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...
