ANALYTICAL SELECTIVITY AND FUNCTIONAL PROPERTIES OF SOME ION EXCHANGE MEMBRANES

Abstract

The development of high performance chemical sensors is a fast growing area in the field of analytical science and technology. Chemical sensors are analytical systems for measuring ions and molecules in solution. The most widely studied chemical sensors are the potentiometric based electrochemical devices i.e., ion-selective electrodes. Potentiometric ion-selective electrodes detect changes in electrode potential or membrane potential arising from the specific adsorption or other interactions of ionic species onto the surface of the sensitive layer. In order to be analytically useful the process must be selective with respect to the species of interest. Ion-selective electrodes find many applications in the analysis of raw materials and for quality control of the products due to the speed, sensitivity, cost, reliability and non consumption of the sample in the process. These are specially useful tool for monitoring environment or waste water and also in biological studies particularly in the field of clinical chemistry where a large number of samples and the need for a rapid method of analysis, rule out many slower, more involved methods. Besides this the device can be used irrespective of the colour, viscosity etc. of the test solution and almost no sample preparation is necessary. 8 Iv A survey of literature as well as market reveals the availability of electrodes for mono and bivalent cations. Sensors for polyvalent cations and anions are still not commercially available. Besides this there is a big question mark on the selectivity of sensors commercially available and also the one being reported in literature. As such investigations, to obtain tailor made material to provide electreactive phase having specific selectivity for a particular ion, are called for. The work included in this thesis deals with the development of sensors with the help of some ionic polymers and inorganic gels exhibiting ion exchange characteristics. The details are presented in the following sequence : An inorganic gel polytungstoantimonate, is reported to possess high selectivity for bivalent viz., Cd *, Zn , Cu , etc. ions. In view of this, it was thought desirable to observe the electroanalytical selectivity of this gel material and explore the utility of the membranes of this substance for the estimation of bivalent cations. Efforts made in this direction are given below: Heterogeneous membranes using araldite as binder were prepared and relevant functional properties like water content, porosity, swelling, electrolyte absorption and conductance w^re determined. The values of water content, porosity and swelling were found to be negligibly small, thus magnifying the importance of exchange sites in the gel membrane. The conductance of the membrane is found to decrease with ionic radii. The electrode exhibits a near Nernstian response for cadmium ions in the range 6.31xlCT5-l. OOxlO^M at PH 5-7 with a slope of 26 mV/decade of concentration. The response time is less than one minute and the potentials stay constant for more than three minutes. The electrode assembly has very limited validity in partially 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 significant effect of surfactants. Titrations involving cadmium nitrate have also been monitored by using this membrane sensor. Same inorganic ion exchanger has also been used for the preparation of Zn2+ selective sensor. It responds to Zn2+ ions in the range of 5.OOxlO"5-!.OOxlO^M at pH 3.5-6.0. Variation of membrane potentials in partially non-aqueous medium has been studied and it is observed that upto 20% non-aqueous content the sensor shows nominal increase in slope with simultaneous slight decrease in working concentration range. Monovalent ions cause a little interference while bivalent and polyvalent cations like Pb , Cd , Al etc. do not interfere. Small additions of surfactant upto 5.0xlO_5M concentration does not cause any interference but higher concentrations of surfactants are likely to interfere. The sensor has also been used as an end point indicator in potentiometric titrations involving Zn2+ ions. A comparative assessment indicates a better performance of the membrane sensor for the evaluation of zinc in comparison to cadmium ions. vl Tin oxide has been reported to possess promising selectivity 3- 2- - for some anions like P04 , Cr04 , V03 etc. Preliminary investigations suggested the utility of the membrane sensor for - 2- the estimation of V03 or Cr04 ions. Functional properties of this membrane sensor are also quite low and comparable to the membranes of the same class. The tin oxide membrane in polystyrene bindergives excellent response for vanadate ions. It can be used to estimate vanadium in the range 2.51xl0~ -1.00x10 M and exhibits very good selectivity towards these ions with reasonably fast response time over a wide pH range 4 to 10, though the behaviour in partially non-aqueous medium is not so good. In the presence of anionic surfactant (upto a concn. of l.00x10~ M) sodium dodeeyl sulphate, no adverse effect, on the working of the sensor is observed. The electrode assembly could also be used as an indicator electrode in the titrimetric estimation of vanadium. Efforts were also made to see the selectivity of this membrane for chromate ions. Although the working concentration range -4 -1 (1.78x10 -1.00x10 M) for chromate ions is somewhat better in comparison to vanadate ions, the other factors like selectivity, effect of non-aqueous medium and the presence of surfactant etc. suggest that the membrane will form a better sensor for the estimation of vanadium. vii The use of organic resins specially chelating polymeric resins are finding importance as scavenger of metal ions. Resins of this type exhibit a very selective behaviour towards some heavy metal ions. In view of the importance of such compounds 2-hydroxy naphthaldoxime formaldehyde resin has been tried as an electroactive phase in the membrane sensors for Pb + ions. Membranes were prepared using PVC as well as polystyrene as binders. Functional properties like water content porosity, swelling and electrolyte absorption could be determined only in the case of polystyrene based membranes and the same are found to be quite low in comparison to the values recorded for inorganic gel membranes comparison. Polystyrene based membrane electrode of the said resin shows linear response in the range 1.58x10 -1.00x10 M concentration of lead ions with a slope of 35 mV/decade of concentration while the PVC based membrane sensor have comparatively lower working -4 -1 2+ concentration range i.e., 3.16x10 -1.00x10 M for the same (Pb ions) with a still higher slope of 40 mV/decade of concentration. Both the electrodes are operational in a pH range 4 to 7. Both the electrodes have a very limited validity in partially non-aqueous medium. Selectivity coefficient values suggest the interference of monovalent ions while bivalent and polyvalent cations are found to have very low values of this parameter. Mixed run plots clearly indicate the utility of the electrode even in the presence of -4 lower concentrations (upto 1.00x10 M) of monovalent ions. Higher concentration of surfactants cause significant interference. Both vlll the membranes can be used as an indicator electrode in potentiometric titrations of Pb ions with EDTA. A comparative assessment reveals a better performances of polystyrene based membrane as a sensor for lead ions in comparison to the PVC based one. The electroanalytical selectivity of one more such resin (2-salicylaldene aminothiopehnol formaldehyde) for cadmium and zinc ions both has been investigated. Membranes were fabricated with the above resin using both polystyrene and PVC as the binder material. Polystyrene based membranes were characterized in terms of water content, porosity, swelling, electrolyte, absorption and conductance. These membranes show response for cadmium ions in a -4 -1 wide concentration range 1.00x10 -1.00x10 M with a slope of 15mV/decade of concentration. In this case also the performance is better than those of PVC based membranes where the range is -4 -1 1.79x10 -1.00x10 M and the slope of the plot is 12mV/decade of concentration. Potentials stay constant over a pW range of 4-6. Similarly for zinc ions the working concentration range -4 -1 (1.26x10 -1.00x10 M) for polystyrene based membrane is better than the PVC based one where the range is from 2.51x10 to 1.00x10 M. Even the slope of the plot is better for polystyrene based membrane. The response of polystyrene based membranes are generally faster in comparison to PVC based membranes in both the cases. IX Cadmium can be estimated in solution having 15% non-aqueous content with a polystyrene based membrane sensor while the PVC membrane does not function in non-aqueous medium. Estimation of zinc, in partially non-aqueous medium, is not possible at all. Selectivity coefficient pattern in the case of cadmium ions suggests the superiority of polystyrene supported membranes over the PVC based ones. Even in the case of zinc ions, polystyrene based membrane electrodes are superior. Polystyrene supported membranes, during the estimation of Cd2+ ions, have higher tolerance limit of monovalent cations and surfactants ions. Polystyrene as well as PVC based membranes act as good end point indicators in potentiometric titrations of Cd2+ and Zn2+ with EDTA. Thus it can be concluded that the membrane sensor incorporating 2-salicylaldene amino thiophenol formaldehyde as an electroactive phase in polystyrene, is a better sensor for cadmium in comparison to zinc and it is quite obvious that the choice of the binder influences the response, slope and selectivity of the membrane. Use of polystyrene as binder exhibits an improved response and selectivity. In the end (last chapter) the applications of polytungstoantimonate and 2-salicylaldene amino thiophenol membranes, for the estimation of Zn and Cd2+ in three waste water samples, are described. The results achieved with the electrodes fabricated in this lab. compare well with the values obtained by the help of more sophisticated techniques. PUBLICATION : "A solid inorganic gel membrane sensor for mercury" Sensors and Actuators, B 13-14, PP. 391-395. CONFERENCES : Presented a paper entitled "A solid inorganic gel membrane sensor for mercury" in the "Fourth international meeting on chemical sensors" held in Tokyo, Japan from September 13-17, 1992. Presented a paper entitled "A solid membrane sensor for cadmium ions" in the "Symposium on recent trends in instrumental methods of analysis" held in Chemistry Department, University of Roorkee, Roorkee, India from March 24 to 26, 1992. A paper entitled "A membrane sensor for the estimation of vanadium as vanadate"was accepted for the presentation in the "Seventh international conference on sensors and actuators" held in Yokohama, Japan in August 1993.