A Study of the Hydrolysis Pathway in Oxalic Acid Catalyzed Reacting TMOS-Water Mixtures Under Ultrasound Stimulation

The hydrolysis of TMOS in oxalic acid catalyzed reacting TMOS-water mixtures, under ultrasound stimulation, was studied by fitting a simplified dissolution and reaction modeling for samples, the hydrolysis rate of which had been measured in a previous work. The reaction pathway represented in a ternary diagram shows a heterogeneous step for the reaction which gradually progresses until complete homogenization of the system. Besides the water dissolved due to the homogenizing effect of the alcohol, ultrasound maintains a virtual and additional dissolution of water located at the interface between the TMOS and water during the heterogeneous step of the reaction. The mean radius of the heterogeneity represented by water dispersed in TMOS was evaluated as around 150 Å. The oxalic acid concentration accordingly increases the hydrolysis rate constant but its fundamental role on the solubility of water in TMOS could not unequivocally be established.     

Effects of the Water Quantity on the Solventless TEOS Hydrolysis Under Ultrasound Stimulation

The acid hydrolysis under ultrasound stimulation of solventless tetraethoxysilane(TEOS)-water mixtures was studied at 40°C, by means of a heat flux calorimetric method, as a function of the initial water/TEOS molar ratio (r) ranging from 2 to 10. The method is based on the time record of the exothermic heat peak of hydrolysis, arising after an induction time under ultrasound stimulation, which is a measure of the reaction rate. The hydrolysed quantity was found to be approximately independent of the water/TEOS molar ratio, even for r < 4. Polycondensation reaction takes place mainly for low water/TEOS molar ratio in order to supply water to allow almost complete hydrolysis. The overall process of dissolution and hydrolysis has reasonably been described by a previous modelling. The dissolution process of water in TEOS, under ultrasound stimulation and acid conditions, was found to be rather dependent of the alcohol produced in the hydrolysis reaction instead of the initial water quantity present in the mixture.     

Determination of oxalate in black liquor by headspace gas chromatography

This study demonstrated a headspace gas chromatographic method (HS-GC) for the determination of oxalate content in black liquor (alkaline aqueous solution of inorganic chemicals and dissolved wood species from the alkaline pulping of wood). The method described in this paper is based on the reaction between oxalic and manganese dioxide in an acidic medium, in which oxalic acid is converted to carbon dioxide that is measured with a GC using a thermal conductivity detector. The challenge in developing this method was ensuring complete conversion of oxalic acid while minimizing the contribution of side reactions between carbohydrates, lignin and manganese dioxide to the carbon dioxide measured. It was found that a complete conversion of oxalate to carbon dioxide can be achieved within 3 min at a temperature of 70 degrees C; a MnO(2):C(oxalate) (concentration of H(2)C(2)O(4)+HC(2)O(4)(-)+C(2)O(4)(2-)) mole ratio of 60 and H(2)SO(4) concentration of 0.005-0.01 mol/L in the headspace vial. The method can detect concentrations as low as 0.39 microg of oxalate. The standard deviation was found to be 7% while recovery experiments with black liquor showed recoveries of 93-108% which were deemed acceptable for analysis of oxalate in an industrial sample such as black liquor.     

A kinetic model for the ultrasound catalyzed hydrolysis of solventless TEOS-water mixtures and the role of the initial additions of ethanol

The acid and ultrasound catalyzed hydrolysis of solventless TEOS-water mixtures are studied, as a function of the initial additions of ethanol to the mixtures, by means of flux calorimetry measurements. A device was specially designed for this purpose. Under acid conditions, our proposed method has been able to resolve hydrolysis from other condensation reactions, by detecting the exothermal hydrolysis reaction heat. The process has been explained by a dissolution and reaction mechanism. Ultrasound forces the dissolution process to start the reaction. The alcohol produced in the reaction helps the dissolution process to further enhance the hydrolysis. Initial amounts of pure ethanol added to the mixtures shorten the start time of the reaction, due to an additional effect of dissolution, and diminish the reaction rate, as a result of the solvent dilution effect. Our dissolution and reaction mechanism modeling describes the main points arising from the experimental data and yields k _H =0.24 M^-1 min^-1 for the second-order hydrolysis rate constant at 39°C.     

Ultrasound assisted alkaline hydrolysis of poly(ethylene terephthalate) in presence of phase transfer catalyst

Alkaline hydrolysis of poly(ethylene terephthalate) (PET) flakes from waste packaged drinking water bottles was carried out with and without influence of ultrasound waves rated 20 kHz frequency and 190 W power. Alkali used for hydrolysis was 10% NaOH (w/w). Tetrabutyl ammonium iodide (TBAI) was used as phase transfer catalyst (PTC) to enhance rate of hydrolysis. The experiment yields terephthalic acid (TA) and ethylene glycol as products of hydrolysis. Minimum time required for ultrasound assisted (UA) reaction and without ultrasound assistance (WUA) reaction to complete was investigated and compared. PTC: PET ratio = 0.03:1 w/w, temperature (90 °C) and NaOH concentration (10% w/w) were kept constant. All reactions were carried out at atmospheric pressure. For UA reaction, time required for 100% conversion of PET was found to be 45 min. For WUA reaction, the time required for 100% conversion of PET was found to be more than 65 min. Yield of TA was found to be >99% on the basis of moles of repeating units of PET fed. Melting point of product was found nearly equal to standard TA. Product TA was confirmed by comparing Fourier-transform infrared spectroscopy (FTIR) spectra of product with that of standard TA. Ratio of PTC to PET was fine-tuned for UA reaction keeping reaction time constant at 45 min.     

Improvement of chemical analysis of antibiotics. V. A simple method for the analysis of tetracyclines using reversed-phase high-performance liquid chromatography

An isocratic high-performance liquid chromatographic method for the determination of tetracyclines is described using a mobile phase containing oxalic acid and C8- and C18-modified silica gel columns. For good separations of tetracyclines, oxalic acid concentrations of above 0.01 and 0.2 M respectively for parent tetracyclines (group I) and impurities in tetracycline (group II) are required. The optimum pH of the aqueous oxalic acids solution in the mobile phase is 2.0. The combinations of the C8-modified silica gel column with methanol-acetonitrile-0.01 M aqueous oxalic acid solution pH 2.0 (1:1.5:5) and the C18-modified silica gel column with methanol-acetonitrile-0.2 M aqueous oxalic acid solution pH 2.0(1:1:3.5) gave satisfactory results for groups I and II, respectively.     

Kinetics of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by alpha-chymotrypsin in aqueous solutions of dodecyltrimethylammonium bromide

The rate of hydrolysis of N-glutaryl-L-phenylalanine p-nitroanilide (GPNA) catalyzed by alpha-chymotrypsin (alpha-CT) has been measured in aqueous solutions of dodecyltrimethylammonium bromide (DTAB) at concentrations below and above the critical micelle concentration, as well as in the absence of surfactant. Under all the conditions employed, the reaction follows a Michaelis-Menten mechanism. The presence of the surfactant leads to superactivity below and above the critical micelle concentration (CMC), with a maximum reaction rate taking place near the CMC when the results are treated in terms of the analytical concentration of the substrate. A similar behavior was observed by working with the enzyme partially deactivated in the presence of 4 M urea. After correction to take into account the partitioning of the substrate between the micelles and the external media, the activity of the enzyme tends to remain almost constant above the corresponding CMCs. This results from a compensation of a decrease in the catalytic constant (k(cat)) and a decrease in the Michaelis constant (K(M)). The behavior of alpha-CT in the hydrolysis of GPNA in DTAB solutions is at variance with that previously reported for the hydrolysis of 2-naphthyl acetate in solutions of the same surfactant (E. Abuin, E. Lissi, R. Duarte, Langmuir 19 (2003) 5374). An explanation of the different effects of the surfactant on the behavior of the enzyme with both substrates is advanced, taking into account the complexity of the mechanism of the alpha-CT-mediated reaction, more specifically, in terms of different rate-limiting steps for the formation of the measured products.     

Determination of trace amount of oxalic acid with zirconium(IV)-(DBS-arsenazo) by spectrophotometry

A novel method is proposed for the determination of trace amount of oxalic acid in the present article. In 1.0M hydrochloric acid medium, oxalic acid can react with the zirconium(IV) in Zr(IV)-(DBS-arsenazo) complex and replaces the DBS-arsenazo to produce a hyperchromic effect at 520 nm. The hyperchromic degree is proportional to the concentration of the oxalic acid added over a defined range. Based on this property, a new method for the spectrophotometric determination of trace oxalic acid was developed. Beer's law is held over the concentration range of 9.0 x 10(-6) to 5.0 x 10(-4)M for oxalic acid with a correlation coefficient of 0.9995. The apparent molar absorptivity of the method is epsilon520 nm = 1.16 x 10(3)L mol(-1)cm(-1) and the detection limit for oxalic acid is 0.815 microg/mL. The developed method was directly applied to the determination of oxalic acid in tomato samples with satisfactory results.     

Metal oxalates in paints: a Raman investigation on the relative reactivities of different pigments to oxalic acid solutions

One degradation phenomenon that occurs in artworks is the formation of metal oxalates on their surfaces. In order to gain insight into the inclination of pigments to produce oxalates, nine pigments including Na, Ca, Fe, Pb and Cu cations were selected to react with oxalic acid solutions at different concentrations (1 M, 0.1 M, 0.01 M and 0.005 M). Micro-Raman spectroscopy was used to detect the different reaction products. Pigments containing calcium (calcite, gypsum and Volterra gypsum) showed a high tendency to form weddellite as well as whewellite, especially at high acidic concentrations; among copper-based pigments (malachite, azurite, verdigris), the formation of moolooite was observed for high concentrations of acid and down to the lowest concentration (0.005 M) in the case of verdigris. Lead oxalate was detected on lead white. No iron oxalates were observed for hematite; the formation of calcium oxalate crystals was observed instead. Ultramarine blue reacted to produce elemental sulfur. According to the results obtained, calcite and verdigris showed the highest reactivity in oxalic acid environments, resulting in a high tendency to form calcium and copper oxalates, even at very low acidic concentrations; this behavior seems to arise from the high solubilities of these pigments in acidic environments.     

CARBOXYLATION OF FORMIC ACID IN AQUEOUS SOLUTIONS UNDER THE INFLUENCE OF U.V.-LIGHT

Abstract— Carboxylation of various organic substances by dissolved carbon dioxide or bicarbonate is achieved by the action of U.V. -Light of 184.9 and 253.1 mμ on water. The transformation of formic acid to oxalic acid was investigated in more detail as a function of u.v.-dose, pH-values and the formate concentration. For an irradiation period of 12 hr of 0.1 M formate, a yield of about 2 m mol.1^--¹ oxalic acid was obtained. A reaction mechanism for the photochemical carboxylation process is presented. The results are of some interest in respect of the synthesis of organic compounds under the primitive earth conditions.     

210Pb and210Po in tobacco-with a special focus on estimating the doses of210Po to man

Studies have been carried out on the solubility of Pu(III) oxalate by precipitation of Pu(III) oxalate from varying concentrations of HNO₃/HCl (0.5–2.0M) solutions and also by equilibrating freshly prepared Pu(III) oxalate with solutions containing varying concentrations of HNO₃/HCl, oxalic acid and ascorbic acid. Pu(III) solutions in HNO₃ and HCl media were prepared by reduction of Pu(IV) with ascorbic acid. 0.01–0.10M ascorbic acid concentration in the aqueous solution was maintained as holding reductant. The solubility of Pu(III) oxalate was found to be a minimum in 0.5M–1M HNO₃/HCl solutions containing 0.05M ascorbic acid and 0.2M excess oxalic acid in the supernatant.     

Potentiometric studies of dithiooxamide with a sulphide-selective membrane electrode

The sulphide-selective electrode is applicable to the determination of dithiooxamide by titration with silver nitrate. The effect of the alkali content of the solutions on the reaction has been studied. The reaction products are silver sulphide, oxalic acid, and nitric acid. If the alkali concentration of the solution is lower than that equivalent to the acid formed during the titration, the amount of sulphide produced by hydrolysis decreases and the equilibrium potential is established more slowly.The determination is rapid and accurate in the presence of 1 M sodium hydroxide.     

以草酸为电子给体在Pt-TiO2上光催化生成氢

 以草酸为电子给体,研究了在Pt－TiO2上光催化生成氢的反应．草酸的存在明显促进了生成氢的反应;二氧化钛负载Pt也明显提高了反应速率,Pt的最佳负载量为w（Pt）＝0．5％．草酸浓度对氢生成反应的影响符合Lang－muir关系式．最佳pH值为2～4．     

Kinetic study of the acid hydrolysis of sucrose and lactose and kinetic determination of sucrose using a periodate-selective electrode

A novel procedure for the kinetic study of the acid hydrolysis of sucrose and lactose has been developed. The parameter observed is the rate of the reaction of periodate with quenched aliquots of the solution in which hydrolysis takes place. This rate measured with a periodate-selective electrode, and is expressed in terms of rate of increase in potential (gDE/Δt). Based on the observed differences in the hydrolysis rate, a kinetic method has been developed for the determination of sucrose in the presence of other carbohydrates in the concentration range 0.01–0.10 M with an average error of 1.4%. The method has been applied to the determination of sucrose in milk products and soft drinks.     

Dissolution of kaolinite induced by citric, oxalic, and malic acids

Kaolinite is a dominant clay mineral in the soils in tropical and subtropical regions, and its dissolution has an influence on a variety of soil properties. In this work, kaolinite dissolution induced by three kinds of low-molecular-weight organic acid, i.e., citric, oxalic, and malic acids, was evaluated under far-from-equilibrium conditions. The rates of kaolinite dissolution depended on the kind and concentration of organic acids, with the sequence R(oxalate)>R(citrate)>R(malate). Chemical calculation showed the change in concentration of organic ligand relative to change in concentration of organic acid in suspensions of kaolinite and organic acid. The effect of organic acid on kaolinite dissolution was modeled by species of organic anionic ligand. For oxalic acid, L(2-)(oxalic) and HL(-)(oxalic) jointly enhanced the dissolution of kaolinite, but for malic and citric acids, HL(-)(malic) and H2L-(citric) made a higher contribution to the total dissolution rate of kaolinite than L(2-)(malic) and L(3-)(citric), respectively. For oxalic acid, the proposed model was R(Si)=1.89x10(-12)x[(25x)/(1+25x)]+1.93x10(-12)x[(1990x1)/(1+1990x1)] (R2=0.9763), where x and x1 denote the concentrations of HL(oxalic) and L(oxalic), respectively, and x1=10(-3.81)xx/[H+]. For malic acid, the model was R(Si)=4.79x10(-12)x[(328x)/(1+328x)]+1.67x10(-13)x[(1149x1)/(1+1149x1)] (R2=0.9452), where x and x1 denote the concentrations of HL(malic) and L(malic), respectively, and x1=10(-5.11)xx/[H+], and for citric acid, the model was R(Si)=4.73x10(-12)x[(845x)/(1+845x)]+4.68x10(-12)x[(2855x1)/(1+2855x1)] (R2=0.9682), where x and x1 denote the concentrations of H2L(citric) and L(citric), respectively, and [Formula: see text] .     

Ester hydrolysis and enol nitrosation reactions of ethyl cyclohexanone-2-carboxylate inhibited by beta-cyclodextrin

Both the ester hydrolysis and the nitrosation reactions of the enol tautomer of ethyl cyclohexanone-2-carboxylate (ECHC) are investigated in the absence and presence of beta-cyclodextrin (beta-CD). The ester hydrolysis reaction is studied in dilute H2O and D2O solutions of hydrochloric acid and in aqueous buffered solutions of carboxylic acids (acetic acid and its chloro derivatives). The pseudo-first-order rate constant increases with both the [H+] and the total buffer concentration, indicating that the hydrolysis is subject to acid and general base catalysis. Substantial solvent isotope effects in the normal direction (kH/kD > 1) for the acid-catalyzed hydrolysis was observed. Addition of beta-CD strongly slows the hydrolysis reaction. The variation of the observed rate constant (k(o)) with [beta-CD] exhibits saturation behavior, consistent with 1:1 binding between the enol of ECHC and beta-CD. The binding is quite strong, and bound ECHC-enol is unreactive. The nitrosation reaction of ECHC in aqueous acid medium, using sodium nitrite in great excess over the concentration of ECHC, yields perfect first-order kinetics, indicating that the slow step is the nitrosation of the enol tautomer. This finding suggests that a great percentage of the total ECHC concentration must exist in the enol form. The nitrosation reaction is of first order in [nitrite] and is catalyzed by the presence of Cl-, Br-, or SCN- ions, which indicates that the attack of the nitrosating agent is the slow step. The nitrosation reaction is also strongly inhibited by the presence of beta-CD because of the formation of unreactive inclusion complexes between the host, beta-CD, and the guest, the enol of ECHC. In alkaline medium, the formation of the enolate ion is observed, which absorbs at higher wavelengths (lambda(max) = 256 nm in acid medium shifts to lambda(max) = 288 nm in alkaline medium). This anion also undergoes ester hydrolysis spontaneously, but shows neither specific basic catalysis nor appreciable effect by the presence of beta-CD. From kinetic and spectroscopic measurements the pKa of the enol of ECHC has been determined as 12.35.     

Computational study on the kinetics and mechanisms for the unimolecular decomposition of formic and oxalic acids

The kinetics and mechanisms for the unimolecular decomposition reactions of formic acid and oxalic acid have been studied computationally by the high-level G2M(CC1) method and microcanonical RRKM theory. There are two reaction pathways in the decomposition of formic acid: The dehydration process starting from the Z conformer is found to be the dominant, whereas the decarboxylation reaction starting from the E conformer is less competitive. The predicted rate constants for the dehydration and decarboxylation reactions are in good agreement with the experimental data. The calculated CO/CO2 ratio, 13.6-13.9 between 1300 and 2000 K, is in close agreement with the ratio of 10 measured experimentally by Hsu et al. (In The 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1983; p 89). For oxalic acid, its isomer with two intramolecular hydrogen bonds is the most stable structure, similar to earlier reports. Two primary decomposition channels of oxalic acid producing CO2+HOCOH with barriers of 33-36 kcal/mol and CO2+CO+H2O with a barrier of 39 kcal/mol were found. At high temperatures, the latter process becomes more competitive. The rate constant predicted for the formation of CO2 and HOCOH (the precursor of HCOOH) agrees well with available experimental data. The mechanism for the isomerization of HOCOH to HCOOH is also discussed.     

Enzyme-like acceleration for the hydrolysis of a DNA model promoted by a dinuclear Zn(II) catalyst in dilute aqueous ethanol

The rates and products of cleavage of methyl (2-chloro-4-nitrophenyl) phosphate (2) promoted by a dinuclear Zn(II) complex (3) of 1,3-bis-N,N'(1,5,9-triazacyclododecyl)propane along with 1 equiv of ethoxide were investigated in ethanol solution containing small amounts of water (8 mM or=1.6 x 10(17) times relative to the background hydroxide reaction, suggesting that complex 3 promotes the hydrolysis at least 1000 times more effectively than ethanolysis.     

Comparative kinetic study for rate constant determination of the reaction of ascorbic acid with 2,6-dichlorophenolindophenol

The rate constant k of the reaction of ascorbic acid with 2,6-dichlorophenolindophenol (DCPI) in oxalic acid solutions is determined, by stopped-flow techniques. Four different methods are used to evaluate the results. The values and errors are compared statistically. The average of the rate constant is 56.5 x 10(3) l. mole(-1), sec(-1) and the overall standard deviation is 0.6 x 10(3) l. mole(-1), sec(-1) or 1.0% relative. The pH-dependence of the rate constant suggests that DCPI reacts with undissociated ascorbic acid.     

Decomplexing madder lakes using oxalic acid: A novel method coupled with microwave or ultrasound processes

The aim of the present study was to establish a mild extraction and universal method to characterize madder lakes of Rubia tinctorum by liquid chromatography coupled with a photodiode array detector . To analyze the lakes, anthraquinone molecules must be decomplexed of metal links. To this end, two processes, one ultrasound and the other microwave, were improved in association with two solutions, an oxalic acid solution (0.5 M in MeOH/H₂O 50/50) and an acetic acid buffer solution (1 M). First, the decomplexation of an alizarin experimental lake was optimized in comparison with a reference method using a strong acid. The microwave process used oxalic acid and increased the decomplexation yield of alizarin (71%) compared with the reference method (31%). Second, different madder experimental lakes, which were prepared using different metal salts, were decomplexed. The obtained results suggest that the use of microwave associated with oxalic acid solution is the most universal method providing a decomplexation of anthraquinones from lakes without hydrolysis of glycosidic compounds occurring.