has been observed at the membrane/solution interface [Sj: the relaxation time of this phenomenon is about 0.1 to 1 ms. In such a case, it becomes necessary to have a special cell to measure the speed of response. As described in the review papers [6,7], several methods have been proposed for studying the transient responses of ISE: electrical methods (complex impedance, voltage step, polarization and exchange current studies. . .) activity step methods l
l
09254005/93/$6.00
In the present work, an activity step method was chosen because it corresponds to a real situation for the sensors. In such methods, a change of the ion concentration is given at the ISE membrane surface. The voltage is then recorded as a function of time. The response time t, is defined as the time necessary to reach the ratio G(of the Nernstian variation (for instance 0.99, 0.9 or 0.5). The shape of the concentration step which is given by the system is then of the highest importance and must be accurately defined, the more so, the faster is the sensor. As mentioned by several authors [6,7], one can distinguish two types of cell to measure the response times: 9 the dipping methods, so called immersion methods: the ISE is conditioned in an initial solution (activity a,) and then, after a fast washing and/or shaking of the sensor, the ISE is immersed into a solution of activity u2. With this method, the activity is not well defined during the transfer. A system with a flowthrough technique is then preferable, for instance a double-jet system with a fast commutation, each jet with a solution of the corresponding activity, a, or a,. the injection methods: the sensor is conditioned in a carrier solution of activity u1 and a small volume of a concentrated solution is quickly introduced into the carrier solution, with a strong stirring action. Several experimental cells are described in the review papers i&71. l
@ 1993
-
ElsevierSequoia.All rights
reserved
174
Experimental A dipping system with pulsed jets was chosen because this type of apparatus is quasi-universal; any ISE device can be tested. The set-up is relatively easy to construct. The apparatus is similar to the one developed by Toth and Pungor [ 81.Both solutions are contained in separate vessels. The jets are thrown out by a neutral gas, for example argon. The commutation of the solution is ensured by a fast shift of the jets (Fig. l(a)). Each jet is thrown perpendicularly to the ISE surface. The distance to the ISE membrane is between one and two ccntimeters Another possibility consists of shifting a mask system in front of both immobilized jets. With such a system, the membrane is not in contact with a solution for too long a time. For this reason, a system with the jets attached to the disk was preferred. Circular shifting
Jat 2
(a)
r’
Reference electmde
(‘-9
Residual dmp
/A
Sensor
” H
(4 Fig. 1. Double-jet cell: (a) double-jet translation system; (b) drop formation; (c)general view: ( 1) inert gas, (2) and (3) manometers, (4) vessel of the C, solution, (5) reference electrode 1, (6) reference electrode 2, (7) vessel of the C, solution, (8) taps, (9) pipes in TygonR, (IO) double-jet driver.
The jet-carrier is very light. It is an openworked disk (Plexiglassa) with a low moment of inertia. The disk is fixed to the axis of a stepper motor. A rubbing system (viscous grease) prevents any oscillation. The connections with both vessels are made by fine and supple pipes (Tygon@). The shift movement is controlled by a microcomputer (Apple IIC? in the first apparatus) with an interface card (Adalab”). In the cells described for instance by Toth and coworkers [7,8], the reference electrode is located near the ISE membrane. A thin film solution ensures an ionic contact between both sensors. The major disadvantage of such cells is the residual drop which remains between the ISE surface and the reference one, especially if the ISE has a spherical shape, as shown in Fig. l(b). Because the drop must be drained, the measured response time is increased, especially for fast sensors. Lindner et al. [9] have improved the set-up by introducing a reference electrode in the part which supports the ISE, the liquid junction is then ensured by a capillary up to the ISE membrane proximity. Such a system is well adapted to plane membranes. Generally, it is assumed that there is no junction potential at the reference electrode. If such a potential occurs, for instance due to a retrodiffusion from the solution to the reference electrode (acid medium), it can result in a delayed response time. To overcome this inconvenience, we have included two reference electrodes, one in each vessel. Both reference electrodes are connected to the electrical ground. A general description is given in Fig. l(c) [lo]. In our device, if a Henderson potential occurs, it takes a steady-state value for each electrode, before the measurement. At the worst, a constant voltage lag is observed (the voltage step does not obey the Nernst law). The movement of the jet support is ensured by an angular change corresponding to one integer step of the motor. The electrical signal is generated by a VIA component (6522 Rockwellfi) which drives a control circuit (L297, SGSG). An amplifier (L298N, SGS@) supplies the power necessary to the motor. A high input impedance differential amplifier with a large common mode rejection ratio (CMRR) adapts the ISE signal. Noise immunity is achieved through shielding. The instrumentation amplifier is configured with two OPA 111 (Burr Brown@) as impedance adapters which are connected to both inputs of a differential amplifier. Frequency compensation is ensured by two 0P07 amplifiers in feedback with capacitors. If the ISE impedance is too high, one of the differential amplifiers (B on Fig. 2) is connected to a similar ISE immersed in a third solution. The third reference electrode is also connected to the ground. With such a system, the common mode voltage is reduced. If the ISE impedance is low, the B-input can
175
100 :
3
L
E
‘Z
. 10
.
. l.
.
3
;\
H $
I
’
i
I
1
10
Pressure
(kPa)
Fig. 3. Response time vs. over-pressure (NASICON sensor)
GffwtandCMRR
Ground
Fig. 2. Instrumentation
amplifier diagram.
be directly connected to the ground (i.e. to the common reference electrodes). The analog to digital conversion is made by a 12 bit successive-approximation ADC (AD 674 A Burr Brown@, minimum conversion time: 15 us). The input voltage is firstly conditioned by a Butterworth filter (f, about 1 kHz) [ 1l] and by a sample and hold circuit (SHC 298 A, Burr Brown@).The sampling rate is set by the converter status line. The ADC time-base is connected to the microprocessor by a special line. Because the Apple Basic language is too slow, assembly subroutines have been written to drive the data acquisition procedure. A detailed description of the apparatus is given in Attari’s thesis [lo].
over-pressure seems to be 8 kPa in our set-up. This pressure was used throughout the studies. Beyond this value, the response time increases. It can be due to a turbulent flow which creates variations in the diffusion layer thickness, the average value being higher. Reference electrodes
The choice of the reference electrodes is very important because their impedance characteristics can cause some difficulties. For instance, saturated calomel electrodes have too high impedances and, in spite of drastic precautions, a 50 Hz noise disturbs significantly the sensor response, as shown in Fig. 4. Such difficulties were not observed with Ag/AgCl, KC1 3 M reference electrodes which are better from this point of view. As discussed above, if the reference electrode is located near the ISE membrane, the effect of the residual drop on the transient response becomes apparent, especially moving from high activity to low activity. For
100
00
Results Hydrodynamic optimization
As the response phenomena can be limited by diffusion in the solution, it is necessary to optimize the jet propulsion to minimize the delay. According to the model of Markovic or Morf [ 12, 131,for a given activity step, it is necessary to reduce the diffusion layer thickness to its minimum value. For NASICON membranes which are very fast, the experimental conditions were optimized by changing the over-pressure inside the vessels. The response times measured in this manner are given in Fig. 3. The best
0
-20
0
20
40
60
60
100
120
Time (ms) Fig. 4. Noise effect observed with calomel reference electrodes (NASICON sensor).
176
instance with NASICON membrane, we have measured 350 ms (t& with such a device and only 15 ms with our device (two reference electrode system) for 1 to 0.1 M NaCl solution. Tests to Na+ response
The responses of the Na+-glass membrane (PNAV from Tacussel@) and the NASICON membrane are plotted in Fig. 5(a) and (b), respectively, from 0.1 to 1 M NaCl concentrations and vice versa. A ratio of one order of magnitude is obtained between both ISE. The response curves can be relatively well fitted according to the relation established by Rechnitz and Hameka [ 141or Johansson and Norberg [ 151for glass ISE E(t) = & + RT/Fexp( -t/r) ln(C,/C2)
(1)
where E, is the final value of voltage, C, /C, is the ratio of both concentrations, z is a time constant, R, T and F having their usual meanings. In this model, r depends on the double capacitance and other electrical characteristics of the system (RC equivalent circuits). The t
260 I
Time (ms)
(a) 130
,
I
I
120
values are about 150 to 200 ms and 10 to 15 ms for the glass membrane and the NASICON one, respectively, with a correlation factor close to 0.99. The response times (to,& are 340-440 ms and 25-37 ms, respectively. With the model proposed by Markovic and Osbum [12] for a diffusion process in the solution film, the response obeys the following relation E(t) = E, + RT/F ln[( 1 - C, /C,) exp( -t/r)]
(2)
where r is a function of the diffusion layer thickness. In this model, the response times are approximately 700 ms and 25-40 ms for the glass membrane and the NASICON one, respectively, with a correlation factor of 0.93 and 0.99. Infuence of the internal reference system
The preceding sensors were built with the internal reference system Ag/AgCI, NaCl 0.1 M, because, as mentioned elsewhere (51, the Na+ ionic exchange between NASICON and aqueous solutions is very fast. For comparison, an ISE built with a solid-state internal reference was also tested, for instance polyethylene oxide (PEO) doped with NaI and AgI [ 161to ensure a thermodynamical reversibility between NASICON and silver used as an electrical connection. The response is then distinctly slower: the response time t,,, is about 500-700ms and the value of 7 is a few 1OOms. The observed difference is too high to be attributed to the geometric area of this new ISE (the diameter is only slightly larger than the preceding one). In such an internal reference system, up to now, we know only the conductivity value and the oxido-reduction kinetics (an ionic exchange study is in progress in our laboratory). The time constants are about 10m4and 1 s, respectively, for both phenomena [ 16, 171.The order of magnitude of the reaction kinetics at the PEO/silver electrode is relatively similar to the response one. This is probably the major limiting phenomenon in the device. On one hand, such a solid-state ISE is advantageous for practical reasons (free position, possibility of high temperature treatment, miniaturization. . .), however, on the other hand, the response time does not seem outstanding. Transient K+ interference
60
04
0
10
20
30
40
For particular applications, for instance in biological monitoring, it may be necessary to know the transient phenomena in the presence of interfering ions, for example KC with our Na+ sensors. For this purpose, transient responses of an Na •t -glass ISE were compared with a NASICON membrane. Up to now, the response was only studied as a function of two separate solutions:
50
Time (ms)
Fig. 5. Transient response to Na+: (a) glass membrane; NASICON membrane.
solution A: aNa= 0.1, aK = 0 (b)
solution B: aNa= 0, aK = 0.1
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175 F g
170
& 2 >"
165 160 155 150 0
12
3 4 Time (s)
(a)
5
6
80
Conclusions
60 F 5 8 4o g 3 20
0
(W
the NASICON one. On the other hand, no transient junction phenomenon can interfere with the response, because our reference electrodes are in steady-state conditions, as discussed above. From our results, we can only attribute this effect to a phenomenon related to the diffusion into the membrane. The NASICON conductivity is very high and the sites are very well calibrated to Na+ ions [5], contrary to the glass membranes, and we think that these phenomena are determining in the selectivity effect by diffusion or by interfacial exchange. Several models are for instance discussed in ref. 7. More experimental results will be necessary to choose an adapted model.
I
0
I
100
I
I
200
300
1
I
I
400
500
600
Time (ms)
Fig. 6. Transient responseto K+ interferingion (separate solution method, ai = 0.1): (a) glass membrane; (b) NASICON membrane.
Our double-jet cell is interesting for fast ISE responses. A similarity is observed between the response time and the characteristic time of ionic exchange at the NASICON/solution interface, showing a correlation between both phenomena. Nevertheless, the response time is still slightly too high and the transient response would be considered as partially limited by a diffusion phenomenon. From our results, NASICON seems to be an advantageous membrane when compared to glass materials. One of the essential points is the fast response. Another point is the absence of a potential overshoot in transient response when NASICON is used as selective membrane under interfering conditions. From a theoretical point of view, this phenomenon is not very clear, nevertheless it could be advantageous for biological studies where K+ and Na+ are competitive ions.
The steady-state response is, according to the Nickolskii relation: E = E” + RTIF ln(a,, + KNa,KaK)
where KNa,K is the selectivity coefficient of the membrane. KNa,K being lower than unity, the voltage in solution B is lower, for both ISE (see for instance refs. 2 and 4 for KNa,K values in these ISE examples). Nevertheless, with the glass electrode, one can observe a potential overshoot (Fig. 6(a)) which is positive in the transfer from solution A to B and negative in the >pposite direction. This phenomenon is very similar to p.he potential overshoots mentioned for instance on AgX ISE [7]. No noticeable overshoot appears on NASICON responses in the scale of our observations, as shown in Fig. 6(b). The overshoot effect cannot be attributed to the diffusion phenomena of ions in the solution, because the used solutions and their propulsion conditions were identical in both experiments. Only the geometries are different: spherical in the glass ISE device and plane in
References
Fabry, J. P. Gros and M. Kleitz, Solid state ionics for ISFET,Symp. Elewochemical Sensors, Rome, Italy, June 12-
1 P.
14, 1984. 2 P. Fabry, J. P. Gros, J. F. Million-Brodaz and M. Kleitz, NASICON an ionic conductor for solid state Na+ selective electrode, Sensors and Actuators, 15 (1988) 33-49. 3 H. Y. P. Hong, Crystal structures and crystal chemistry in the system Na, +.Zr,Si,P,_,O,,, Mater. Rex Bull., 11 (1976) 173-182. 4 A. Caneiro, P. Fabry, H. Khireddine and E. Siebert, Performance characteristics of sodium super ionic conductor prepared by sol-gel route for sodium ion sensors, Anal. Chem., 63 (1991) 2550-2557. 5 P. Fabry and E. Siebert, in S. Yamauchi (ed.), Chemical Sensors Technology, Vol. 4, Kodansha, Tokyo, 1992, pp. I1 l124. 6 J. Koryta and K. Stulik, Ion-Selective Elecrrodes, Cambridge University Press, Cambridge, 2nd edn., 1983. 7 E. Lindner, K. Toth and E. Pungor, Dynamic Characteristics of Ion-Selective Electrodes, CRC, Boca Raton, FL, 1988.
178 8 K. Toth and E. Pungor, Recent results on the dynamic response of precipitate based ion-selective electrodes, Anal. Chem., 64 (1973) 417-421. 9 E. Lindner, K. Toth, J. Jeney, E. Pungor, R. P. Buck and P. Kemeny, in E. Pungor (ed.), Ion-Se/e&e Elecfrodes, Vol. 5, Pergamon, Oxford, 1989, pp. 459-479. M. Attari, Pilotage automatique de tests de capteurs $ ions, Thesis, Grenoble, 1991. J. D. Chatelain and R. Dessoulavy, Electronique, Presses Polytechniques Romandes, Lausanne, 1982. P. L. Markovic and J. 0. Osbum, Dynamic response of some ion selective electrodes, AIChe J., 19 (1973) 504-510. W. E. Morf, E. Lindner and W. Simon, Theoretical treatment of the dynamic response of ion-selective membrane electrodes, Anal. Chem.. 4711975) 1596-1601. 14 G. A. Re&itz ‘and H. F. Hameka, The theory of glass electrode response, Z. Anal. Chem., 214 (1965) 252-257. 15 G. Johansson and K. Norberg, Dynamic response of the glass electrode. J. Electroanal. Chem.. 18 (1968) 239-250. C. Montero-OcamG anh M.’ Armand, Polymer 16 P. Fabj, electrolyte as internal ionic bridge for ion solid-state sensors, Sensors and Actuators, 15 (1988) l-9. 17 P. Fabry and M. Armand, Mise au point dBlectrodes sp& cifiques tout-solide, Jourties d’Etudes Electrochimie et Environnement, Gifsur Yvette, Apr. 17-18, 1991, Proc. SEE, Paris, 1991, pp. 85-92.
Bibliographies Mokhtar Attari was born on August 14, 1962 in Algiers (Algeria) and was educated at the ‘Ecole Nationale Polytechnique’ in Algiers. He received his Doctor degree in 1991 from the University of Grenoble on instrumentation and measurement. He is now ‘Maitre Assistant’ at the University of Science and Technology of Algiers. He specializes in electrochemical instrumen-
tation (ISE conditioners, data acquisition and signal processing, dynamic analysis of ISE) and, more recently, in biomedical instrumentation and electrochemical noise generated by ISE. Pierre Fabry was born in Saint Chamond (France) in 1943. He received the degree of Doctor in Physics in 1976 from the Scientific and Medical University (Joseph Fourier) of Grenoble. He is presently assistant professor in this University (Department of Chemistry) and he is head of a Sensor Group in the ‘Laboratoire d’Ionique et d’Electrochimie du Solide de Grenoble’ (ENSEEG-INPG). His research interest is in solid-state electrochemistry and more specially in the use of solid conductors for ion analysis in biomedical or environmental applications. Hubert Mall2 was born in Omans (France) in 1934. He is assistant professor in the Scientific and Medical University of Grenoble (Department of Physics). He has worked in the LIME since its foundation and his research activity is in the field of electronic and microcomputing. Georges Quezel was born in ChambCry (France) in 1932. He graduated from the Scientific and Medical University of Grenoble with a specialization in solidstate physics. In 1984, he founded the ‘Laboratoire d’Instrumentation, Micro-Informatique et Electronique de Grenoble’ (LIME-UJF-G) which deals with industrial and biomedical instrumentation. From that time, his activity has been principally devoted to sensor research.