Wittaya Ngeontae

Chemically modified silica gel with aminothioamidoanthraquinone for solid phase extraction and preconcentration of Pb(II), Cu(II), Ni(II), Co(II) and Cd(II).

Silica gel chemically bonded with aminothioamidoanthraquinone was synthesized and characterized. The metal sorption properties of modified silica were studied towards Pb(II), Cu(II), Ni(II), Co(II) and Cd(II). The determination of metal ions was carried out on FAAS. For batch method, the optimum pH ranges for Pb(II), Cu(II) and Cd(II) extraction were >/=3 but for Ni(II) and Co(II) extraction were >/=4. The contact times to reach the equilibrium were less than 10min. The adsorption isotherm fitted the Langmuirs model showed the maximum sorption capacities of 0.56, 0.30, 0.15, 0.12 and 0.067mmol/g for Pb(II), Cu(II), Ni(II), Co(II) and Cd(II), respectively. In the flow system, a column packed modified silica at 20mg for Pb(II) and Cu(II), 50mg for Cd(II), 60mg for Co(II), Ni(II) was studied at a flow rate of 4 and 2.5mL/min for Ni(II). The sorbed metals were quantitatively eluted by 1% HNO(3). No interference from Na(+), K(+), Mg(2+), Ca(2+), Cl(-) and SO(4)(2-) at 10, 100 and 1000mg/L was observed. The application of this modified silica gel to preconcentration of pond water, tap water and drinking water gave high accuracy and precision (%R.S.D.</=9). The method detection limits were 22.5, 1.0, 2.9, 0.95, 1.1mug/L for Pb(II), Cu(II), Ni(II), Co(II) and Cd(II), respectively.

Highly selective preconcentration of Cu(II) from seawater and water samples using amidoamidoxime silica

Chemically modified silica containing amidoamidoxime group was studied as a sorbent for solid-phase extraction (SPE) and preconcentration of Cu(II) prior to determination by flame atomic absorption spectrometry (FAAS). The sorbent showed an extremely high selectivity towards Cu(II) in the pH range of 4-6, while the extraction of Pb(II), Cd(II), Ni(II) and Co(II) was low. The adsorption isotherm followed the Langmuir model and the maximum sorption capacity of 0.0163 mmol Cu(II)g(-1) was achieved. In the flow system, Cu(II) was completely retained on a column containing 40 mg of the modified silica at the flow rate of 4.0 mL min(-1) and quantitatively eluted by 5 mL of 1% (v/v) HNO(3). No interference from Na(+), K(+), Mg(2+), Ca(2+), Cl(-) and SO(4)(2-) at 10, 100 and 1000 mg L(-1) was observed. When applied for preconcentration and determination of Cu(II) in tap water, pond water, and seawater, the recoveries were 96, 101, and 95%, respectively, with high precision (% relative standard deviation (R.S.D.)<4) and low method detection limit (9 microg L(-1)).

Backside Calibration Chronopotentiometry: Using Current to Perform Ion Measurements by Zeroing the Transmembrane Ion Flux

A recent new direction in ion-selective electrode (ISE) research utilizes a stir effect to indicate the disappearance of an ion concentration gradient across a thin ion-selective membrane. This zeroing experiment allows one to evaluate the equilibrium relationship between front and backside solutions contacting the membrane by varying the backside solution composition. This method is attractive since the absolute potential during the measurement is not required, thus avoiding standard recalibrations from the sample solution and a careful control of the reference electrode potential. We report here on a new concept to alleviate the need to continuously vary the composition of the backside solution. Instead, transmembrane ion fluxes are counterbalanced at an imposed critical current. A theoretical model illustrates the relationship between the magnitude of this critical current and the concentration of analyte and countertransporting ions and is found to correspond well with experimental results. The approach is demonstrated with lead(II)-selective membranes and protons as dominating interference ions, and the concentration of Pb(2+) was successfully measured in tap water samples. The principle was further evaluated with calcium-selective membranes and magnesium as counterdiffusing species, with good results. Advantages and limitations arising from the kinetic nature of the perturbation technique are discussed.

Synthesis and Fluorescence Sensing Properties of Calix[4]arenes Containing Fluorophores

New fluoroionophores 4 and 5 derived from calix[4]arene triester monoacid chloride with 2-amino-4-(1,3-benzothiazol-2-yl)phenol and 4-aminoquinaldine, respectively, have been synthesized. A preliminary test showed that the fluorescence intensity of 5 was very low, so only 4 was subjected to cation recognition investigation. In methanol, the fluorescence intensity of 4 was quenched by Na+. The fluorescence intensity decreased linearly with increasing Na+ concentration with a stability constant of log K = 2.91 ± 0.08. No significant response was observed for other alkali metal ions under the same experimental conditions.