Ni-Zn ferrite spinel: characterization and evaluation of its suitability for ion selective electrodes

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Ion selective electrodes (ISE) constitute an important place in the expanding areas of analytical research. This is not surprising, because ISE offer a wide variety of possible applications in many disciplines and, most importantly, they are relatively inexpensive and easy to operate [1, 2]. In recent years, a variety of solid state membrane electrodes have been developed, each of which has its own impact on the analysis of various species [3—5]. The work reported here was aimed at exploring the possibility of utilizing Ni—Zn ferrite spinel for solid state ion selective electrodes and involved the characterization of Ni—Zn ferrite spinel by surface analytical techniques, electrical conductivity measurement and potential response studies. A 1:1 molar ratio of NiO/ZnO and Fe2 03 was used to prepare the Ni—Zn ferrite spinel. In all cases the Fe2 03 content was fixed at 1.0 molar and the molar ratios of NiO and ZnO were varied in such a way that their total content was 1.0 molar. Five different molar ratio combinations of NiO and ZnO, namely 0.1 and 0.9; 0.3 and 0.7; 0.5 and 0.5; 0.7 and 0.3; and 0.9 and 0.1, were chosen for the present study and the spinel samples prepared using each proportion were assigned a formula based on the amount of NiO, ZnO and Fe203 present in them. The NiO, ZnO and Fe203 (in suitable molar ratios) were thoroughly mixed and ground with 50% polyvinyl alcohol (PVA). A weighed quantity of 300 mg of the mixed and ground sample was then compacted and pressed at 8 tcm2. Sintering was carried out in an automatic temperature control programmed electric furnace at four different temperatures, 1100, 1200, 1350 and 1450 °C. During sintering the temperature of the furnace was increased at a rate of 200 °C h—' and after attaining the desired temperatures the samples were retained for 2 h. Two kinds of cooling procedures were adopted: (i) rapid cooling in air and (ii) slow cooling in furnace. The prepared Ni—Zn ferrite samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and Xray diffraction (XRD). EDX analysis was performed with a Link Analysis AN 185S model using a tungsten filament and at an accelerated potential of 20 kV. The measurements were made for a period of 100 s. Based on the integrated intensity values, the amount of each element present was calculated and the oxygen content was obtained after subtracting the total metal content. XRD measurement was carried out with a Shimadzu model VD-IA X-ray diffractometer using CuKa and a nickel filter at an accelerated voltage of 30 kV. The diffraction patterns obtained were analysed after the ASTM (file card no. 8-234) set for the Ni—Zn ferrite spinel. SEM evaluation of the samples were made after they were suitably polished (mechanically by fine abrasive papers and 1 tm and 0.3 tim size alumina powder, followed chemically by 86% phosphoric acid) and sputtered with a gold coating. The electrical conductivity of the prepared Ni—Zn ferrite samples was measured using a two-point probe method described elsewhere [6]. The potential response studies were made with the use of a pH/ion meter. Disk samples of Ni—Zn ferrite were mounted on the bottom of a glass tube of 9 cm length, 1.4 cm diameter and having an inner diameter of 0.5 cm using epoxy resin. The prepared electrodes were tested for any leakage, as such a leakage can drastically affect the potential response. A 0.1 M solution of KC1 was used as internal filling solution for the electrode. Ag/AgC1 electrode and saturated calomel electrodes served as the internal and external reference electrodes. The potential response of the Ni—Zn ferrite electrodes towards the Bi3 and 2_ in the concentration ranges from 10 to iO M were assessed. The interference for the analysis of Bi3* and S2 from various cations and anions was also studied. The composition of the Ni—Zn ferrite spinel prepared at various sintering temperatures was determined using EDX. There was not much change in the Ni, Zn, Fe and 0 contents of the spinel up to a sintering temperature of 1200 °C. However, there was a considerable decrease in the zinc content at higher sintering temperatures, and at 1450 °C this decrease in zinc content amounted to about 50% of the initial zinc content used before sintering (Table I). The loss of zinc at higher sintering temperatures is obvious at all proportions of the NiO, ZnO and Fe2 03 used in the present study and hence can very well be attributed to the sublimation process. XRD measurements were performed on the prepared spinel samples to confirm their formation and crystal structure. For all proportions used in this