A Lipophilic Sol−Gel Matrix for the Development of a Carbonate-Selective Electrode

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Abstract
Organic-inorganic hybrid sol-gel matrixes were used as hosts for trifluoroacetyl-p-decylbenzene (TFADB), a traditional ionophore for carbonate. The sol-gel precursor was prepared by the reaction of (3-isocyanopropyl)triethoxysilane with ethylene glycol. Hexadecyltrimethoxysilane (HDTMOS) was employed as a co-precursor. An appropriate amount of tridodecylmethylammonium chloride (TDMAC) and 2-nitrophenyloctyl ether (NPOE) were used as membrane components. On mixing with an acidic catalyst, the sol-state precursors slowly gelled, yielding a membrane in which the active components, TFADB and TDMAC, were encapsulated. Infrared, (1)H, and (29)Si MAS NMR spectrometers were employed to monitor the sol-gel process and the degree of polymerization. The performances of the sol-gel membrane-based electrodes were compared to those of TFADB-based poly(vinyl chloride) (PVC) membrane electrodes. Membranes with a molar ratio of TFADB:TDMAC (1:0.14) showed extended lifetime and stable baseline potential. The response slope toward carbonate was approximately 27 mV/decade between 10(-)(5) and 10(-)(3.5) M at 18 degrees C. Interestingly, selectivity toward carbonate over salicylate and other lipophilic anions was improved, clearly deviating from the Hofmeister selectivity pattern. Responses toward small inorganic anions including chloride and sulfate were negligible. The selectivity coefficients measured by the matched potential method in 0.1 M tris-sulfuric acid buffer, pH 8.75, were log = -0.3, log = -4.2, and log = -2.5.