Salicylate electrode

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%. 

---

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,... 

What is the maximum acceptable concentration of benzoate if this ion-selective electrode is to be used for analyzing samples containing as little as 10 M salicylate with an accuracy of better than 1% ... 

Historically, the first capacitors using an electrocfiemical system were the electrolytic capacitors. Built like film capacitors, they have electrodes made of aluminum foil on which by electrochemical oxidation a thin film of aluminum oxide (i.e., 10 to lOOnm thick) is grown to serve as the dielectric. Solutions are used as the electrolyte which aid self-repair of the oxide film on aluminum after accidental damage. Such electrolytes are solutions of salts of a number of orgaiuc acids (trifluoroacetic, salicylic, and some others). Because of the small thickness of the oxide layer, electrolytic capacitors have a markedly higher capacity than film capacitors. They can thus be used in the microfarad range. 

Fig. 4.3. Potentiometric titration of phenol and salicylic acid. Solvent pyridine-dimethylfuran-diethylamine (4 7 9, v/v) titrant (C4H9)3CH3NOH, 0.1 Nin IPA glass-calomel electrode (aq.).

Fig. 4.4. Potentiometric titration of salicylic and benzoic acid. Titrant (C4H9)4NOH, 0.1 A in methanol-benzene glass-calomel electrode (pyridine).

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

Another aspect of tin as a constituent of electrode material is shown by tin(IV)TPP complexes incorporated into PVC membrane electrodes. These increase the selectivity to salicylate over anions such as Cl-, Br- I-, I()4, Cl()4, citrate, lactate and acetate. The specificity is attributed to the oxophilic character of the Sn ion in TPP at the axial coordination sites. Indeed, carboxyl groups incorporated into the membrane polymer compete for these binding sites. The complete complex structure is important. Substitution of TPP with octaethylporphirine results in loss of salicylate selectivity231. Preparation and analytical evaluation of a lead-selective membrane electrode, containing lead diethyldithiocarbamate chelate, has also been described232. 

The catalytic reduction of o-nitrophenol by the surface of a Ti(IV)/Ti(III) redox system on a Ti/Ti02 electrode has been used in a preparative-scale operation [522]. 5-Nitro-salicylic acid [523] and... 

Below the outer membrane is a filter, usually composed of an anionic polymer, e.g. based on salicylate. Its precise composition and dimensions (thickness, pore size, amount and type of plasticizer, fillers, etc.) are optimized in order to tailor the diffusion rates of material crossing the filter from the analyte solution toward the working electrode of the sensor. Ideally, some uncharged molecules, such as H2O2, will traverse the filter so fast that, in effect, the filter is invisible to... 

Figure 15.10 Selectivity pattern of PPy(NO ) electrodes. The anions tested include (1) nitrate, (2) bromide, (3) perchlorate, (4) salicylate, and (5) phosphate A is the difference between the steady-state potential and the starting potential. Reprinted from Hutchins and Bachas (1995). Copyright 1995 American Chemical Society.

Numerous methods for the synthesis of salicyl alcohol exist. These involve the reduction of salicylaldehyde or of salicylic acid and its derivatives. The alcohol can be prepared in almost theoretical yield by the reduction of salicylaldehyde with sodium amalgam, sodium borohydride, or lithium aluminum hydride by catalytic hydrogenation over platinum black or Raney nickel or by hydrogenation over platinum and ferrous chloride in alcohol. The electrolytic reduction of salicylaldehyde in sodium bicarbonate solution at a mercury cathode with carbon dioxide passed into the mixture also yields saligenin. It is formed by the electrolytic reduction at lead electrodes of salicylic acids in aqueous alcoholic solution or sodium salicylate in the presence of boric acid and sodium sulfate. Salicylamide in aqueous alcohol solution acidified with acetic acid is reduced to salicyl alcohol by sodium amalgam in 63% yield. Salicyl alcohol forms along with -hydroxybenzyl alcohol by the action of formaldehyde on phenol in the presence of sodium hydroxide or calcium oxide. High yields of salicyl alcohol from phenol and formaldehyde in the presence of a molar equivalent of ether additives have been reported (60). Phenyl metaborate prepared from phenol and boric acid yields salicyl alcohol after treatment with formaldehyde and hydrolysis (61). 

J.H model is, indeed, the common behavior. Figure 1 reports the degradation rate of phenol [36], a poorly adsorbed compound, and that of CHBA [35], a strongly absorbed compound, as a function of their initial concentration. A peaked reaction rate is observed, in contrast to the saturative LH model, also for CHCI3 and dodecane (see Fig. 2 in Ref. 37). For the photocurrents measured in photoelectro-chemical oxidation experiments of methanol and salicylic acid on anatase film electrodes, a saturation curve for the poorly adsorbed methanol and a peak at an intermediate concentration for strongly adsorbed salicylic acid were also observed as a function of the substrate concentration [38]. 

Solvent extraction offers unique advantages among separation techniques. A system based on extraction into a polymer [poly(vinyl chloride)] as solvent was examined here because of possible advantages in speedy simplicity, sample size, solvent handlingy etc.f especially when coupled with flow injection and an amperometric detector. Solutes examined included salicylic acid and 8-hydroxy quinoline. The apparatus typically consisted of 0.8-mm i.d. X 170-cm coiled tubing that could be connected directly to the injection loop of a flow-injection amperometric detector system containing a nickel oxide electrode. 

Kwan et al. [27] l-Lactate Yoghurt milk, soda, sport drinks, and healthy supplement Salicylate hydroxylase (SHL), L-lactate dehydrogenase (LDH), and pyruvate oxidase (PyOD)/entrapped by a poly(carbamoyl) sulfonate (PCS) hydrogel on a Teflon membrane Clark-type oxygen electrode ... 

Gajovic et al. [64] L-malate Fruits, fruit juices, ciders and wines NAD(P)+-dependent L-malate dehydrogenase oxaloacetate decarboxylating with salicylate hydroxylase (SHL)/ in gelatine membrane sandwiched between a dialysis membrane and a PET membrane Clark-electrode ... 

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. 

In dicyanocobalt(III) a,b,c,d,e,g-hexamethyl-f-stearylamide cobyrinate (derivative 3) the six peripheral amide groups of vitamin B12 have been replaced with methyl ester groups, and the proximal base of the vitamin at the f-position with a stearylamide group (11). Electrodes prepared with this ionophore and DOS as the plasticizer were also selective for thiocyanate and nitrite over the rest of the anions tested. The main anionic interferent was salicylate. In all cases, the response of the electrodes to the preferred anions was sub-Nemstian. Overall, the selectivity pattern obtained with ionophore 3 is similar to that of 2 and to that of the hydrophobic cobyrinate-based electrodes reported previously (5, 12, 13). This observation suggests that in all cobyrinate ionophores the anions interact with the cobalt(III) center, and that the side chains of the corrin ring have a small effect on the selectivity of this interaction. 

Figure 3. Selectivity pattern of an ISE based on ionophore 2. The electrode was exposed to the following anions salicylate (1), thiocyanate (2), nitrite (3), perchlorate (4), iodide (5), benzoate (6), bromide (7), bicarbonate (8), hydrogen phosphate (9), nitrate (10), chloride (11), sulfate (12). (Reproduced with permission from ref. 10. Copyright 1989 Alan R. Liss.)...

Although the ISEs based on cobyrinates have good selectivity for nitrite over several anions, they also respond to salicylate and thiocyanate. To eliminate this interference, the nitrite-selective electrode based on ionophore 2 was placed behind a microporous gas-permeable membrane (GPM) in a nitrogen oxide gas-sensor mode (75). NOx was generated from nitrite in the sample at pH 1.7 and, after crossing the GPM, was trapped as nitrite by an internal solution that was buffered at pH 5.5 (0.100 M MES-NaOH, pH 5.5, containing 0.100 M NaCl). The internal solution was "sandwiched" between the nitrite-selective electrode and the GPM. 

Figure 6 shows the selectivity behavior of this NOx gas sensor. The sensor had a sub-Nernstian response toward nitrite, with slopes in the range of -45 to -50 mV/decade. Further, the response observed with salicylate and thiocyanate was diminished substantially, as compared to that obtained with the original nitrite-selective electrode (Figure 3). In addition, the gas sensor described here does not suffer interferences from nitrate, bicarbonate, acetate, benzoate, or chloride. These excellent selectivity properties of the sensor are a combination of the selectivity characteristics of the nitrite-selective electrode and the additional discrimination provided by the GPM.

The potentiometric behavior of electrodes based on these films was studied (Figure 8). These ISEs presented sub-Nemstian slopes for thiocyanate (from -40 to -53 mV/decade, depending on the buffer used), and had detection limits of 5xl0 7 M. The response time of the electrodes was typically less than 25 s. The selectivity pattern observed was thiocyanate > perchlorate > iodide > nitrite - salicylate bromide > chloride > bicarbonate > phosphate. This anion-selectivity behavior does not follow the Hofmeister series, with thiocyanate and nitrite being the ions that deviate the most from it. This indicates that there is a selective interaction of the immobilized porphyrin with the two anions. 

Salicylate is determined in blood serum using immobilized salicylate hydroxylase electrodes (243-245). This enzyme is a mixed function monooxygenase that converts salicylate to catechol in the presence of NAD(P)H and molecular oxygen ... 

Either the CO2 formation is followed potemiometrically (243) or the O2 consumption is measured amperometrically at an oxygen electrode (245). In the first method, the enzyme is physically immobilized with a dialysis membrane. The response is linear in the range 5-300 pg/mL of salicylate. The second technique uses chemically immobilized enzyme (GA -F BSA) attached to a pig intestine mounted on the tip of the O 2 electrode. Samples containing from 10 pM to 2 mM salicylate were analyzed. An elegant microelectrode (244) has the enzyme and the cofactor immobilized in the electrode matrix (carbon paste) and the catechol formation is monitored at -F 300 mV versus Ag/AgCl. The electrode consists of a disposable strip, allowing measurements to be made on a drop of blood within 1 min. 

Many functional materials and compounds with new stmctures and properties, such as Sr[C6H4(C02)2] Tb a-Zn(0C6H4C02) Tb salicylate-doped zinc benzoate [Zn(Bzo)2 SalJ luminescent materials and non-crystalline tin-based composite oxide negative electrode materials, etc. have been obtained by means of the rheological phase reaction method [11-14]. 

---