Computer-based automation of concentration measurements with ion-selective electrodes

An integrated computer system consisting of an expandable ionanalyzer and a PC has been used to automate concentration measurements with ion-selective electrodes (ISEs). The ionanalyzer determines ionic concentrations using a reference electrode coupled with an ISE. The measurements and calibrations are carried out in a thermostated sample changer equipped with a flow cell. The data obtained by the ionanalyzer are sent via a standard RS 232-C interface to a microcomputer. In this paper, we describe the automatic data acquisition system and the subsequent processing of the measurements. One program (Transorion) automatically collects the measurements carried out by the ionanalyzer, giving a real-time graphic representation of the measurement on the computer screen. A second program (Graforion) facilitates the management of the data stored by the first program, and listing and graphics of these can be obtained on the printer/plotter. The method has been used to study continuous concentration changes in an aqueous solution of potassium iodide.


Introduction
This paper reports an automatic method for the measurement of ionic concentrations using ion-selective electrodes (ISEs). Ionic  V24) output, which gives the concentration, the corresponding electrical potential difference and facilitates the necessary ISE calibrations. The measurements carried out by the ionanalyzer are received by a microcomputer using a program (Transorion), and can be stored on a hard or floppy disk and displayed on the computer screen. A second program (Graforion) gives the data read-out, listing the values obtained and/or providing graphics.

Experimental
The experimental device consists of a reactor (with stirring system), a liquid circulation system, a thermostat circuit, a measurement system (consisting of electrodes, a sample changer and the ionanalyzer and a data collection and recording system.

Reaclor
The solution to be analyzed was placed in a thermostated reactor. The solution was homogenized using a paddlestirrer driven by a variable-speed motor, which was controlled by an electro-optical device; we used a stirring rate of 100 rev min-1. The experiment was conducted at 25-0 + 0"1 C, with an aqueous solution of potassium iodide at an initial concentration of0"01 M; this concentration was increased by adding a 0" M solution of the same electrolyte from a reservoir (M1 in figure 1), keeping the volume of the solution in the reactor (R) constant by extraction to a waste reservoir (M2  2). This electrode response is fed to an ionanalyzer (Orion, EA940) where the temperature, the concentration and the associated electrical potential difference are displayed in a continuous form. By means of an output RS 232C the data obtained are sent to a microcomputer, where they are stored for further use (figure 1).
Data acquisition and recording system An information system is Used for acquiring, storing and processing the data. This system consists of a microcomputer (Olivetti M24) with three RS 232C interfaces, a high resolution graphics screen, a printer (Olivetti DM 286/2) and a plotter (Roland DXY 880); the operating system is MS DOS. A program (Transorion) automatically collects the measurements carried out by the ionanalyzer. It manages the serial asynchronous communications interface and continually receives and processes the data received from the ionanalyzer. When these data make up a complete measurement the program stores and displays them graphically on the screen. The transmission of this information can be interrupted at any time so that additional information can be entered through the keyboard or electrode calibrations carried out. It is also possible to interrupt the program to return to the main menu. Hence the program allows the real time graphic representation of the measurements and can be interrupted at any moment to realize other functions. The user can interrupt the measurement in order to carry out any other process (e.g. electrode calibration). When calibrating the electrodes it is possible to store on disk the calibration information, and when the user considers the process to be finished, he can return to the main menu.
A second program (Graforion) facilitates the management of the data stored by the Transorion program and its task is to list and display graphically the measurements obtained. When a file name is given by the user, see the characteristic parameters of the process and the values of measurements at specific times, generating a total or partial listing and a graphic representation of the measurements. Alternatively, a copy can be obtained by printer or plotter. Figure 4 shows the flowchart of the Graforion program. When the process is over it is possible to return to the beginning of the program to select another file without returning to the main menu.

Results
The method developed has been applied to the study of the concentration change in an aqueous solution of potassium iodide prepared with doubly distilled, degassed and deionized water, with an initial value of 0"01 M. The solution was continuously renewed by a peristaltic pump, which extracted a volume of solution with a tube while introducing by another tube the same volume of solution at 0" M; the reverse process (from 0" to 0"01 ) was then carried out.
The concentration measures were continuously obtained with an ISE versus a reference electrode over a period of approximately 5 h. Concentration changes with time for the solution inside the reactor were recorded. The results obtained are shown in figure 5. this time, the associated electrical potential difference and, finally, the temperature.

Conclusion
The technique described in this paper is an efficient and reliable method for measuring concentration changes with ISEs. The incorporation of a sample changer with measuring cell, thermostated in parallel with the reactor, allows the periodic calibration of ISEs and the measurement ofproblem concentrations at the same temperature, and can therefore eliminate errors.