A Solid-Contact Indium(III) Sensor based on a Thiosulfinate Ionophore Bull. Korean Chem. Soc. 2013, Vol. 34, No. 4 1153 http://dx.doi.org/10.5012/bkcs.2013.34.4.1153 A Solid-Contact Indium(III) Sensor based on a Thiosulfinate Ionophore Derived from Omeprazole Mohammad Nooredeen Abbas * and Hend Samy Amer Analytical Laboratory, Applied Organic Chemistry Dept., National Research Centre, Dokki, Cairo, Egypt * E-mail: dr.noordldin@yahoo.com Received October 25, 2012, Accepted January 21, 2013 A novel solid-contact indium(III)-selective sensor based on bis-(1H-benzimidazole-5-methoxy-2-[(4-methoxy-3, 5-dimethyl-1-pyridinyl) 2-methyl]) thiosulfinate, known as an omeprazole dimer (OD) and a neutral ionophore, was constructed, and its performance characteristics were evaluated. The sensor was prepared by applying a membrane cocktail containing the ionophore to a graphite rod pre-coated with polyethylene dioxythiophene (PEDOT) conducting polymer as the ion-to-electron transducer. The membrane contained 3.6% OD, 2.3% oleic acid (OA) and 62% dioctyl phthalate (DOP) as the solvent mediator in PVC and produced a good potentiometric response to indium(III) ions with a Nernstian slope of 19.09 mV/decade. The constructed sensor possessed a linear concentration range from 3 × 10 -7 to 1 × 10 -2 M and a lower detection limit (LDL) of 1 × 10 -7 M indium(III) over a pH range of 4.0-7.0. It also displayed a fast response time and good selectivity for indium(III) over several other ions. The sensor can be used for longer than three months without any considerable divergence in potential. The sensor was utilized for direct and flow injection potentiometric (FIP) determination of indium(III) in alloys. The parameters that control the flow injection method were optimized. Indium(III) was quantitatively recovered, and the results agreed with those obtained using atomic absorption spectrophotometry, as confirmed by the f and t values. The sensor was also utilized as an indicator electrode for the potentiometric titration of fluoride in the presence of chloride, bromide, iodide and thiocyanate ions using indium(III) nitrate as the titrant. Key Words : Indium(III), Solid-contact sensor, Thiosulfinate, Omeprazole dimer, FIA Introduction Electrochemical potentiometric sensors are known to be excellent, low cost tools for the selective, sensitive and rapid determination of a vast variety of analytes for various ap- plications. 1 Solid-contact ion sensors are especially interest- ing because they exhibit very low detection limits and are not limited by the presence of an inner reference solution found in conventional sensors. 2 Ionophore based ion-selec- tive sensors have gained attention during the last decade because of their high selectivity, 3 and sulfur-containing ionophores are extremely interesting. 4 Thiosulfinates are the salts of thiosulfinic acids that contain a -S-S(O) functional group. The biological activities of onion (Allium cepa L.), garlic (Allium sativum L.) and other edible Allium are attri- buted to thiosulfinates, 5 which are volatile sulfur compounds responsible for the pungency of these vegetables. The thio- sulfinates, or alkane(ene) thial-S-oxides, are formed by the alliinase enzyme present in the plant from their respective S-alk(en)yl cysteine sulfoxides. Whereas synthesized thio- sulfinates with electron-withdrawing substituents on the aromatic ring cannot be stably isolated from the reaction mixture, those with a releasing substituent in the para- position showed good stability. 6 Omeprazole, a widespread proton pump inhibitor (PPI) containing sulfur, nitrogen and oxygen, was previously investigated as an ionophore for a gadolinium-selective electrode. 7 However, omeprazole is not a stable compound because it readily degrades in aqueous solution, especially at low pH values, and its maximum stability is at pH 11. 8,9 Omeprazole’s main acid-catalyzed decomposition product is sulfonamide, which reacts im- mediately with the sulfenic acid intermediate to form a dimer through the thiosulfinate functional group, as shown in Figure 1. The existence of the dimer compound was confirmed by a differential pulse polarographic study with 3.00 × 10 -5 M omeprazole in a buffered solution at pH 4. A strong peak was assigned to the dimer hypothesized to be the major product from the acid decomposition of omeprazole. 10 Therefore, we decided to investigate the stable thiosulfinate degradation product of omeprazole, i.e., the omeprazole dimer 11 (Fig. 1), as a potential ionophore for heavy metal cations. Fortunately, it showed high selectivity for indium(III) ions. Indium is a rare element with a trivalent oxidation state and is extremely stable to atmospheric exposure. Indium is commonly used to make indium-tin oxide (ITO) for manu- facturing liquid crystal displays (LCD's), accounting for 50% of its annual consumption. 12 Indium is also used to make photovoltaic devices, semi-conductors, high-speed transistors, specialized solders and metal alloys. 13 Several methods are used to identify indium(III), including atomic absorption spectrometry, 14 voltammetry, 15 spectrophoto- metry 16 and HPLC. 17 However, only a few liquid or conv- entional ion selective electrodes (ISEs) have been described for indium(III), 18-20 and they exhibit similar drawbacks to