Molecular iodine preconcentration and determination in aqueous samples using poly(vinylpyrrolidone) containing membranes

Membranes for preconcentration of molecular iodine were developed by two different routes: (i) UV-grafting of 1-vinyl-2-pyrrolidone in the pores of microporous poly(propylene) host membrane (grafted membrane), and (ii) physical immobilization of preformed poly(vinylpyrrolidone) (PVP) in a plasticized cellulose triacetate matrix to form the polymer inclusion membrane (PVP-PIM). The UV-grafted PVP-membrane was found to be hydrophilic (water uptake capacity = 166 wt.%), while the PVP-PIM was found to be highly hydrophobic (≈2 wt.%). PVP-PIM was found to uptake only I2 from aqueous sample whereas I2 and I3− were sorbed in the grafted membrane. This selectivity of PVP-PIM towards I2 was attributed to its hydrophobicity that allows only neutral I2 to interact with PVP in the membrane matrix. Thus, the selective preconcentration and quantitative determination of I2 in aqueous sample was carried out using PVP-PIM. As PVP-PIM was optically transparent, the characteristic absorbance of PVP–I2 complex (λmax = 361 nm) could be used for quantitative determination of I2 in the membrane. The instrumental neutron activation analysis (INAA) of the I2-loaded PIM samples indicated that 82% could be sorbed into the PIM samples from the solution within 10 min of equilibration time. This membrane was applied to I2 determinations in the samples of 131I radiotracer. The concentration level of iodine species in these samples were in sub-ppb level. Therefore, these samples were ideal for testing the preconcentration efficiency of the membrane towards I2 by monitoring the radioactivity of 131I. The amounts of I2 in the aqueous samples were standardized by conventional solvent extraction of I2 with the chloroform for validating the preconcentration efficiency of PVP-PIM. The detection limit of I2 in aqueous samples by INAA hyphenated with PVP-PIM was found to be 0.3 ppb for a sample size of 25 mL.