STUDIES WITH INORGANIC lONEXCHANGE GELS AND THEIR MEMBRANES

Abstract

The development of analytical methods has followed closely.introduction of new instruments and devices for identification, estimation and separation of various class of compounds. Techniques such as distillation, crystallization, ultrafiltra tion, solvent extraction, chromatography, .electrophoresis and ion exchange are widely used to obtain diverse separations in numerous fields. Amongst the methods used for isolation, the ion exchange technique is quite fruitful specially for the separation of ionic species. Recent trend in this field has been to develop new exchanger materials possessing adequate stability and specific selectivity, [inorganic ion exchangers are preferred over organic ones in view of their selective exchange and stability at high temperature as well as in presence of ionizing radiations.[The development of new inorganic ion exchangers is getting additional impetus with the realization that their membranes may also undergo a selective exchange and may be used as ion selective electrode for the ion, for which the exchanger is selective. Significant research efforts, have been made to develop inorganic materials such as hydrous oxides and salts of isopolyacids in many laboratories in this country and abroad. A survey of literature reveals that out -11- of all these insoluble salts of polybasic acids the metal tellurites have been sparsely studied as ion exchangers. Besides this, most of the products reported so far, possess cation exchange characteristics and absolutely no efforts have been made to explore the possibility of using inorganic gels as anion exchangers. The only available reference are the investigations of some hydrous oxides (of Zrv Th, Sn etc.), which possess pH dependent cation or anion exchange properties. During the course of investigations on the sol-gel behaviour of some of these compounds, it could be possible to prepare a positively charged gel of zirconium, thorium and stannic tellurite. These gels have good chemical and thermal stability and possess promising anion exchange characteristics. Preparation of these gels, their character ization by routine methods and investigations on their exchange as well,as membrane properties, formulate the subject matter of this thesis. The salient features of these investi gations are described in subsequent paragraphs. The tellurites of zirconium, thorium and tin have been prepared by mixing metal salt with freshly prepared solution of potassium tellurite (Reference 3gf Part II). A number q£ samples of each compound were prepared under different conditions viz., the concentration, ratio of reactants, acidity and the order of mixing. Out of these, the sample exhibiting best exchange capacity and stability was prepared in bulk and used for further investigations. These compounds were characterized by chemical analysis, X-ray diffraction, -111- thermogravimetry and infrared spectroscopy. On the basis of chemical analysis data the products zirconium, thorium and stannic tellurites have been assigned tentative mole cular formulae: ZrO.TeO,.3HpO, Th(TeO,)p.4HpO and Sn(TeO,)p.3H«0 respectively. Water content of these compounds was determined by thermogravimetric analysis. Infrared spectra shows that no structural changes occurs on exchange. The X-ray diffraction studies of all these exchangers reveal that tellurites of zirconium and tin are amorphous in nature and in the case of thorium tellurite, weak diffraction lines are obtained, indicating a poorly crystalline nature of the product. It was not possible to estimate the anion exchange capacity of these salts, by observing the release of anions. As such the uptake capacity for CI has been determined and the same is found to be 0.98, 1.12 and 0.84 miHiequivalent per gram for the tellurites of zirconium, tin and thorium respectively. The cation exchange capacity of these materials has also been estimated and the same is found to be negligibly small. In order to know the selectivity of these exchangers, distribution coefficient values for a large number of anions were determined by titrimetric, spectrophotometric and radiometric methods. The K, values show that these compounds are highly selective for CrO^~, Cr?0^~, So|~ and MoO^ etc. High K, values for some ions and low for a large number of ions on these gels, make it possible to achieve some -ivimportant separations. In order to know the mechanism of uptake of ions and to determine the conditions for the elution of adsorbed species, the variation of K. values for chloride was investigated as a function of N0~ ion 3 concentration. The uptake is found to be nonstoichiometric in nature thereby revealing that the anion is not only taken up by ion exchange mechanism but some other process e.g., physical adsorption also takes place. On the basis of K^ measurements, various separations of analytical interest have been performed on the column of these gels. The binary separations of CrO?~ or CrpO^~ from a large number of other anions were achieved on the column of zirconium tellurite with quantitative recovery. Similarly the binary separation of SOf on the column of 2— stannic tellurite and that of MoO, on the column of thorium tellurite from a large number of other anions have also been performed. The elution curves in almost all the cases are fairly sharp and the recovery from synthetic mixtures is almost 100 percent. Besides this it has also been possible to use it for four to six cycles without any loss in separat ion efficiency. Since these exchangers can be used in alkaline range the products are much more superior to hydrous oxides, which behave as anion exchanger only in acidic media, and the selectivity and separation efficiency of the same is also relatively poor, in comparison to these substances. The membranes of these ion exchangers are also expected to show a good electrochemical behaviour and have been -V— tried1 for ion activity measurements. Only heterogeneous membranes of these compounds could be prepared, using 15//. polystyrene for zirconium and thorium and 30'/. araldite for stannic tellurite, as binder. The optimum amount of polystyrene and araldite required to prepare membranes of adequate stability was determined by trial. Before using the membranes as electrodes, these were characterized with regard to functional properties viz., water content, porosity, swelling, electrolyte absorption and conductance. These properties vary in the order thorium) zirconium) stannic tellurite. The low values of these properties in case of stannic tellurite membrane suggests that the diffusion across it mainly take place through the exchange sites present in the membrane phase. The comparative selectivity order of these membranes for a particular ion in potential response is expected to be stannic> zirconium > thorium tellurite. Details of membrane characterization are given in Part III of this thesis. For measurement of the membrane potential, following cell was set up, Reference

Test I Membrane j Reference f:| Reference electrode I' solution! 1 solution //electrode It is observed that 15*/. polystyrene supported zirconium tellurite membrane forms a good electrode for the potentiometric estimation of chromate or dichromate in the concentra tion range 10~ to 7x10"^ the slope of the two linear plot -vivaries from 22 to 23 mv, thereby showing the nonNemstian behaviour of the membrane. The response time is 20 to 25 seconds and remains stable for about 20 minutes. Standard deviation of potential in the working concentration range is +0.2 mv while the membrane can be used for about four months without observing any drift in potentials. The electrode is tolerant of pH change in the range 3-6 for CrO^2- and 8-H for CrpO„2- ions. A large number of bivalent and polyvalent anions viz., So|~, S0?~, Mc0?~, WO?", Aso|~, p°43-» -Pe(CWv)4^— etc. do not cause any sort of interference. Only few monovalent ions if present in large amounts can interfere with the working of this electrode assembly. It can also function well in the presence of cations other +• + than Na and K . Lead, silver and barium interfere at levels at which they decrease the chromium concentration by precipitation. The electrode system can also be used in nonaqueous media and in presence of surfactants. The utility of this electrode has also been observed in the estimation of Cr in wastes from electroplating and tannery industry. Fifteen percent polystyrene supported thorium tellurite membrane has been used to measure molybdate ion activity. The working pH range is 7-9. The plot showing the variation of membrane potential with log activity is perfectly -4 linear in the concentration range 0.1 to 1.7x10 M, with a nonNemstian slope of 28.0 mv per decade of concentration. Static response time is 20-30 seconds and the standard -viideviation of the data is +0.3 mv. The selectivity coefficient values being quite less than one, for most of other bivalent and polyvalent anions, reflect the selectivity of the electrode for the primary ion under consideration. Mono valent anions do not cause interference unless the same are present in significant amounts. The electrode system can be used in nonaqueous media as well. It has also been successfully used as an end point indicator involving the titration of molybdate ions with thorium nitrate. A thirty percent araldite supported stannic tellurite membrane electrode has been fabricated and. tried for sulphate ion activity measurement. The plot showing the membrane potential with log concentration or activity is perfectly linear in the concentration range 0.1 to 1.1x10~^m with a nonNemstian slope of 30.1 mv per decade of concentration'. The working pH range is 7-10 and the static response time is 20-25 seconds. The standard deviation of the data is about +0.2 mv only. A large number of bivalent and polyvalent anions do not interfere. Monovalent anions can interfere if present in the system in larger amounts. Cations like Na+, K+, Li+, Zn?+, Cd2+, Mn?+, Mg2+, Al^~!" do not interfere brrt silver, lead and mercury cause serious interference. The electrode system is also tolerant of small amounts of surfactant and can be used in nonaqueous media (upto 25"/.) It has been successfully used to estimate the sulphate -viiiion activity in wastes from tannery and paper and pulp industry and as an indicator electrode for titrations involving sulphate ions. The detailed results of these investigations are given in the last chapter of this thesis