Membrane optode for mercury(II) determination in aqueous samples.

A color changeable optode for Hg(II) was prepared by the immobilization of a dye 4-(2-pyridylazo)resorcinol (PAR) and a liquid ion-exchanger trioctylmethylammonium chloride (Aliquat-336) in the tri-(2-ethylhexyl) phosphate plasticized cellulose triacetate matrix. Hg(II) and CH(3)Hg(+) from aqueous samples could be quantitatively preconcentrated in this transparent optode producing a distinct color change (lambda(max)=520 nm) within 5 min equilibration time in bicarbonate aqueous medium or 30 min in natural water. Whereas optode sample without Aliquat-336 did not change its color corresponding to Hg-PAR complex on equilibrium with the same aqueous solution containing Hg(II) ions. The uptake of Hg(II) was found to be pH dependent with a maximum (>90%) at a pH 7.5. The uptake of ions like Cu(II), Fe(II), Zn(II) and Pb(II) was negligible in the optode where as the uptake of Cd(II) and Zn(II) ions was 10-15% at pH 7.5. The optode developed in the present work was studied for its analytical application for Hg(II) in the aqueous samples by spectrophotometry, radiotracer ((203)Hg), Energy Dispersive X-ray Fluorescence (EDXRF) analyses and Instrumental Neutron Activation Analysis (INAA). The minimum amount of Hg(II) required to produce detectable response by spectrophotometry, INAA and EDXRF were found to be 5.5, 1 and 12 microg, respectively. This optode showed a linear increase in the absorbance at lambda(max)=520 nm over a concentration range of 0.22-1.32 microg/mL of Hg(II) ions in aqueous solution for 5 min. The preconcentration of Hg(II) from large volume of aqueous solution was used to extend the lower limit of concentration range that can be quantified by the spectrophotometry of optode. It was observed that preconcentration of 11 microg Hg(II) in 100mL (0.11 microg/mL) in aqueous samples gives a distinct color change and absorbance above 3 sigma of the blank absorbance. The optode developed in the present work was found to be reusable.

Molecular iodine selective membrane for iodate determination in salt samples: chemical amplification and preconcentration.

A molecular iodine selective membrane has been used for preconcentration of I(2) generated in situ by iodometric reaction of IO(3)(-) with excess I(-) in acidic medium (pH 1-2). This iodometric reaction amplifies the iodine content six times resulting in enhancement of analytical response ranging from three times for molecular methods to six times for elemental methods. The chemical conditions of this iodometric reaction were optimized for quantitative generation and subsequent sorption of I(2) in the membrane samples (96 +/- 3%). The homogeneous transparent membrane was prepared by immobilizing I(2)-complexing polyvinylpyrrolidone (PVP) in the plasticized cellulose triacetate matrix. Four different analytical methods were examined for quantitative determination of IO(3)(-) in iodized salt samples by preconcentrating it as I(2) in the membrane matrix. These methods were: (1) spectrophotometry of the PVP-I(2) complex formed in the membrane matrix, (2) a radiotracer method using I(-) tagged with (131)I radiotracer, (3) instrumental neutron activation analysis (INAA), and (4) energy-dispersive X-ray fluorescence (EDXRF) analysis. The IO(3)(-) contents thus determined in the iodized salt samples by the membrane-based radiotracer method were compared with the total iodine determined in salt samples by epithermal instrumental neutron activation analysis (EINAA). The membrane-based method for iodate determination in salt samples has advantages over conventional analytical methods, for example preconcentration and chemical amplification, and is free from interference from anions.

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 ( approximately 2 wt.%). PVP-PIM was found to uptake only I(2) from aqueous sample whereas I(2) and I(3)(-) were sorbed in the grafted membrane. This selectivity of PVP-PIM towards I(2) was attributed to its hydrophobicity that allows only neutral I(2) to interact with PVP in the membrane matrix. Thus, the selective preconcentration and quantitative determination of I(2) in aqueous sample was carried out using PVP-PIM. As PVP-PIM was optically transparent, the characteristic absorbance of PVP-I(2) complex (lambda(max)=361 nm) could be used for quantitative determination of I(2) in the membrane. The instrumental neutron activation analysis (INAA) of the I(2)-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 I(2) determinations in the samples of (131)I 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 I(2) by monitoring the radioactivity of (131)I. The amounts of I(2) in the aqueous samples were standardized by conventional solvent extraction of I(2) with the chloroform for validating the preconcentration efficiency of PVP-PIM. The detection limit of I(2) in aqueous samples by INAA hyphenated with PVP-PIM was found to be 0.3ppb for a sample size of 25mL.

Selective preconcentration and determination of iodine species in milk samples using polymer inclusion sorbent.

A method to determine low levels of iodine species namely I(-) and IO(3)(-) in aqueous samples was developed and applied to milk and milk powder samples. It is based on selective preconcentration of I(-) in polymer inclusion sorbent (PIS) and neutron activation analysis (NAA) of I(-) sorbed in PIS. The PIS was found to be highly selective for I(-) in presence of IO(3)(-) and other anions commonly present in the milk samples. In order to preconcentrate total I(-)+IO(3)(-) content in the PIS, IO(3)(-) was reduced to I(-) using a mixture of acetic acid and ascorbic acid. It was found that total iodine content in milk could be determined with epithermal neutron activation analysis (ENAA). A scheme was developed to determine I(-), IO(3)(-) and total iodine. The developed method was applied to milk reference materials (NIST SRM-1549 and IAEA-RM-153 milk powder) and a commercially available milk powder. The scheme for estimation of iodine in different forms was validated by using reference material NIST SRM-1549.