Anal. Chem. 1983,55,2089-2093 (7) Nagashima, S.Anal. Cbim. Acta 1977, 97, 303-306. (8) Nagashlma, $3. Anal. Chim. Acta 1978, 99, 197-201. (9) Nagashima, 15.; Ozawa, T. I n t . J . Envlron. Anal. Chem. 1981, 70, 99-106. (10) , . "Standard Methods for the Examination of Water and Wastewater". 13th ed.; American Water Works Association: New York, 1971; Part 207. (11) "Standard Mlethods for the Examination of Water and Wastewater",
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15th ed.; Arnerlcan Water Works Association: Washington, DC, 1080; Part 412. (12) Nagashima, S. Water Res. I W 3 , 77, 833-834. (13) Ogata, Y.; Sawaki, Y. J . Am. Chem. SOC. 1972, 94, 4189-4196. (14) Oaata. Y.: Gawaki, Y . J . Ora. Chem. 1972, 37,2953-2957. .
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RECEIVED for review April 20, 983. Accepted July 8, 19183.
Synthesis of Silica- Immobilized 8-Quinotinol with (Aminophenyl)trimethoxysilane Monte A. Marshall and Horacio A. Mottola* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
A more efflcleint and tlme-savlng synthetic route to sllicalmmoblllzed 8-quinolinol has been developed that results In a product exhibltlng a relatively large exchange capacity. A comparison of dlfferent sllica supports has been performed and the resulting slllca-bound 8-qulnolinol materlals have been characterized with regard to exchange capaclty, carbon content, acid-base properties, and stablllty. The dependence of capaclty on surface area and pore size Is dlscussed and a comparison i o conventionally prepared silica-lmmoblllzed 8-qulnolinols is made. A posslble explanatlon of apparent discrepancies in the heterogeneous acid dissoclatlon constants of slllca-immoblllzed 8-qulnollnol Is also proposed.
8-Quinolinol, also known as oxine (HOx), has been used extensively in the past in immobilized form for the preconcentration of trace levels of metal ions from high ionic strength aqueous samples (1-4). Generally the extracted metals are eluted as a mixture with aqueous acid and the coeluted metals are then determined in situ although sometimes a partial separation is achieved. Other applications of immobilized HOx include liquid chromatographic separation of metal ions (5), ion-pair separaition of certain anions (6),and, more recently, the metal-assisted separation of phenols (7). Silica has been the aiupport of choice in most of these applications. Immobilization reactions on silica are relatively simple, especially when compared to imrnobilizations involving organic polymers. Also, silica exhibits the good mechanical strength and swelling stability required for use in high-pressure liquid chromatographic systems. Probably of greatest importance, however, is the fast metal ion exchange kinetics observed with silica-based chelating ion exchangers, the lack of which precludes the use of many organic polymer-based chelating ion exchangers (4, 8-11). Essentially d l silica-based HOx materials have been prepared utilizing one of two similar procedures described by Hill ( I ) and Sugawslra et al. (12). These immlobilizationprocedures involve a rather lengthy synthesis (3 to 4 days) consisting of silylation of the silica iiurface with an aliphatic aminosilane, aromatic nitro group introduction, reduction of immobilized ArN0, to immobilized ArNH,, diazonium salt formation, and finally diazo coupling to HOx. All of the above steps, with the exception of diazo coupling to HOx, have been adopted from the procedure described by Weetall for enzyme immobilization (13). However, unlike enzyme immobilizations which often require a spacer between the support and the enzyme, immobilization of relatively small molecules such as
HOx should not be expected to require such a spacer. Therefore, although some effort has been made to optimize the above synthesis (14),little effort has been made to provide a more direct route to silica-immobilized HOx. Diazo coupling offers some distinct advantages over other possible synthetic routes to silica-immobilized HOx. The azo bond, once formed, can be cleaved with a reducing agent and the resulting aromatic amine can be rediazotized and coupled to another ligand (1). Also, d e azo dye formed in the coupling reaction, being brightly colored, allows for a qualitative evaluation of the success of the synthesis. Yellow or light orange products have low exchange capacities and thereffore further evaluation of these materials is generally not useful. A new, less circuitous, route to silica-immobilized HOix is described here which retains the desirable features of diazo coupling reaction while reducing the preparation time and yielding products having relatively high exchange capacities. E X P E R I M E N T A L SECTION Reagents. All reagents were analytical grade and were used
as received unless otherwise indicated. All water used was deionized by reverse osmosis followed by distillation in a borosilicate glass still equipped with a quartz immersion heater (Corning Model AG-la, Corning, NY). (Aminopheny1)trimethoxysilane (Petrarch Systems Inc., Bristol, PA) was obtained as mixed isomers and stored under refrigeration. Organic solvents were stored over molecular sieves. Silica gel (Woelm TLC grade, ICN Pharmaceuticals, Clevehd, OH), Porasil iC (Waters Associates, Milford, MA), and Controlled-Pore Glass (CPG00500, Electro-Nucleonics,Fairfield, NJ) were dried at 120 OC for 12 h before use and were stored iin an oven at 120 O C . Apparatus. All pH measurements were made with an Orion Research Model 601A pH meter (Cambridge,MA) equipped with an epoxy-body combination electrode (Sensorex, Westminster, CAI. Atomic absorption measurements were carried out with the use of a Perkin-Elmer Model 290B (Norwalk, CT) atomic absorption spectrophotometer. A custom-made shaker based on the use of a vacuum driven windshield wiper motor was used for batch studies. Immobilization Procedure. A 10% solution of (aminopheny1)trimethoxysilanein dry toluene (20 mL) was added to the dry silica material (2.0 g) and vacuum was,then applied in some cases to remove any trapped air. The reaction mixture was refluxed for 4 h, filtered through a filter funnel, and dried overnight at 80 O C in an oven. This results in the formation of an arylamine silica. This arylamine silica was then diazotized and coupled in essentially the same manner as previously described (14). First, 100 mL of 2% sodium nitrite in 2 M hydrochloric acid was added to the arylamine silica at 0 "C and reaction was allowed to occur for 30 min. This reaction results in the formation of diazonium
0003-2700/83/0355-2089$01.50/0 0 1983 American Chemical Sociery
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salt-silica. Degassing was performed in some cases. The diazonium salt-silica was quickly fiitered, washed with three 25-mL portions of cold water, and added to 100 mL of a 2% solution of HOx in ethanol. Degassing was also performed in some cases at this point. Appearance of a deep red color indicates the formation of the azo-coupled HOx. This coupling reaction was carried out for 30 min, after which the silica-immobilizedHOx was filtered and washed sequentially with ethanol, 0.1 M hydrochloric acid, and water. This material was then air-dried and stored in a desiccator. Capacity Determinations. Several different methods were used to determine capacities. Copper(I1)was used as a metal ion probe to give an indirect measure of the amount of immobilized HOx on the silica surface. Typically, 20.00 mL of a 150 pg/mL standard stock solution of copper(I1) (prepared from CuS04.5H20) in 0.2 M acetate buffer (pH 5) was added to approximately 0.2 g of silica-immobilized material in a 30-mL screw cap vial. This mixture was then allowed to equilibrate for 30 min, after which an aliquot of the liquid was removed and the capacity determined from the change in copper concentration observed. The copper(I1) uptake of a silica blank was also determined in order to confirm that the silica support was not responsible for any significant amount of the observed capacity. The capacity value obtained by using this procedure is believed to be the best estimate of the capacity, and unless otherwise noted, all capacity values reported have been obtained in this manner. Alternatively,the equilibrated silica-immobilized material was sometimes fiitered and rinsed with water and the extracted copper(I1) eluted with 100 mL of 0.1 M hydrochloric acid followed by 15 mL of 1.0 M hydrochloric acid in some cases. The capacity was then determined by measuring the amount of copper eluted. All copper determinations were carried out at the 3247-A line of copper with an Intensitron No. 2252 hollow-cathode lamp (Perkin-Elmer). Calibrations were performed by using a series of standards prepared from a 993 pg/mL copper atomic absorption standard stock solution (Aldrich). A Perkin-Elmer Model 240 elemental analyzer was used for carbon determinations which gives a direct measure of the total amount of immobilized organic material. Stability Studies. The silica-immo'pilized HOx prepared was characterized with regard to both hydrolytic and storage stability. Hydrolytic stability studies were carried out by mixing approximately 0.15-0.20 g of silica-immobilized HOx with 20 mI, of appropriate solutions adjusted to integral pH values between approximately -1 and 12. Buffer solutions were used when possible. Hydrochloric acid (pH -1 and 0), 0.1 M potassium chloride (pH 1and 2), 0.1 M phthalate buffer (pH 3 and 4), 0.1 M acetate buffer (pH 5 and 6), 0.1 M phosphate buffer (pH 7, 8, and 9), 0.1 M carbonate buffer (pH 10 and ll), and 0.1 M sodium carbonate (pH 12) were used. After equilibration for a period of time (24 h or 72 h), the solid was removed by filtration, rinsed with water, and dried. The capacity of the dried material was then determined. Visual inspection of the filtrate was also performed. Storage stability was checked by measuring the capacity after 4 months of dry storage in a desiccator. Titrations. Potentiometric pH titrations were performed on some of the prepared materials. Usually, about 0.2 g of the silica-immobilized material was added to 25 mL of water. The titration was then performed with 0.01 M hydrochloric acid or 0.01 M sodium hydroxide. In some cases, the bound material was rinsed with pH 7 buffer solution or 0.1 M hydrochloric acid prior to titration in order to ensure that the material was in the neutral (HOx) form or in the cationic (HzOx+)form, respectively. Unless indicated otherwise pK, values were estimated from the pH at the half-equivalence volume. In some cases, a graphical method similar to one described previously (15) was used t o estimate pK,,. In this method, it is assumed that at pH
