Analytical applications of ion selective devices

Abstract

Ion selective electrodes, ISEs, and ion sensitive field effect transistors, ISFETs, are small, relatively simple to operate and easily automated sensors and, therefore, have a wide range of uses e. g. for field measurements in portable detectors, for on-line measurements in industrial flow systems and in clinical work. Several flow systems were studied for use with ion selective devices. New design ISE flow cells, designed at Newcastle, were found to minimise dead space and carry-over of sample solutions, allowing more rapid sample throughput. An ISFET flow cell studied, however, was found to have serious design faults. The constant volume dilution method of calibration and selectivity determination was shown to be a simple easy-touse method but must be implemented with caution. The selectivity of sensors to the primary ion was determined, where applicable, and the optical sensitivity of ISFETs was examined. Potassium concentrations in fertilizers were determined, using ISEs, in both flow systems described above; more accurate results were obtained using the newer flow-cells. Failure of ISEs after prolonged use in fertilizer solutions is believed to be have been caused by Donnan Breakdown due to HPO 2- ions. A computer controlled titration system was developed which can be used for volumetric or coulometric titrations. Coulometry, an absolute method, is particularly suitable for titration of sub micro-litre samples and for chemically labile species as sample manipulation is minimised and avoids addition of solution reagent, obviating CO 2 contamination of hydroxide. The advantages of coulometry were exploited in work to confirm the second dissociation constant for hydrogen sulphide. Aerial oxidation and sample carbon dioxide uptake are common problems associated with sulphide solutions. Using degassed water for sample preparation, keeping all solutions under nitrogen and using a sulphide anti-oxidant buffer it was possible to reduce sulphide oxidation. Coulometry was used to generate hydrogen ions and potentiometric measurements of the pH and sulphide ion concentrations, made simultaneously, were used to calculate the pK 2d of hydrogen sulphide for a range of 50 gl sodium sulphide solutions. A non-linear least squares programme, SUPERQUAD, was used to obtain a better value for pK 2d. Though a coulometric option exists in SUPERQUAD, it is not often implemented. ISE titration results have not been used much with SUPERQUAD; this work examined the potential of expanding the application of SUPERQUAD. Values of pK 2d of 12.08 ± 1.0 and 11.83 ± 0.4 were obtained by visual inspection and SUPERQUAD refinement, respectively. These values agree well with the text-book value, of 11.96, and demonstrate the accuracy of coulometry. The auto-titration system developed has advantages in many areas, particularly in clinical chemistry where determinations of available species in sub micro-litre samples, delivered in a flow system are required.