Aqueous Acetate Buffer Research Articles

Extraction Equilibria of Copper(II) with D2EHPA in Kerosene from Aqueous Solutions in Acetate Buffer Media

Extraction equilibria of copper(II) from aqueous acetate buffer solutions with di-(2-ethylhexyl)phosphoric acid (D2EHPA) dissolved in kerosene and the stripping equilibria were investigated. Results showed that acetate ions can greatly improve the copper(II) extraction efficiency. The distribution coefficients were significantly dependent on the concentration of acetate ions and the pH value in the aqueous phase because of various mechanisms of extraction and complex formation in the organic phase. With the initial pH value of 4.44 and the acetate ion concentration of 0.18 mol·L-1, the maximum distribution coefficient was observed. In addition, mechanisms of copper(II) extraction in acetate buffer media were established. In the stripping process, the ability to back-extract copper(II) from the organic phase is HCl > H2SO4 > H3PO4.

Properties of Oxide Formed on Aluminium in Aqueous Acetate Buffer

The properties of oxide formed on pure aluminium in acetate buffers under controlled potential, temperature, and pH are characterized by in situ electrochemical techniques and spectroscopic visual ellipsometry (VISSE). Monitoring the current density (cd) during transition between different applied potentials gives direct information about Faradaic changes in the oxide properties, while the ellipsometer monitors the changes in the actual thickness and refractive index of the film. Electrochemical impedance spectroscopy (EIS) reveals, in addition to film properties, information about the charge transfer processes and adsorption at steady-state conditions. VISSE and ex situ transmission electron microscopy (TEM) indicate the presence of a two layered oxide structure, consisting of the barrier and a porous hydrated layer. The thickness and density of these two layers are monitored in situ during growth and dissolution. The thickness of the barrier layer depends reversibly on the applied potential, while the thickness of the hydrated layer is less affected by the potential. The absorption sensitive parameter is correlated with the buffer concentration and therefore believed to be related to the acetate.

Optimisation of the derivatization in cellulose‐type chiral selectors for enantioseparation by centrifugal partition chromatography

Cellulose was chemically modified with hydrophobic dodecanoyl groups followed by 3,5-dimethylphenylcarbamoyl substituents forming mixed ester/carbamate derivatives in order to improve the solubility in lipophilic solvents compared to the corresponding homosubstituted cellulose tris(3,5-dimethylphenylcarbamate). Two mixed derivatives of different degree of substitution were prepared, tested as chiral selectors (CSs) in counter-current chromatography (CCC) and compared to the homo-substituted derivative. Alternatively, cellulose tris(3,5-dichlorophenylcarbamate), was synthesised and tested with the same purpose. The racemic drugs pindolol and warfarin were used as test compounds. Biphasic organic/aqueous solvent systems, composed of methyl isobutyl ketone, t-butyl methyl ether or ethyl acetate and aqueous ammonium acetate or sodium phosphate buffer, were used. Centrifugal partition chromatography, a variety of CCC, in the classical elution mode and the pH-zone-refining displacement mode was applied. The enantioseparation of pindolol and warfarin was achieved in the latter conditions. The presence of dodecanoyl chains on the CS increased solubility in the organic solvents used. The selectivity of the mixed dodecanoyl/3,5-dimethylphenylcarbamoyl cellulose derivative with low degree of dodecanoyl groups (degree of substitution 0.3/degree of substitution 2.6, respectively) was similar to that of the homo-substituted derivative. Furthermore, the loading capacity for pindolol was increased by a factor of three compared to cellulose tris(3,5-dimethylphenylcarbamate). Nevertheless, the increasing degree of substitution with dodecanoyl groups on the CS, although improved solubility in the stationary phase, contributed negatively to the enantioselectivity, where warfarin was more affected than pindolol.

Effect of electrochemical and mechanical parameters on the lubrication behaviour of Al 2O 3 nanoparticles in aqueous suspensions

The effect of alumina nanoparticles suspended in an aqueous acetate buffer solution on the tribocorrosion behaviour of a 316L steel/alumina sliding contact was investigated using a reciprocating wear test rig equipped with an electrochemical cell. Through the addition of 10% volume of alumina nanoparticles to an acetate buffer solution it was possible to reduce the coefficient of friction by a factor of 2 and the stainless steel wear by a factor of 10 and this over a range of electrochemical and mechanical conditions. The presence of alumina particles caused a significant wear on the alumina counter part. The lubricating behaviour of the suspension was comparable to a commercial oil in water emulsion used for metal cutting. The lubricating effect was attributed to a third body formed by nanoparticles trapped within the contact. The agglomeration state of the alumina particles and their size were found to play a critical role.

Enantiomeric quantification of ( S)-(+)-methamphetamine in urine by an immunoaffinity column and liquid chromatography–electrospray-mass spectrometry

A method using an immunoaffinity column (IAC) and liquid chromatography–electrospray ionization mass spectrometry (LC/MS) for on-line detecting the presence of MA in the effluent was developed for the quantitative and enantiomeric determination of ( S)-(+)-methamphetamine ( d-MA) in urine. The IAC was made in our laboratory and utilized in the LC/MS to simultaneously extract and separate enantiomers of MA from urine samples. An aqueous ammonium acetate buffer was used as the mobile phase. Urine samples were spiked with racemic deuterated methamphetamine (MA-d 14) as internal standard (IS), filtered through a membrane, and injected into the LC/MS without any further pre-treatment. Protonated molecular ion of MA and MA-d 14 ( m/ z 150 and 164) were isolated and further fragmented, the respective product ions, m/ z 119 and 130, were collected for quantitative determination. This is an improvement of our previous method (A.C. Lua, Tsong-Yung Chou, J. Chromatogr. A 967 (2002) 191). In the previous method, MA was separated with HPLC, the efflux was fractionated and each fraction was either determined with an immunoassay or GC/MS. Monitoring of MA in the efflux is tedious and time consuming. Urine samples spiked with different concentrations of d-MA were measured by this method. A linear relationship exists in the 150–1050 ng/mL range, and the detection limit (defined as signal-to-noise ratio 3) of d-MA was determined to be 18 ng/mL. The linearity of the method for d-MA can be described by the equation ( Y = 1.415 × 10 −3 X + 0.034, correlation coefficient: r 2 = 0.999). Within run, accuracy and precision ( n = 6, relative error: −7.2 to +4.0% and relative standard deviation: 3.8–9.3%) of the method are fairly good.

Stability indicating simultaneous determination of domperidone (DP), methylparaben (MP) and propylparaben by high performance liquid chromatography (HPLC)

A simple, specific and precise high performance liquid chromatographic method has been developed and validated for the simultaneous determination of methylparaben (MP), propylparaben (PP), and domperidone (DP) in oral suspension. Isocratic mobile phase consists of 0.5% w/v aqueous ammonium acetate buffer:methanol, 40:60 (v/v). Column containing octylsilyl chemically bonded to porous silica particles (Optimapak, OP C8, 150 mm × 4.6 mm, 5 μm, stainless steel analytical column from RS tech) is used as stationary phase. The detection is carried out using variable wavelength UV–vis detector set at 280 nm. The solutions are chromatographed at constant flow rate of 1.0 mL/min. The method separates MP, PP, DP and droperidol (DR) impurity in less than 12 min with good resolution, peak shapes and minimal tailing. Retention times (RT) for MP, PP, DP and DR are about 3.4, 7.0, 9.0 and 10.9 min, respectively. Linearity range and percent recoveries for MP, PP and DP are 90–270, 10–30, 50–1500 μg/mL and 100.30%, 100.78% and 100.48%, respectively. Method was validated according to ICH guidelines and proved to be suitable for stability testing, homogeneity testing and quality control of these compounds in pharmaceutical preparations.

Identification of Curcuma and Safflower Dyes by Voltammetry of Microparticles Using Paraffin-Impregnated Graphite Electrodes

Electrochemical identification of natural insoluble curcuma and safflower dyes in microsamples usable for microchemical examination of works of art and archaeological artifacts via solid state voltammetry is described. Upon attachment to paraffin-impregnated graphite electrodes in contact with aqueous acetate and phosphate buffers, curcuma and safflower yield well-defined square wave voltammetric responses with well-defined oxidation peaks in the potential region of +0.65–+0.25 V vs. AgCl/Ag allowing for unambiguous identification of such dyes in microsamples.

Cyclophane-lectin Conjugates as a New Class of Water-soluble Host

A conjugate composed of tetraaza[6.1.6.1]paracyclophane bearing carboxylic acids and lectin, a carbohydrate binding protein, was prepared. The specific saccharide-binding abilities as well as the secondary structural features of the lectin were not disturbed, when the cyclophane were covalently bound to the lectin. The conjugate was found to act as a water-soluble host for inclusion of anionic guest molecules such as 6-p-toluidino-naphthalene-2-sulfonate (TNS) and 8-anilinonaphthalene-1-sulfonate (ANS) in aqueous acetate buffer (pH 4.0) with binding constants of 4.2 × 104 and 1.5 × 104 dm3 mol−1, respectively. The obtained binding constants were much larger than those by untethered water-soluble cyclophane. A highly desolvated microenvironment was provided by the cyclophane cavity on the protein surfaces so that the tight host–guest interaction, which brought about the marked motional repression of the entrapped guests, became effective. The conjugate also showed molecular discrimination capabilities toward the anionic guests through electrostatic repulsion mechanism originating from acid-dissociation equilibrium of carboxylic acids of the cyclophane branches.

Electroreduction of (1S,2S)-2-amino-1-(4-nitrophenyl)-propane-1,3-diol derivatives. Behaviour of electrogenerated species and applications to organic synthesis

Hydroxylamines electrogenerated from (1S,2S)-2-amino-1-(4-nitrophenyl)-propane-1,3-diol derivatives (derivatives of p-nitrophenylserinol) are unstable in methanol–acetate buffer and practically stable in acetonitrile–aqueous acetate buffer. This latter medium was used to carry out subsequent reactions in situ involving the triacetylated p-hydroxylaminophenylserinol and various reagents. An azoxy compound was the major product isolated after reaction with maleic or phthalic anhydrides. In contrast, a benzoxazine dione was normally obtained with phthaloyl dichloride and a N-sulphonylated hydroxylamine was produced with p-toluenesulphonyl chloride.

Synthesis and chromatographic properties of α-Schiff bases (6-imino)-β-cyclodextrin bonded silica for stationary phase of liquid chromatography

Two new chiral stationary phases (CSPs), 6-deoxyisopropylimino-β-cyclodextrin bonded on the silica gel (YBCDs) and heptakis[2,6- o-diamyl-6-deoxyisopropylimino]-β-cyclodextrin bonded on the silica gel (WYBCDs), were prepared. The chromatographic properties of two CSPs (YBCDs and WYBCDs) were tested. Good resolutions for disubstituted benzene isomers were obtained with YBCDs. l- and d-amino acids mixtures were well separated on both YBCDs and WYBCDs. The influence of pH of aqueous triethylammonium acetate buffer (TEAA) was also studied on the two CSPs. Experimental results show that WYBCDs enhances the retention of amino acids. It is proposed that the amylation modification above the cyclodextrin (CD) partially occludes the mouth of the CD cavity and sterically influences the formation of inclusion complexes, leading to the change in solute retention.

Electrochemical Hydrogenation of Acetophenone and Methyl-4-isobutylacetophenone in a Two-phase Water–Organic System Containing -Cyclodextrin

The possibility of the formation of inclusion complexes based on methyl-4-isobutylphenyl ketone, β-cyclodextrin (β-CD), and o-hydroxy propylated β-cyclodextrin in an aqueous phosphate buffer is demonstrated by methods of NMR spectroscopy and cyclic voltammetry. Preparative electroreduction of acetophenone and methyl-4-isobutylphenyl ketone is performed on a lead electrode in aqueous phosphate and acetate buffers in the absence and presence of β-CD and the composition of products of electrochemical reactions is ascertained. It is shown that the products of reduction of ketones are corresponding alcohols and pinacones. The possibility of asymmetric electrochemical hydrogenation of ketones in the presence of β-CD, which yields relevant chiral alcohols, is shown.

Determination of atractyloside in Callilepis laureola using solid-phase extraction and liquid chromatography–atmospheric pressure ionisation mass spectrometry

A selective analytical method based on high-performance liquid chromatography, combined with atmospheric pressure ionisation mass spectrometry, was developed for the detection of atractyloside. The analysis was performed on an Xterra Phenyl column utilising a gradient elution profile and a mobile phase consisting of 10 mM aqueous ammonium acetate buffer–methanol–acetonitrile. The calibration curve of the method (1 ng/ml–160 μg/ml) was best described by a second order polynomial function ( r 2 = 0.998) but displayed good linearity in the range of 100 ng/ml–1 μg/ml ( r 2 = 0.999). The limit of detection for the atractyloside standard was determined and found to be 100 pg/ml and the limit of quantification of atractyloside in tuber matrix was found to be 250 pg/ml. The relative standard deviation of the method was on average below 5% ( n = 8). The method was successfully applied to the analysis of Callilepis laureola tubers and unknown powdered samples for the presence of atractyloside.

Influence of a buffered solution on the adsorption isotherm and overloaded band profiles of an ionizable compound

The overloaded band profiles and the adsorption isotherms of propranolol were acquired at 23 °C, on the endcapped C 18-Kromasil stationary phase, using two aqueous solutions of methanol as the mobile phase. The first solution contained 40% methanol and no buffer. The second contained an aqueous acetate buffer at C buffer=0.20 M and pH=5.9. In both cases, 33 isotherm data points were acquired by frontal analysis (FA), to achieve an accurate description of the isotherms in the concentration range between 1.54×10 −3 and 1.54×10 −1 mol/l of propranolol. The isotherms obtained were best described by a bi-Langmuir and a bi-Moreau isotherm model, depending on whether the mobile phase was buffered or not. This shows that the adsorption of propranolol takes place on two different types of sites, a behavior similar to the one already observed with phenol and caffeine on the same column. The presence of the buffer in the mobile phase drastically changes the adsorption mechanism of propranolol. Weak adsorbate–adsorbate interactions (two and three times RT on the low- and the high-energy sites, respectively) take place in the absence of buffer but vanish when the mobile phase is buffered. As expected, the adsorption constants on the abundant low-energy sites with or without buffer are comparable because the mobile phase composition was adjusted to give similar retention times in the two cases. On the other hand, the adsorption of propranolol on the high-energy sites is stronger in presence of the buffer. The difference probably comes from ion-pair formation in the adsorbed phase between the propranolol cation and the acetate anion. The change in total saturation capacity of the adsorbent (22%) compared to that for phenol is explained by the difference in methanol content of the mobile phase.

Catalytic Monolayer Voltammetry and In Situ Scanning Tunneling Microscopy of Copper Nitrite Reductase on Cysteamine-Modified Au(111) Electrodes

We have studied the adsorption and electrocatalysis of the redox metalloenzyme blue copper nitrite reductase from Achromobacter xylosoxidans (AxCuNiR) on single-crystal Au(111)-electrode surfaces modified by a self-assembled monolayer of cysteamine. A combination of cyclic voltammetry and in situ electrochemical scanning tunneling microscopy (in situ STM) directly in aqueous acetate buffer, pH 6.0 has been used. High-resolution in situ STM shows that cysteamine packs into ordered domains with strip features of a periodic distance of 11.7 ± 0.3 A. No voltammetric signals of the nitrite substrate on this surface could be detected. A strong cathodic catalytic wave appears in the presence of nitrite. The catalytic current follows a Michaelis−Menten pattern with a Michaelis constant of Km ≈ 44 μM, which is close to the value for AxCuNiR in homogeneous solution. The apparent catalytic rate constant based on a dense monolayer is kcat = 6−10 s-1. This is significantly lower than two reported values of 185 s-1 and...

Simple high-performance liquid chromatographic method for the determination of acyclovir in pharmaceuticals

An assay method for the determination of acyclovir from pharmaceutical preparations has been developed for assessment of product quality utilising high-performance liquid chromatography. The chromatographic conditions comprised a reversed-phase C 18 column (250×4.6 mm i.d.) with a mobile phase of acetonitrile-20 mmol l −1 aqueous ammonium acetate buffer of pH 4.5 (40:60). The flow rate was 0.8 ml min −1 and UV detection was used at 250 nm. Calibration graph was linear in the range 1.98–59.4 μg ml −1. The method has been validated according to current guidelines including assay of pharmacopoeial standard tablets. Recoveries ranged from 96.64 to 99.53%. The exipients present in the tablets did not interfere with the method.

Stability of Sulfonamides, Nitrofurans, and Chloramphenicol Residues in Preserved Raw Milk Samples Measured by Liquid Chromatography

A stability study was made of 10 antimicrobials: 6 sulfonamides, 3 nitrofurans, and chloramphenicol residues in raw milk samples preserved with 0.1 % potassium dichromate (K2Cr2O7) and 0.05% mercuric bichloride (HgCl2) during cold storage for 7 days. Preserved milk samples fortified with 50 ppb of each antimicrobial were analyzed by liquid chromatography (modified AOAC Method 993.32). Drugs were extracted with chloroform-acetone after solvent evaporation residues were dissolved with aqueous sodium acetate buffer solution (0.02M, pH 4.8), and fat was removed with hexane. Sulfonamides and chloramphenicol were detected at 275 nm (UV) by using a gradient system of sodium acetate buffer solution-acetonitrile starting at 95 + 5 (v/v) and finishing at 80 + 20 (v/v). Nitrofurans were detected at 375 nm (UV) isocratically with sodium acetate buffer solution-acetonitrile (80 + 20, v/v). Residues stability was measured through recovery data. Sulfamethoxazole, sulfachloropyridazine, nitrofurazone, furazolidone, and furaltadone residues remained stable in the presence of either preservative for 7 days. Sulfamethazine and chloramphenicol were not affected by K2Cr2O7, but had significant losses (p <0.05) when HgCl2 was used: 26.2 and 13.4%, respectively. Average recoveries of sulfamonomethoxine, sulfamerazine, and sulfathiazole significantly decreased by Day 7, with losses of 17.1, 17.2, and 23.2% for K2Cr2O7, and 23.3, 20.7, and 48.0% for HgCl2, respectively. During 5 days of cold storage all antimicrobials tested, except sulfathiazole, remained stable in milk samples preserved with 0.1 % K2Cr2O7 or 0.05% HgCl2.

The first one-dimensional heteropoly tungstovanadate coordination polymer: [(V VO 4)W VI8M 4O 36(V IVO) 2] n− (M=0.71V IV+0.29W VI, n=4.68)

An aqueous ammonium acetate buffer solution (pH 4) containing vanadate, tungstate and dithionite ions, on heating at 100 °C for 1 h yields one-dimensional polyoxometalate chain compound (NH 4) 4.68[(V VO 4)W VI 8M 4O 36(V IVO) 2]·12H 2O (M=0.71V IV+0.29W VI) 1, which is characterized by single-crystal X-ray structure determination, redox titration, and IR spectroscopic studies. Crystallographic data for 1, H 42.72N 4.68O 54V 5.84W 9.16, tetragonal, space group I 4 ̄ , a=15.8132(5), c=11.1114(5) Å, Z=2, V=2778.5(2) A ̊ 3 .

Synthesis and characterization of a reduced heteropoly-tungstovanadate: (NH4)7[VvO4W 10 IV V 2 IV O36].ca. 22H2O

The compound (NH4)7[ VvO4W VI10 V VI2 O36]·ca.22H2O (1) has been synthesized from an aqueous ammonium acetate buffer (pH 4) containing sodium vanadate, sodium rungstate_and sodium dithionite. Compound (1) crystallizes in a cubic space groupFm — 3, witha = 22.2001(6) Å and Z = 8. The anion [VvO4W VI10 V IV2 O36]7- is a typical Keggin type structure with VVO4 as the central tetrahedron. (1) has further been characterized by elemental analyses, redox titration, IR, EPR, and electronic spectroscopy and room temperature magnetic moment measurement.

Study of the stability of promethazine enantiomers by liquid chromatography using a vancomycin-bonded chiral stationary phase

Three chiral stationary phases based on macrocyclic antibiotics (teicoplanin, vancomycin and ristocetin A) have been tested for chiral separations of promethazine. The vancomycin phase permits the best, baseline enantioseparation of promethazine, with a mobile phase of a 80:20 (v/v) mixture of methanol with a 1% aqueous triethylamine acetate buffer of pH 4.1 and with the analysis time not exceeding 15 min. The limits of detection amount to 27.5 and 31.0 ng/ml for the earlier and later eluting enantiomers, respectively. This separation system, that also permits a sufficient resolution between the promethazine enantiomers and their degradation products, has further been used for the monitoring of the effects of light, temperature and the promethazine concentration in solution on the stability of methanolic promethazine solutions over a period of 19 days. It has been found that the stability of more concentrated solutions is primarily affected by the temperature, whereas the effects of the temperature and light are comparable with more dilute solutions. After 19 days, a solution of 0.5 mg/ml promethazine stored in darkness at a low temperature still contained 84.0% of the original amount of the enantiomers; this value was 89.6% for a solution with the ten times lower promethazine concentration. If the solutions were stored in darkness but at laboratory temperature, the respective values decreased to 38.1 and 62.6% and for the solutions exposed to light at laboratory temperature they decreased even more to 36.7 and 52.6% of the initial promethazine amount.

Synthesis and chromatographic properties of a novel chiral stationary phase derived from heptakis(6-azido-6-deoxy-2,3-di- O-phenylcarbamoylated)-β-cyclodextrin immobilized onto amino-functionalized silica gel via multiple urea linkages

A novel chiral stationary phase (PPHCDN7) was prepared by immobilization of heptakis(6-azido-6-deoxy-2,3-di- O-phenylcarbamoylated)-β-cyclodextrin (PPHCD) onto the surface of amino-functionalized silica gel via multiple urea linkages derived from an extended application of the Staudinger reaction. A wide range of structurally divergent racemic drugs and other compounds were successfully separated into their enantiomers under both normal and reversed-phase conditions. β-Adrenergic blockers and racemic tertiary, secondary and primary amines were readily separated using a mixture of methanol and aqueous triethylammonium acetate buffer. The optimal pH value for the separation falls in the range of 4.65 to 6.30. With atropine and isoproterenol, good enantioseparations with separation factors of α>5 were easily attainable.