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The conventional anion-selective electrodes based on quaternary ammonium or phosphonium salts always display the so-called Hofmeister selectivity pattern. 1 Since the 1980's, research interest in anti-Hofmeister anion selective membrane electrodes has increased more and more. Recently, electrodes based on derivatives of vitamin B12 and transitional metal complexes of porphyrins, and phthalocyanines have been reported. [2][3][4][5][6] These electrodes demonstrated potentiometric anion selectivity sequences apparently deviating from the Hofmeister sequences. Such deviations result from the unique interaction of the central metals with an anion rather than from the hydration free energy and the relative solubility of the individual anions in the solvent mediator.Due to the vital importance of selective SCN -determination, especially in food, biological and water samples, there are several reports on the preparation of SCN -selective membrane electrodes based on a variety of ion carriers.5,7-13 However, most of these SCN -sensors have one, two or, in some cases, all of the following problems: a) high detection limit, b) narrow working concentration range and/or c) serious interfering effect of such anions as ClO4 -, I -and IO4 -.In this work we report a coated graphite membrane electrode based on Ni(II)-2,2′,4,9,9′,11-hexamethyltetraazacyclotetradecanediene perchlorate (NiL) for potentiometric monitoring of ultra trace amounts of SCN -. Experimental ReagentsReagent grade dibutyl phthalate (DBP), benzyl acetate (BA), acetophenone (AP), hexadecyltrimethylammonium bromide (HTAB), tetrahydrofuran (THF), and high relative molecular weight (PVC) were purchased from Aldrich and used as received. Nickel(II)-2,2′,4,9,9′,11-hexamethyltetraazacyclotetradecanediene perchlorate (NiL) was synthesized and purified as described elsewhere.14 Potassium salts of all anions used (all from Merck) were of the highest purity available; they were used without any further purification except for vacuum drying The construction, performance characteristics, and application of a novel polymeric membrane coated on a graphite electrode with unique selectivity towards SCN -are reported. The electrode was prepared by incorporating Ni(II)-2,2,4,9,9,11-hexamethyltetraazacyclotetradecanediene perchlorate into a plasticized poly(vinyl chloride) membrane. The influences of membrane composition, pH and foreign ions were investigated. The electrode displays a near Nernstian slope (-57.8 mV decade -1 ) over a wide concentration range of 1 × 10 -7 -1 × 10 -1 M of SCN -ion. The electrode has a detection limit of 4.8 × 10 -8 M (2.8 ng/cm 3 ) SCN -and shows response times of about 15 s and 120 s for low to high and high to low concentration sequences, respectively. The proposed sensor shows high selectivity towards SCN -over several common organic and inorganic anions. The electrode revealed a great enhancement in selectivity coefficients and detection limit for SCN -, in comparison with the previously reported electrodes. It was successfully applied to the d...

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Silver has long been an important metal in commerce. It is valued for its resistance to corrosion and for its use in alloys, medicine and jewelry. 1 The recovery of silver from waste solutions has commercial importance, and improved methods for accomplaced this, are of interest. Recent information about the interaction of silver with essential nutrients, especially Se, Cu, vitamin E and vitamine B12, have focused attention on its potential toxicity. 2,3 Improved means of lowering silver concentrations are needed, since the acceptable level, 10 ng/dm 3 , is exceeded in many culinary water supplies. The analysis of Ag + ions at low concentration in the presence of other cations at high concentration, requires very sensitive techniques.The most common methods for the determination of silver ion are flame and furnace atomic absorption spectrometry, 4 spectrophotometry, 5 stripping voltammetry and potentiometry. 6 Unfortunately, their sensitivity is usually insufficient for the low concentrations in environmental samples.Consequently, a preconcentration and matrix elimination step is usually required. The most widely used techniques for the separation, preconcentration of trace amounts of silver are liquid-liquid extraction, 7 coprecipitation with tellurium, 8,9 absorption of silver on powdered dithizone, 10 and rhodamine, 11 ion exchange resin, 12 fire assay with platinum metal in the presence of cuprous sulfide as collector 13 and solid phase extraction (SPE). 14 The solid-phase extraction techniques reduce solvent usage and exposure, disposal cost, and the extraction time for sample preparation. 15 Recently, octadecyl-bonded silica SPE disks have been utilized for the extraction and analysis of many different organic and environmental matrices. [16][17][18][19] Moreover, SPE disks modified by suitable ligands have been successfully used for the separation and sensitive determination of metal ions. [20][21][22] There are several literature reports dealing with the preconcentration of silver ions based on SPE, most of which employ chelating groups bound to resins or other polymeric supports; these include 2-mercaptobenzothiazolyl on poly(acrylonitrile) fibers, diethyldithiocarbamate on activated carbon and sulfhydryl on cotton microcolumns. [23][24][25] However, to the best of our knowledge, there is only one report on the use of modified octadecylsilica membrane disks for the separation and preconcentration of Ag + with a detection limit of 50 ng/dm 3 . 14 In this work, a simple and reliable method for the rapid extraction and determination of ultra-trace amounts of Ag + ions using an octadecyl-bonded silica membrane disk modified with tetramethyl tetrathia fulvalen (TMTTF) and graphite furnace atomic absorption spectrometry is presented. Experimental ReagentsAcetic acid, nitric acid, sodium thiosulfate and high-purity of methanol (all from Merck) were used as received. Analyticalgrade silver nitrate and other salts (all from Merck) were of the highest purity available, and were dried in a vacuum over P2O5. A simpl...

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A rapid method for the extraction and monitoring of picogram level of silver ions using uniform silanized mesopor (MCM-41) modified by a thiocompound and inductively coupled plasma atomic emission spectrometry (ICP-AES) is presented. The preconcentration factor of the method is 500, and detection limit of the technique for silver is 6.5 pg/ml. The time and the efficiency of the extraction, pH and flow rate, type and minimum amount of acid for stripping of silver from modified MCM-41 was tested. The break through volume of the method was found 2500 ml. The maximum capacity the modified MCM-41 was found to be 11.0 ± 1.2 μg of Ag + /mg modified MCM-41.