STUDIES WITH ION SELECTIVE MEMBRANE ELECTRODES

Abstract

Ion-Selective electrodes are extensively in use from the point of view of low investment and operational costs. Other attractive features of the I.S.Es. are- the speed with which they permit a sample to be analysed. Furthermore, analysis may be made without difficulty of even coloured and viscous samples containing a high concentration of suspended solids. The sensors are utilised in biological, biochemical, environmental and other analytical research. Recent trend in this field has been to develop tailor made materials possessing adequate stability and specific selectivity for the fabrication of desired sensors. The development of new ion exchangers is getting additional impetus with the realization that their membranes may undergo selective exchange and may be used as ion selective electrodes. There has been vigorous research activity, in the last few decades, in this direction and new materials have been prepared and studied for their sorption characteristics and selectivity with the sole objective of the development of some specific sensors. These products provide a suitable matrix for the fabrication of solid membrane sensors selective for cations and anions. A Polytungstoantimonate gel possessing good ion exchange characteristics and promising selectivity for bivalent ions has been prepared and studied for column separations etc. (1) The product exhibits high selectivity for Hg2+, La3+, Cu2+ and Pb2+ idns, which is reflected by their high K, values. In view of

this, it was thought desirable to explore the electroanalytical selectivity of this substance. Detailed investigations were subsequently made to prepare sensors for mercury and lanthanum ions. The estimation of Hg and La is difficult in presence of various ions as the known methods are susceptible to interferences from other ions and halides. As such it was important to develop electrodes for Hg2+ and La3+ ions, incorporating the above mentioned electroactive phase. Heterogeneous membranes using polystyrene as binder were prepared and relevant functional properties like water content, porosity, swelling, electrolyte absorption and conductance were determined. The values of water content, porosity and swelling are negligibly small, thus magnifying the importance of exchange sites in the exchanger material. The conductance of the membrane is found to decrease with increasing ionic radii. •The electrode exhibits a Nernstian response to Mercury in the range 10-1 to 5xlO~5M at pH 4-6 with the slope of 25mV/decade of concentration. The response time is less than one minute and the potentials stay constant for more than ten minutes. The electrode has been tested to be successful in both aqueous and 50% non-aqueous medium. Monovalent ions cause some interference in the working of this assembly, but bivalent and polyvalent ions cause negligible interferences. This electrode has no significant effect of surfactants. Precipitation titrations involving mercuric chloride has also been monitored by using this membrane #sensor. Anions also cause no interference. (iii) Same inorganic exchanger has also been used for the preparation of La selective sensor. It can estimate La3+ even in the presence of various cations, thus magnifying its importance. La ion can be estimated in the range of 10_1 to -5 10 M at pH 4-7. Variation of membrane potentials in partially non-aqueous medium has been studied and it is observed that upto 50% non-aqueous medium the sensor is unaffected. Bivalent and polyvalent cations like Zn2+, Cd2+, Al3+, Cr3+ etc. do not interfere. Small additions of surfactant causes significant shift in membrane potentials. This however can be overcome by treating the membrane with the surfactant solutions of some optimum concentration for a definite time. The membrane sensor has also been used as an end point indicator in potentiometric titrations involving La ions. Anions have practically no disturbance in the working of the electrode sensor. Fourth chapter deals with the organic gel membranes. Preparation and characterisation of the organic polymers is dealt with, in second chapter. Two new chelating ion exchange resins (2-Salicylidene aminothiophenol formaldehyde and salicyaldoxime formaldehyde) are prepared and characterised. These resins exhibit very good uptake for a number of metal ions. Investigations reveal that these polymers exhibit promising selectivity for bivalent ions like copper, lead, zinc etc. Consequently efforts were made to use these chelating polymers for the fabrication of membrane sensors selective to copper and lea'd ions. (iv) Relevant functional properties of both unloaded and loaded membranes were determined. The values of water content, porosity, swelling and electrolyte absorption were found to be much higher in the case of unloaded membrane, where as specific conductance is higher for loaded membrane. Solid membrane electrode made from unloaded resin shows linear response in the range 10~1-5xl0"5M of copper and lead ions with a slope of 23mV/decade of concentration. Working pH range for both copper and lead ions is 6-8. Effect of non-aqueous medium is also studied and there is no significant change in the working of the electrode up to 35% non aqueous content. Monovalent ions cause some interference in the working of this electrode, but bivalent and polyvalent ions cause tolerable interferences. The effect of surfactants on the estimation of copper and lead ion concentration have also been observed. The presence of surfactant reduces the potentials and a very sharp fall in slopes of the plots is also observed. The above mentioned disturbances can be overcome by treating the membranes in surfactant solutions. Precipitation titrations involving copper and lead ions have been monitored by using these sensors. Anions cause no interference and electrode can function well in their presence also.

Membranes made out of loaded resin showed significant improvement over the ones prepared from the unloaded resins. With loaded membranes, copper can be estimated in the range 10 -5xl0~ M at pH 4-8. A better slope (28 mV/decade of concentration) is observed in the plots obtained with these membranes. In the case of lead ions, the membrane response is linear and slope=25 mV/decade of concentration in the concentration range of 10_1M-10~6M at pH 4-8. The presence of non agueous content can be tolerated up to 35%. Interferences of various ions have been reported and it is found that polyvalent ions do not interfere at all, whereas monovalent ions show some minute interference. Surfactants have practically no interference and even the slopes and potentials are well maintained. The membrane sensors are good end point indicators in potentiometrie titrations for copper and lead ions. The unloaded resin was also used to fabricate PVC based solid matrix type membrane sensor. This membrane was also tested for both copper and lead ions. The PVC based membrane exhibits a better linearity range, slope and selectivity as compared to the other two polystyrene based membranes. It was found to be Nernstian in the range 10~ -10~6M for both copper and lead ions with pH range 4-8 and 4-7 respectively. This sensor was found to be# much more selective and tolerant to higher concentration of non-aqueous medium and surfactants. The sensors showed much sharper end points in the titrations of lead nitrate and copper sulphate solutions.

I Besides this the electroanalytical selectivity of one more such resin(Salicyaldoxime formaldehyde) for lead ions has been investigated. Membranes are fabricated with the above resin using both polystyrene and PVC as the binder material. Polystyrene membranes were characterised in terms of water content, porosity, swelling, electrolyte absorption and conductance. The polystyrene membrane shows a Nernstian response to lead ions in the range 10" -10~ M, (slope=27mV/decade of concentration). The electrode functions in the pH range 3-6. Presence of non-aqueous content up to 35% does not effect its functioning. Small additions of surfactants cause large shifts in membrane potentials. Membrane after being treated with these surfactants becomes immune to their effect and exhibits a wider range of response to lead ions. Interferences of polyvalent ions is negligible, where as monovalent ions do cause some serious interferences. Successful titrations of lead nitrate solutions have been monitored using this membrane electrode. PVC based Salicyaldoxime formaldehyde membrane shows a Nernstian response to lead ions in the range 10-1-10"*5M (slope=28mV/decade of concentration). The valid pH range for the electrode is 4-8. The electrode can function well in the presence of 50% non-aqueous content. Trivalent ions have quite low values of selectivity coefficient thereby indicating their non-interference. For monovalent ions the values are slightly higHer (5^7) and tthey might interfere if present in high

t concentration. The effect of surfactants is not significant and the same can be overcome by conditioning the membranes with the detergent solution. The sensor gives sharp end points in the titration of lead ions. The last chapter deals with the analytical applications of I.S.Es. to estimate metal ions in river water. The developed sensors have been successfully used for the analysis of ions like Hg2+, 2+ 2 + Cu and Pb in Hindon river water. The con centrations of metal ions obtained with the developed sensors compare well with the reported results published in government bulletins. 1. Sen, A.K. and Sudhangsu, B.D., "Studies on Polytungsto antimonate Ion exchanger: Part I - Synthesis, properties and Ion exchange behaviour of Polytungstoantimonate", Indian J. Chem., 25A, 669, 1986