An automatic system for determining the effects of temperature on the hysteresis curves of ion-selective electrodes

This paper describes an automatic system which measures the effect of temperature variations on the response of ion-selective electrodes (hysteresis curves). The system is managed by a computer program which plots hysteresis curves following a pre-established temperature cycle, from setting and controlling the temperature of the water-bath, to acquiring the response potentials of up to five electrodes after temperature stabilization.


Introduction
The pH sensitive glass electrodes on the market have an internal reference solution with a fixed composition in order to place the concentration of the isopotential point in the middle of the operational range of measured values, thus minimizing the effects of the temperature variation on the electrode response [1,2]. There has seemingly been little effort made to study the effect of the temperature on the response of other ion-selective electrodes in order to improve their performance. The early literature on ion-selective electrode cc.ntain: .n!y a few papers on the subject, published in the early 1970s [3][4][5].
Work at the authors' laboratory on minimizing the effect of temperature on the response of ion-selective electrodes by adjusting the position of the isopotential point [6], then showed that for a PVC membrane calcium sensitive electrode with internal reference solution, the concentration of the isopotential point could be adjusted by varying the solution's composition [7].
The possibility of optimizing the construction procedure of all-solid-state ion-selective electrodes to bring the concentration of the isopotentiai point to the range of operational concentration values was also investigated, both for plastic [7][8][9] and crystalline [10][11][12] membrane electrodes. Temperature effects on all-solid-state electrodes are complex, because the devices involve several materials with different thermal properties and a badly defined internal reference. The results obtained showed that the effect of the temperature variation on the response of all-solid-state ion-selective electrodes depends on the nature and composition of the conductive support used, its internal electric contact and the type of membrane [6]. No systematic relations were found between these construction parameters of the electrodes and the coordinates of their isopotential points, so the optimization of the construction procedure of all-solid-state electrodes Correspondence to A. Madado. has to be trial-and-error [5,6], involving a large amount of experimental work and time-consuming experiments. A hysteresis experiment for all-solid-state ion-selective electrodes involving cycles of five temperature values (10 -+ 50 --10C in 10C steps), for example, typically requires seven to eight hours of practically continuous monitoring of temperature and the corresponding response potentials of the electrodes [10]. Therefore, it was decided to assemble a system for automatic handling of experiments to determine the temperature hysteresis curves of ion-selective electrodes in isothermal cells (indicator and reference electrodes both immersed in the same cell, the temperature of which is varied) [13]. In favourable situations, these curves provide full information about the temperature behaviour ofthe electrodes, including the position of the isopotential point [14]. The assembly of the system, the software for its operation, and the results obtained when it was used to plot hysteresis curves for all-solid-state PVC membrane ammonium electrodes with nonactin as sensor, are described in this paper. Description of the system Assembly of the system The automatic system, shown in figure 1, consists of a Techne thermoregulator, model TU-16D, in a waterbath of 5 capacity; a Metrohm thermostated doublewall cell with 50 ml of capacity; a home-made high impedance circuit box, which allows parallel or, rather, almost simultaneous successive readings of differences of potential for up to five electrodes, against the same reference electrode; a Compaq Prolinea 325s, PC with a Lab Master DMA card (Scientific Solutions Inc.).
The water-bath temperature is controlled by the PC. The temperature is set by an analogue signal generated, via software, by the A/D converter of the Lab Master DMA card, and sent to the thermoregulator. An analogue signal sent by the thermoregulator to the A/D converter of the card is used to monitor the temperature. The response potentials of the electrodes are also acquired by the A/D converter of the Lab Master DMA card through the high impedance circuit box.

Control software
The control software was developed in Quick Basic, The A/D converter allows 40,000 potential measurements per second to be read. In the present application, and for noise reduction purposes, the program averages 500 readings per point. This number of readings allows response potentials with standard deviations lower than -+-0"05 mV to be acquired. Averaging 500 readings per point corresponds to 80 potential readings per second, changes in the response potentials of up to five electrodes can be followed. Therefore, the high impedance circuit box was built for this number of electrodes.  The global performance of the system was evaluated by monitoring the variation with temperature of the response potentials of ammonium ion-selective electrodes to obtain hysteresis curves. The electrodes were immersed in solutions of ammonium chloride of constant concentration and the temperature was varied from 10 to 40 C and back again to 10 C, in 10 C steps.

Electrodes and reagents
All-solid-state nonactin ammonium selective electrodes were prepared by casting a PVC membrane, mm thick, on a graphite/epoxy conductive support [6,15]. The solution used for preparing the sensing membrane of the ammonium electrode consisted of 3% of nonactin (Fluka, Ammonium Ionophore I), 33% of PVC (Fluka), and 64% of dibutylsebacate (Fluka, Selectophore), all dissolved in THF (Merck). The graphite powder used for the preparation of the conductive support was from Merck (<50 mm). The epoxy resin used to bind the graphite in the conductive support was HUNF, from Epoxy Technology, Inc. (1 graphite to epoxy weight proportion). When not in use, the ammonium electrodes were conditioned in a 0"IM ammonium chloride solution.

Results and discussion
Control of the water-bath temperature Figure 3 shows the temperature values stored during about 11 h of monitoring, when the water-bath control set was fixed, respectively to 25 and 45 C.    The times required for stabilization of the temperature of the bath show that thermal stability was reached in 3-12 min, depending on the difference between the initial and the set temperatures and on the direction of temperature change.
In conclusion, the system allows the temperature of the water-bath to be controlled with sufficient stability and precision for the application.

Hysteresis experiments
Typical results obtained in a hysteresis experiment for an ammonium selective electrode in 0"5 mM ammonium chloride are presented in figure 4. Figure 4(a) shows the hysteresis curve and figure 4(b) shows the response potential of the electrode versus time, obtained at the different temperatures. These last plots allow the time at which the stabilization of response is attained to be identified and the corresponding average response potentials after stabilization at each temperature, which are used to plot the hysteresis curve, to be calculated. To obtain a response hysteresis curve of the type shown in figure 4(a), the system requires approximately 150 min. However, the operator is only involved for 30 min, most of this time is used for preparation and for assembling the electrodes in the potentiometric cell. The results obtained show that the system described is adequate for studying hysteresis due to temperature variations in the response of ion-selective electrodes.
The system reduces the time needed for obtaining a hysteresis curve by about a half. In addition operator time is reduced by 90% compared with manual operation of a classical potentiometric system. The system can also be adapted for automatic calibration of a set of electrodes at several temperatures by connection to an automatic burette.