Salicylate-selective membranes using

The preparation of an ion-selective electrode for salicylate is described. The electrode incorporates an ion-pair of crystal violet and salicylate in a PVC matrix as the ion-selective membrane. Its use for the determination of acetylsalicylic acid in aspirin tablets is described. A similar experiment is described by Creager, S. E. Lawrence, K. D. Tibbets, C. R. in An Easily Constructed Salicylate-Ion-Selective Electrode for Use in the Instructional Laboratory, /. Chem. Educ. 1995, 72, 274-276. 

Creager and colleagues designed a salicylate ion-selective electrode using a PVC membrane impregnated with tetraalkylammonium salicylate. To determine the ion-selective electrode s selectivity coefficient for benzoate,... 

As described above, significant progress in the design of anion and gas selective membrane electrodes has been made. While further work is needed to understand fully the response mechanisms and to improve the performance of the new thiocyanate, salicylate, and sulfite selective membrane electrodes, each of these sensors appears to offer adequate selectivity for use in real sample measurements. In addition, by carefully... 

The most widely used sensor for chloride ions in clinical analyzers is based on an ion-exchanger, a quaternary alkylammonium chloride, dispersed in a plastic membrane. It is not an ideal sensor due to the interference of lipophilic anions (e.g., salicylates, bromides) and lip-ophylic cations (e.g., bacteriostatic agents, anesthetics) and a relatively poor selectivity towards hydrogen carbonates (bicarbonates). However, compared to charged anion- and neutral carrier-based membranes that have been tested, it is still the best-suited for automated analyzers. 

A hydrophobic cobyrinate (Figure 2, structure 2) was used to prepare solvent polymeric membranes (10). The typical membrane composition was 1% (w/w) ionophore, 66% (w/w) plasticizer and 33% (w/w) polymer. Electrodes prepared with this ionophore, dioctyl sebacate (DOS) and poly(vinyl chloride) (PVC) presented, at pH 6.6, the selectivity pattern shown in Figure 3. The response of the electrodes was near-Nernstian for salicylate, thiocyanate, and nitrite. Their selectivity behavior clearly deviates from that of the Hofmeister series, with nitrite being the anion that presents the larger deviation. 

Recently [43] Gao et al. applied a zeolite-fiOlled polyvinyl alcohol (PVA) membrane in esterification and acetalization reactions. Zeolites NaA, KA and CaA as well as NaX were loaded into PVA up to 27 wt% and the composites tested in selective water removal during reaction. A pervaporation cell with a membrane area of 22.9 cm was coimected to a collection system kept at a vacuum of 0.1 mm Hg. A sulfonated resin was used as Bronsted acid catalyst in the esterification mixture (120 ml). Figure 28 shows the progress of the esterification of salicylic acid and methanol at 60°C. The reaction is accelerated considerably as a result of the water removal. 

PAMPA-pKa fiux optimized design (pOD)-permeabiiity Iso-pH mapping unstirred PAMPA was used to measure the effective permeability, Pe, as a function of pH from 3 to 10, of five weak monoprotic acids (ibuprofen, naproxen, ketoprofen, salicylic acid, benzoic acid), an ampholyte (piroxicam), five monoprotic weak bases (imipramine, verapamil, propranolol, phenazopyridine, metoprolol), and a diprotic weak base (quinine). The intrinsic permeability, Po, the UWL permeability, Pu, and the apparent pKa (pKa.fiux) were determined from the pH dependence of log Pg. The underlying permeability-pH equations were derived for multiprotic weak acids, weak bases, and ampholytes. The average thickness of the UWL on each side of the membrane was estimated to be nearly 2000 p, somewhat larger than that found in Caco-2 permeability assays (unstirred). As the UWL thickness in the human intestine is believed to be about forty times smaller, it is critical to correct the in vitro permeability data for the effect of the UWL. Without such correction, the in vitro permeability coefficient of lipophilic molecules would be indicative only of the property of water. In single-pH PAMPA (e.g., pH 7.4), the uncertainty of the UWL contribution can be minimized if a specially selected pH (possibly different from 7.4) were used in the assay. From the analysis of the shapes of the log Pe-pH plots, a method to improve the selection of the assay pH, called pOD-PAMPA, was described and tested. From an optimally selected assay pH, it is possible to estimate Pg, as well as the entire membrane permeability-pH profile. 

Because the carbonate selectivity provided by these ionophores is often not satisfactory enough, supplementary approaches have been used to reduce interference from highly lipophilic anions, such as salicylate and perchlorate. These include coating of the ionophore-doped sensing membrane with a hydrophilic, porous layers that slows down the access of large, lipophilic anions such as salicylate to the sensing membrane. " ... 

A new salicylate eledrode based on polymer membranes was used in a flow injection analysis system [66]. The electrode membrane contains 29.2-31.0% of PVC, 5.8-6.3% of tetraoctylammonium salicylate, 58.5-62.7% of o-nitrophenyloctylether and 6.5% of p-ferf-octylphenol. The tubular electrode was stored for approx. 6 months in a sodium salicylate solution. The electrode shows the slope of response curve close to theoretical — 56.0 0.6 mV decade in the range of 5 x 10" -10" mol L the response time is up to 5 s. The electrode can be used in a pH range of 6-9. The log K selectivity coefficients are 2-2.9 for acetates, 1.8-2.2 for chlorates, 0.7-1.0 for nitrates and 0.6-1.3 for acetosalicylates. The tubular salicylate electrode can be used for determination of acetylsalicylic add, after its previous analysis to salicylate, in multicomponent preparations and effervescent tablets (Anadin Extra, Aspirin, Dolviran, Alka-Seltzer). The results of potentiometric measurements are consistent with the method used in pharmacopoeia 100.7 to 103% of the compound was obtained, with standard deviation of RSD 0.6-1.8%. 

When the large tricaprylammonium cation (Aliquat 336 S, General Mill, USA) dissolved in 1-decanol (0.01 to 0.1 M) is used, the resulting liquid membrane electrode responds to many anions. In order to obtain stable potentials, the organic phase must be shaken ahead of time with an approximately 1 M solution of the sodium salt of the anion to be measured [150]. In this way, electrodes have been reported in the literature which are selective for HPO " [151], CrO ", ZnCl , PdCll" [152], MnO, Cr20 -, lOj [143],salicylate-[153],HgCl -, Hgir, FeCU [154] and COi" [147] anions. 

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