STUDIES ON SOME NEW INORGANIC ION EXCHANGERS AND THEIR MEMBRANES

Abstract

The separation of a desired component from a mixture is required in many analytical procedures and in a number of industrial processes. Amongst the various methods used for separation and isolation processes, the ion-exchange technique is widely used specially for the separation of ionic species. Both organic and inorganic ion-exchangers have been used for various separations. However, the recent trend in the field of ion-exchangers is to develop new inorganic ion-exchangers which exhibit selective exchange. Thus, inorganic ion-exchangers are preferred over organic ones in view of their selective exchange and stability at high temperature and in presence of ionizinq radiations. If selective exchange is not required in an application, organic exchangers may be preferably used as they show greater reproducibility and large regeneration capacity. The development of new inorganic ion-exchangers is getting additional impetus with the realization that they can be used as suitable materials for preparing membranes to be used as ion-selective electrodes. The survey of literature shows that metal hexacyanoferrates(III) have been sparsely studied as ion-exchangers. Therefore, hexacyanoferrates(III) of Co(II), Cu(II), Zr(IV), Sn(II) and Sn(IV) have been prepared, characterized and studied as ion-exchangers. Further, araldite based membranes of cobalt, copper and stannous hexacyanoferrates(III) have also been investigated as ion-selective electrodes. The results of these investigations are briefly summarised here. The metal hexacyanoferrates(III) have been prepared by mixing metal salt solution with freshly prepared potassium hexacyanoferrate(III) solution. A number of samples of each compound were prepared under different conditions (11) of concentration, ratio of reactants and the order of mixing. The sample showing best exchange capacity has been prepared in bulk and used for further investigations. These compounds were characterized by chemical analysis, X-ray diffractions thermogravimetry and infrared spectroscopy. On the basis of chemical analysis, cobalt and copper hexacyanoferrates(III) [CoFiC and CuFiC, respectively] were assigned tentative molecular formulae K[CoFe(CH)J .5H20 and K[CuFe(CN)g].5H20 whereas no molecular formulae could be assigned to zirconium, stannous and stannic hexacyanoferrates(III) [ZrFiC, Sn(II)FiC and Sn(IV)FiC9 respectively]. The water content of these metal hexacyanoferrates(III) was determined from thermogravimetric analysis. The infrared spectra shows that no structural change occurs on exchange. The X-ray diffraction studies of all these exchangers reveal that CoFiC, CuFiC and ZrFiC are crystalline and found to be primitive cubic with cell edge of 5.17, 5.03 and 5.09 A°9 respectively. The exchangers were equilibrated with 1M HN03 and their ion-exchange capacity, measured in terms of H+ions realeascd, was determined for alkali metal ions and found to increase with decrease in hydrated ionic radii. The details of synthesis and characterization of these exchangers are given in Chapter II. In order to know the selectivity of these exchangers, distribution coefficient values for large number of metal ions were determined. A perusal of K. values (Chapter III) shows that CoFiC, CuFiC and ZrFiC are highly selective to Cu2+9 Cs+ and Bi +, respectively whereas Sn(II)FiC and Sn(IV)FiC show selectivity for Pb2+. High «d values for some ions and low for large number of ions on these exchangers makes it possible to achieve some important separations. In order to know the mechanism of (iii) . uptake of the metal ions and also to know the conditions for the elution of adsorbed metal ions from the column, the variation of K. values for a number of ions was investigated in presence of acid and salt solutions. The variation of Kd values of Cs+, Rb+, Ag+, Cu2+, Co2+, Zn2+ and Fe3+ on CoFiC; Cs+9 Rb+, Ag+ on CuFiC; Cs+, Rb+S Ag+ and Bi3+ on ZrFiC; Cs+, Rb+, Ag+ and Pb2+ on Sn(II)FiC and Ag+, Zn2+S Co2+, Pb2+ and Bi3+ on Sn(IV)FiC with NH^ and H concentration shows that these ions are taken up on these exchangers not only through ion-exchange mechanism; some other mechanism i.e. precipitation, molecular adsorption may also be operative. On the basis of K^ measurements, various separations of analytical and radiochemical interest have been performed on the columns of these exchangers. The binary separations of Cu ,Cs+ and Rb+ from large number of metal ions were achieved on the column of CoFiC with quantitative recovery. Cs+ and Rb+ were eluted with 10M HN03, Cu2+ with 1M HN03 and other ions with water (pH 2-3). The binary separations of Cs+ and Rb+ from other metal ions have been also carried out on CuFiC column. Cs+ and Rb+ were eluted with 5M HN03 and other ions with water of pH 2-3. These two exchangers are superior to the existing exchanqors, ammonium molybdophosphate (AMP) and cobalt and nickel hexacyanoferrates(II), for obtaining separation of Cs , in that (i) no mechanical carrier is required for making columns and (ii) Cs+ elution is net difficult, while using AMP (Reference 45, Chapter III) mechanical carrier is required and Cs+ elution is difficult on the columns of cobalt and nickel hexacyanoferrates(II) (References 7,9, Chapter I'll). The column of zirconium hexacyanoferrate(III) has been found useful for selective separation of Bi from large number of metals. Bismuth was eluted with 2M HN03 and other metal ions with water (pH 2-3). Recovery in all cases is almost cent percent. Sn(II)FiC and Sn(IV)FiC exchangers (iv) have been used for the separation of Pb from many metals of interest. The adsorbed Pb2+ from Sn(II)FiC column was eluted with 1M HNO^Cu2* ncl Ba2+with 0.1M HN03 and all other metal ions with water (pH 2-3). A ternary sepa ration of Pb- Cu- Zn was also carried out on Sn(II)FiC column. Recovery of 2+ Pb and all other metal ions is quantitative. The column of Sn(II)FiC has been also used for preconcentration of lead ions. Binary separations 2+ of Pb from a number of metal ions have been also achieved on the column of Sn(IV)FiC exchanger. Lead was eluted with 1M HN03 and other metal ions with water (pH 2-3) with 98% recovery of lead and 100% for other metal ions. It is worth mentioning that Sn(II)FiC is superior than Sn(IV)FiC exchanger. Many other binary separations of various cations pairs, having low separation factors are also possible with the help of these exchanger materials. The membranes of these ion-exchangers are expected to act as ion-selective electrodes. Therefore, the heterogenous membranes of CoFiC, CuFiC and Sn(II)FiC were prepared using araldite as binder. The optimum amount (40%) of araldite required to prepare membranes of adequate stability was found by trial. Before using the membranes as electrodes, they were characterized with regard to functional properties viz., water content, porosity, swelling^electrolyte absorption and conductance. These properties vary in the order Sn(II) FiC >CoFiC>CuFiC. The low values of these properties in case of CuFiC membranes s.uggest that the diffusion through these membranes is wholly taking place through exchange sites present in the membrane phase. In view of high porosity and swelling of Sn(11)FiC membrane it is expected that this membrane would not function as highly selective electrode. This conclusion was later confirmed by potential measurements. The order of selectivity of these membranes in potential response is expected (v) to be CuFiC>CoFiC>Sn(II)FiC. Details of membranes characterization are given in Chapter IV. For measurement of the membrane potential, following cell was set up » External satd. calomel electrode Test solution Membrane Internal solution (0.1M metal salt solution) Internal satd. calomel electrode The salt solution used in the above cell was Cu(N03)2 for CoFiC, CsN03 and T1N03 for CuFiC and Pb(N03)2 for Sn(II)FiC membranes. The membranes were equilibrated with 0.1M respective salt solution. The potential of the cell was measured for various concentrations of the test solution. The results indicate a linear potential response in the concentration range 10"1 -10°4M of Cu2+ for CoFiC membrane; 10" -10" M of Cs+ and 10_1-5xlO"4M of Tl+ for CuFiC membrane and 10" -10" Mof Pb H for Sn(11)FiC membrane. Further, the potential response has been found to be near Nernstian for CuFiC membranes and non-Nemstian for CoFiC and Sn(II)FiC membranes. The potential generated across these membranes was found to be reproducible and response time was low. The useful pH range has been found to be 3-6 for CoFiC and CuFiC (Cs+ selective) and 4-6 for CuFiC (Tl+ selective) and Sn(II)FiC membrane electrodes. These membrane electrodes can also boused in partially non-aqueous" media (upto 25% v/v acetone or alcohol). The performance of these membrane electrodes was also assessed in presence- of large number of ions. The selectivity coefficient values show ?+ + + that CoFiC membrane is selective for Cu , CuFiC for Tl and Cs as compared (vi) to large number of metal ions. Thus they can be used for the determination of these ions in mixtures. However, some ions lik^i Co , Pb and A1 + + 2+ interfere in the determination of copper; Rb , Tl and Cu in the estimation of Cs+ and Pb2+ and Cu2+ in the estimation of Tl+ by these electrodes. The selectivity of Sn(II)FiC membrane is poor and it can be only used for Pb2+ estimation in pure solution and in presence of Hg . These membranes have also been used as indicator electrodes for ond point detection in potentiometric titrations. Although titration plots are not of classical shape due to interference but exhibit sharp break points which correspond to the stoichiometry of the reaction and hence can be used for concentra tion determination. The results of these investigations, which show that membranes of CoFiC, CuFiC and Sn(II)FiC act as moderately selective electrodes, are given in the last chapter of this thesis.