Applicability of 0.05 mol kg potassium hydrogen phthalate as reference value pH standard in water–organic solvent mixtures

The applicability of 0.05 mol kg⁻¹ potassium hydrogen phthalate (KHPh(S), S=the solvent under study) as reference value pH standard (RVS) was studied in water–organic solvent mixtures. Phthalic acid in various aqueous organic solvent mixtures was titrated potentiometrically with Bu₄NOH (MeOH), using a pH-ISFET and a glass electrode as pH-sensors, and the buffer capacity of the HPh⁻ solution was roughly estimated from the difference between the first and second half-neutralization potentials. The influence of potassium ion to the buffer capacity was also studied, titrating KHPh and Et₄NHPh with CF₃SO₃H and Bu₄NOH (MeOH) and comparing the titration curves. The KHPh solution loses its buffer capacity if the solvent mixture has an aprotic property. For aqueous mixtures of protophilic aprotic solvents (e.g. DMSO and DMF), it is unsuitable as RVS if the water content is less than ca. 40 (v/v)%. The necessity of selecting subsidiary pH standard(s) is suggested for the solvent systems in which KHPh is inapplicable.     

Reference value standards for pH measurements in 10, 30, 50, and 70% (w/w) 2-propanol/water solvent mixtures at temperatures from 288.15 to 318.15 K

Reference value standards, pH (RVS), for 0.05 mol kg^?1 potassium hydrogenphthalate (KHPh) reference buffer solutions in 10, 30, 50 and 70% (w/w) 2-propanol/water solvent mixtures at temperatures from 288.15 to 318.15 K are determined from reversible e.m.f. measurements of the cell Pt¦H₂¦KHPh + KCl¦AgCl¦Ag¦Pt. The consistency of the present results is confirmed by multilinear regression analysis of the pH values obtained for each solution composition and temperature, allowing appropriate interpolation of pH (RVS) values within the range of the experiment. The ancillary values of the standard e.m.f. of the cell Pt¦H₂¦HCl¦AgCl¦Ag¦Pt are optimized through multilinear regression analysis of the available data in the literature, and the ancillary values of the first ionization constant of o-phthalic acid (H₂Ph; benzene-1,2-dicarboxylic acid) in these solvent mixtures are evaluated from reversible e.m.f. measurements of the cell Pt¦H₂¦H₂Ph + KHPh + KCl¦AgCl¦Ag¦Pt.     

Thermal properties of ternary systems: The calculation of the standard enthalpy of solution for compounds in binary mixtures

An equation free of fitting parameters is proposed for calculating the standard heats of solution for compounds in nonaqueous binary mixtures. The parameters of the equation are the standard heats of solution of a compound in the components of the mixed solvent. Nonlinear ΔH ⁰(x) trends are reconstructed for solutions of water in i-PrOH + MeOH and MeCN + MeOH, t-BuOH in MeCN + MeOH, squalane in CHCl₃ + CCl₄ and C₆H₆ + CHCl₃, and hexadecane in MeOH + i-Pr₂O and in mixtures of butyl acetate, ethyl acetate, and 1,4-dioxane with 1-octanol. The standard heats of solutions are calculated for water in alcohol + alcohol, alcohol + aprotic solvent, and aprotic solvent + aprotic solvent mixtures     

Reference value standards for pH measurements in 5, 15 and 30% (w/w) acetonitrile/water solvent mixtures at temperatures from 288.15 to 308.15 K

Reference value standards, pH (RVS), for 0.05 mol kg^?1 potassium hydrogenphthalate (KHPh) reference buffer solutions in 5, 15 and 30% (w/w) acetonitrile/water mixed solvents at temperatures from 288.15 to 308.15 K are determined from reversible e.m.f. measurements of the cell Pt¦H₂¦KHPh + KCl¦AgCl|Ag|Pt. Values of the first ionisation constant of o-phthalic acid (H₂Ph; benzene-1,2-dicarboxylic acid) in these mixed solvents are also determined from analogous measurements. The consistency of the results is analysed by multilinear regression of the quantity p(a_HγCl) as a function of both solution composition and temperature. The standard pH (RVS) values determined are given by the equation pH (RVS) = 4.0080 + 6.330x + 16.177x² ? 115.3x³ + 0.3089u ? 201.0ux² + 909ux³ + 13.04v, where x is the mole fraction of acetonitrile in the mixed solvent, u = z/(1 + z), v = [ln(1 + z) ? u], z = (T ? θ)/θ, and θ = 298.15 K.     

Standard pH values for potassium hydrogenphthalate reference buffer solutions in 10, 30 and 50% (w/w) 1,4-dioxane/water mixed solvents at temperatures from 288.15 to 318.15 K

Standard pH(S) values for 0.05 mol kg^?1 potassium hydrogenphthalate (KHpH) reference buffer solutions in 10, 30 and 50% (w/w) 1,4-dioxane/water solvent mixtures within the temperature range 288.15–318.15 K are determined from e.m.f. measurements of the cell without transference Pt|H₂|KHPh + KCl|AgCl|Ag|Pt. The consistency of the results is analysed by a recently described method of multilinear regression of the quantity p(a_Hγ_Cl) as a function of both solution composition and temperature. The standard pH(S) determined can be reproduced to within ±0.01 by the equation pH(S) - 4.004 + 3.309w + 0.408z + 1.037w³ - 14.95zw² + 27.1zw³, where w is the weight fraction of dioxane in the solvent mixture,z = (T –θ)/θ, and θ - 298.15 K. Values of the first ionization constant of phthalic acid (H₂Ph; benzene-1,2-dicarboxylic acid) in the above solvent mixtures are also determined from e.m.f. measurements of the cell without transference Pt|H₂|H₂Ph + KHPh + KCl|AgCl|Ag|Pt.     

Impact of organic solvents on the resolution of synthetic peptides by capillary electrophoresis

The effect of variations in the concentrations of different organic solvents, including acetonitrile, methanol, ethanol, propanol and isopropanol, with aqueous buffer electrolytes of defined composition and pH on the electroosmotic flow velocity, v(EOF), of uncoated fused silica capillaries and on the electrophoretic mobility, mu(e), of synthetic peptides in high-performance capillary electrophoresis (HPCE) has been systematically investigated. In these experiments, the volume fractions of the organic solvent in the aqueous buffer electrolyte were changed from psi = 0.0 to 0.80. The addition of these organic solvents to the aqueous buffer electrolyte reduced the electroosmotic flow (EOF) of the system, but to significantly different extents. For the protic solvents as the alkyl chain of the alcohol increased, at the same volume fraction the greater was the influence on the electroosmotic flow. However, for the aprotic solvent, acetonitrile, the EOF did not change substantially as the volume fraction was varied. The electrophoretic mobility of synthetic peptides under the different buffer electrolyte conditions showed similar trends, confirming that the content and type of the organic modifier can be rationally employed to subtly manipulate the separation selectivity of synthetic peptides. These results, therefore, provide fundamental insight into the experimental options that can be used to maximise resolution of synthetic peptides in HPCE with aqueous buffer-organic solvent mixtures as well as a basis to select optimal binary or ternary buffer electrolyte compositions for the analysis of peptides when hyphenated techniques, such as HPCE-electrospray ionisation mass spectrometry (ESI-MS), are contemplated for the analysis of peptide samples of low abundance as can often be experienced in proteomic investigations.     

The Standard Partial Molar Volumes of Ions in Solution. Part 2. The Volumes in Two Binary Solvent Mixtures with No Preferential Solvation

Standard partial molar volumes of ions were obtained from literature data on 1:1 electrolytes in mixtures of propylene carbonate (PC) with acetonitrile (MeCN) and of water (W) with methanol (MeOH) at 298.15 K. The hypothesis was examined that when the solvents in the mixtures do not differ too much in their polarity and/or hydrogen-bonding ability, only negligible preferential solvation occurs in the solvent shell around the ion where electrostriction takes place. Given the solvent-independent intrinsic volume of an ion, the electrostriction, calculated by the shell-by-shell method, permits the examination of this proposition. This hypothesis was indeed validated by the calculated standard partial molar ionic volumes in the dipolar aprotic mixtures and in the protic aqueous methanol.     

Investigation of folic acid stability in fortified instant noodles by use of capillary electrophoresis and reversed-phase high performance liquid chromatography

A single enzyme treatment with alpha-amylase, prior to the quantification of added folic acid (FA) in fortified instant fried Asian noodles with analysis performed by capillary zone electrophoresis (CZE) and reversed-phase high performance liquid chromatography (RP-HPLC) with UV detection, is described. The method was validated and optimized for capillary electrophoresis (CE) with separation achieved using a 8 mM phosphate-12 mM borate run buffer with 5% MeOH at pH 9.5. FA was well separated from matrix components with nicotinic acid (NA) employed as an internal standard. In a comparative study, separation of FA was performed using HPLC with a mobile phase consisting of 27% MeOH (v/v) in aqueous potassium phosphate buffer (3.5 mM KH(2)PO(4) and 3.2 mM K(2)HPO(4)), pH 8.5, and containing 5 mM tetrabutylammonium dihydrogen phosphate as an ion-pairing agent. For both methods, excellent results were obtained for various analytical parameters including linearity, accuracy and precision. The limit of detection was calculated to be 2.2 mg/L for CE without sample stacking and 0.10 mg/L with high performance liquid chromatography (HPLC). Sample extraction involved homogenization and enzymatic extraction with alpha-amylase. Results indicated that FA was stable during four main stages of instant fried noodle manufacturing (dough crumbs, cut sheets, steaming and frying).     

Solvent effect on the kinetics of oxidation of [Co(en)2amS]2+ by IO4−

The dependence of the rate constants for the oxidation of [Co(en)2amS]2+ (amS=–SCH2CH2NH2 2– by IO4– on solvent composition has been established for aqueous mixtures of MeOH, i-PrOH, t-BuOH, urea (U), ethylene carbonate (EC), acetonitrile (AN), dimethyl formamide (DMF) and acetone (Me2CO). Addition of aprotic cosolvents leads to marked decrease in the reaction rate in comparison to monohydroxylic alcohols. Alcohols inhibit the reaction rate in order MeOH < i-PrOH < t-BuOH. The change in solvation on going from the initial to the transition state are discussed in terms of the Gibbs transfer function, Gt0.     

极性非质子溶剂与甲醇或1,2-二氯乙烷的汽液平衡

使用双沸点仪测定了丙酮、乙酸乙酯、对二氧六环、乙腈或三乙胺与甲醇或1,2→二氯乙烷以及二者混合物等十一组二元体系在99.3 kPa下的汽液平衡数据(T,x,p), 计算了有关体系的过量吉布斯自由能。结果表明, 六种非质子溶剂与甲醇组成的二元系GE>0; 乙腈或三乙胺与1,2-二氯乙烷组成的二元系GE>0, 而丙酮、乙酸乙酯或对二氧六环与1,2-二氯乙烷的二元混合物GE<0。从同种分子间或不同种分子间的缔合作用对上述结果进行了讨论。本文还在固定极性非质子溶剂(第三组分)物质的量浓度的条件下, 测定了非质子溶剂+1,2-二氯乙烷+甲醇三元混合物的汽液平衡数据, 考察了非质子溶剂的加入对甲醇+1,2-二氯乙烷二元系GE的影响。     

Standards for pH measurements and first ionization constants ofo-phthalic acid. Methylcellosolve-water solvent mixtures from −10 to 45°C

From reversible emf measurements of the cell Pt/H₂/KHPh+KCl/AgCl/Ag/Pt, reference value pH-metric standards, pH _RVS , for 0.05 mol-kg^–1 potassium hydrogenphthalate (KHPh) reference buffer solutions in 20, 50, and 80 wt % methylcellosolve (2-methoxyethanol, CH₃O-CH₂CH₂OH)-water solvent mixtures from –10 to +45°C, have been determined in compliance with the criteria endorsed by IUPAC. The corresponding values of the first ionization constant ofo-phthalic acid (H₂Ph, benzene-1,2-dicarboxylic acid), which is a key quantity for the acquisition of pH _RVS standards, have been determined from emf measurements of the reversible cell Pt/H₂/H₂Ph+KHPh+KCl/AgCl/Ag/Pt.     

Determination of low molecular weight organic acids in soil,plants, and water by capillary zone electrophoresis

Determination of low molecular weight organic acids in soils and plants by capillary zone electrophoresis was accomplished using a phthalate buffer and indirect UV detection mode. The influence of some crucial parameters, such as pH, buffer concentration and surfactant were investigated. A good separation of seven organic acids was achieved within 5 min using an electrolyte containing 15 mmol L(-1) potassium hydrogen phthalate, 0.5 mmol L(-1) myristyltrimethylammonium bromide (MTAB), and 5% methanol (MeOH) (v/v) at pH 5.60, separation voltage -20 kV, and temperature 25 degrees C. The relative standard deviation (n=5) of the method was found to be in range 0.18-0.56% for migration time and 3.2-4.8% for peak area. The limit of detection ranged between 0.5 micro mol L(-1) to 6 micro mol L(-1) at a signal-to-noise ratio of 3. The recovery of standard organic acids added to real samples ranged from 87 to 119%. This method was simple, rapid and reproducible, and could be applied to the simultaneous determination of organic acids in environmental samples.     

Determination of the pH of binary mobile phases for reversed-phase liquid chromatography

The measurement of pH in chromatographic mobile phases has been a constant subject of discussion during many years. The pH of the mobile phase is an important parameter that determines the chromatographic retention of many analytes with acid-base properties. In many instances a proper pH measurement is needed to assure the accuracy of retention-pH relationships or the reproducibility of chromatographic procedures. Three different methods are common in pH measurement of mobile phases: measurement of pH in the aqueous buffer before addition of the organic modifier, measurement of pH in the mobile phase prepared by mixing aqueous buffer and organic modifier after pH calibration with standard solutions prepared in the same mobile phase solvent, and measurement of pH in the mobile phase prepared by mixing aqueous buffer and organic modifier after pH calibration with aqueous standard solutions. This review discusses the different pH measurement and calibration procedures in terms of the theoretical and operational definitions of the different pH scales that can be applied to water-organic solvent mixtures. The advantages and disadvantages of each procedure are also presented through chromatographic examples. Finally, practical recommendations to select the most appropriate pH measurement procedure for particular chromatographic problems are given.     

Counter-current chromatographic separation of nucleic acid constituents with a hydrophilic solvent system

Nucleic acid constituents such as nucleobases, nucleosides and nucleotides were separated by counter-current chromatography using type J coil planet centrifuge. The separation was performed with a hydrophilic solvent system composed of 1-propanol/800 mM potassium phosphate buffer (pH 7.4) (1:1, v/v) by eluting the lower aqueous phase at a flow-rate of 0.5 ml/min. Eight selected nucleic acid constituents (4.0 mg, 0.5 mg of each), uridine monophosphate (UMP), adenosine monophosphate (AMP), deoxyadenosine monophosphate (dAMP), uridine, urasile, deoxy uridine, adenosine and adenine were well resolved within 160 min.     

Column-switching high-performance liquid chromatographic determination of clarithromycin in human plasma with electrochemical detection

A column-switching HPLC method was described for the direct analysis of clarithromycin in human plasma using electrochemical detector without sample pre-purification step. Plasma samples were diluted with washing solvent, i.e. acetonirile-methanol-0.05 M potassium phosphate buffer (pH 7.0) (5:2:93, v/v) and then, injected to the precolumn. After plasma proteins had flowed out from the precolumn, clarithromycin and internal standard (roxithromycin) were eluted to a Luna 2 C(18) column and separated with acetonitrile-methanol-0.05 M potassium phosphate buffer (pH 7.0) (41:6:53, v/v). Electrochemical oxidation of clarithromycin occurred at 0.87 V vs. Ag/AgCl reference electrode with glassy carbon electrode. The calibration curve was linear in the concentration range 0.1-4 mug ml(-1) with correlation coefficient of 0.998. This method showed excellent precision (RSD 3.8% at 0.1 mug ml(-1)) and accuracy (+/-2%) with the total analysis time per sample of 30 min. The present method was successfully applied to the pharmacokinetic study of clarithromycin in volunteers receiving a single oral administration of clarithromycin.     

Simply enhancing throughput of free‐flow electrophoresis via organic‐aqueous environment for purification of weak polarity solute of phenazine‐1‐carboxylic acid in fermentation of Pseudomonas sp. M18

Herein, a simple novel free‐flow electrophoresis (FFE) method was developed via introduction of organic solvent into the electrolyte system, increasing the solute solubility and throughput of the sample. As a proof of concept, phenazine‐1‐carboxylic acid (PCA) from Pseudomonas sp. M18 was selected as a model solute for the demonstration on feasibility of novel FFE method on account of its faint solubility in aqueous circumstance. In the developed method, the organic solvent was added into not only the sample buffer to improve the solubility of the solute, but also the background buffer to construct a uniform aqueous‐organic circumstance. These factors of organic solvent percentage and types as well as pH value of background buffer were investigated for the purification of PCA in the FFE device via CE. The experiments revealed that the percentage and the types of organic solvent exerted major influence on the purification of PCA. Under the optimized conditions (30 mM phosphate buffer in 60:40 (v/v) water‐methanol at an apparent pH 7.0, 3.26 mL/min background flux, 10‐min residence time of injected sample, and 400 V), PCA could be continuously purified from its impurities. The flux of sample injection was 10.05 μL/min, and the recovery was up to 93.7%. An 11.9‐fold improvement of throughput was found with a carrier buffer containing 40% (v/v) methanol, compared with the pure aqueous phase. The developed procedure is of evident significance for the purification of weak polarity solute via FFE.     

Reductive heterocyclizations via indium-iodine-promoted conversion of 2-nitroaryl imines or 2-nitroarenes to 2,3-diaryl-substituted indazoles

While N-(2-nitrobenzylidene)anilines produced mixtures of 2,1-benzisoxazoles and 3-anilino-2-aryl-2H-indazoles in the presence of indium and iodine in MeOH, N-(2-nitrobenzylidene)anilines were transformed into 3-anilino-2-aryl-2H-indazoles as the predominant major product through the change of the solvent from protic MeOH to aprotic THF. In an indium-mediated one-pot reductive reaction, 2-benzaldehydes and anilines in THF were also successfully transformed into the corresponding indazoles.     

Membrane sampling with microdialysis coupled to HPLC/UV for on-line simultaneous determination of melamine and cyanuric acid in non-dairy coffee creamer

An on-line microdialysis/high-performance liquid chromatography method was developed for the simultaneous determination of melamine and cyanuric acid in non-dairy coffee creamer. To collect these analytes from aqueous samples, the microdialysis system featured a microdialysis probe incorporating a polyarylethersulfone membrane and employed 0.05 M HCl in 0.1% (v/v) MeOH as the perfusate, with optimal efficiency obtained at a flow rate of 1 μL min(-1). The chromatographic conditions were optimized when using a reverse-phase phenyl column and a mobile phase of phosphate buffer solution in 10% (v/v) MeOH, buffered at pH 3.0. Good linearity relationship (r(2) > 0.9987), intra- and inter-day precisions (RSDs < 6.6%), recoveries (96.9 - 105.0%), and limits of detection (melamine, 3 ppb; cyanuric acid, 150 ppb) were observed for the two analytes. This method has been successfully applied to simultaneous determination of melamine and cyanuric acid in commercial creamers with the recoveries in the range of 97.5 to 102.6%.     

Hydroxy- and alkoxy-bridged dinuclear uranyl-Schiff base complexes: hydrolysis, transamination and extraction studies

The reaction of uranyl nitrate with 1,3-bis(salicylideneamino)-2-propanol (H(3)L1) and 1,3-bis(3,5-di-tert-butylsalicylideneamino)-2-propanol (H(3)L2) in the presence of triethylamine (Et(3)N) yielded hydroxy- and alkoxy-bridged dinuclear complexes; [(UO(2))(2)(L1)(OH)(MeOH)(2)].(MeOH)(2) (.(MeOH)(2)) and [(UO(2))(2)(L2)(OH)(MeOH)(2)].(MeOH)(2) (.(MeOH)(2)). The crystal structures of .(DMF)(2) and .(DMF)(2) exhibit an unsymmetrical central U(2)O(2) core involving bridging alkoxy- and hydroxy-oxygen atoms. The geometry around the uranium center in .(DMF)(2) and .(DMF)(2) is that of a distorted pentagonal bipyramid with the solvent molecule occupying the fifth coordination site. The flexible nature of the ligand backbone is more pronounced in .(DMF)(2) compared to .(DMF)(2), yielding two molecules per unit cell in different conformations. Under similar reaction conditions, using ethylenediamine as a base, the respective Salen-based uranyl compounds, [UO(2)(Salen)(MeOH)] () and [UO(2)(Bu(t)(2)-Salen)(MeOH)] () are obtained due to transamination of the ligand backbone. Complexes .(MeOH)(2) and .(MeOH)(2) when reacted with an excess of ethylenediamine failed to yield the respective Salen-based complexes, and , respectively. The new compounds have been characterized using solution (NMR and UV-Vis) and solid-state (IR, X-ray crystallography) techniques. Hydrolysis of .(MeOH)(2) and .(MeOH)(2) in the pH range 1-14 was studied using UV-Vis spectroscopy and compared with the hydrolysis of and [UO(2)(Salophen)(MeOH)] (). A two-phase extraction study suggests quantitative removal of uranyl ions from the aqueous phase at higher pH conditions.     

Solvent-derived metal oxides in electrospray mass spectrometry of metal salt solutions

The electrospray mass spectra of MX₂and MX₃salts (where X is typically halide or nitrate) in protic and aprotic solvents, and solvent mixtures were examined. Comparisons of species responses with equilibrium aqueous solution concentrations were made. For MX₂salts, a good correlation between MOH⁺ response and MOH⁺ solution concentration was observed under conditions where the collision energy was nominally zero. Cu(II) is easily reduced in acetonitrile to Cu(I); Cu(I) was the principal species observed in the electrospray mass spectrum of Cu(II) in acetonitrile whereas Cu(II) was the principal species observed in that of Cu(II) in dimethyl sulphoxide. Gas-phase reactions between solvated clusters produced by electrospray and a second solvent vapour were examined. M³⁺clusters were the principal ions observed when an aqueous solution of MX₃was sprayed in the presence of aprotic solvent vapours.