Retention of ionizable compounds on HPLC. 6. pH measurements with the glass electrode in methanol-water mixtures

The relationship, delta values, between the two rigorous pH scales, S(S)pH (pH measured in a methanol-water mixture and referred to the same mixture as standard state) and S(W)pH (pH measured in a methanol-water mixture but referred to water as standard state), in several methanol-water mixtures was determined (delta = S(W)pH-S(S)pH). Delta values were measured using a combined glass electrode and a wide set of buffer solutions. The results are consistent with those obtained with the hydrogen electrode. This confirms the aptness of the glass electrode to achieve rigorous pH measurements in methanol-water mixtures. An equation that relates delta and composition of methanol-water mixtures, and allows delta computation at any composition by interpolation, is proposed. Therefore, S(S)pH can be achieved from the experimental S(W)pH value and delta at any mobile phase composition. S(S)pH (or S(W)pH) values are related to the chromatographic retention of ionizable compounds through their thermodynamic acid-base constants in the methanol-water mixture used as mobile phase. These relationships were tested for the retention variation of several acids and bases with the pH of the mobile phase. Therefore, the optimization of the mobile phase acidity for any analyte can be easily reached avoiding the disturbances observed when W(W)pH is used.     

Hydrogen peroxide yields in methanol-water mixtures gamma-irradiated at room temperature and 77 K

Hydrogen peroxide yields in air-saturated methanol-water mixtures with methanol: water volume ratios varying from 199 to 1000, produced by⁶⁰Co gamma-irradiation boith at room temperature and at 77 K have been determined. At room temperature, G(H₂O₂) values are almost constant over the entire range of methanol: water ratios. However, for irradiations at 77 K, G(H₂O₂) values increase with increasing methanol concentration up to the volume ratio of 982.     

Effect of solvent composition on the association behavior of pectin in methanol-water mixtures

Turbidity, small-angle neutron scattering (SANS), and dynamic light scattering measurements have been carried out on semidilute systems of pectin in methanol-water media of various composition ratios. Structural and dynamical properties of pectin dissolved in water-methanol mixtures (case I) are compared with the corresponding conditions when pectin was dissolved in water before the prescribed amounts of methanol were added (case II). The turbidity rises as the percentage of methanol in the mixture increases and this trend is much stronger when the first procedure is used to dissolve pectin. The wavelength dependence of the turbidity indicates that larger association complexes are formed for the samples prepared according to case I. These findings indicate that at a given methanol-water composition, the preparation procedure in case I gives rise to poorer thermodynamic conditions of the system. Local structures probed by SANS experiments do not reveal any significant differences between the systems prepared by the two different procedures. The dynamic light scattering (DLS) results show that increasing methanol concentration in the mixture promotes the formation of association complexes and the disengagement relaxation time of individual chains or clusters is longer at higher percentage of methanol for case II. This can be attributed to stronger entanglement effects in case II. The features from DLS can be rationalized in the framework of the coupling model for constrained and interconnecting systems.     

Preparative-scale isoelectric trapping separations in methanol-water mixtures

The typically low aqueous solubilities of small, hydrophobic organic ampholytic molecules limit the production rates that can be achieved in their isoelectric trapping (IET) separations and call for the use of hydro-organic mixtures as solvents. The compatibility of methanol-water mixtures and poly(ethylene terephthalate) substrate-supported isoelectric polyacrylamide hydrogels, developed for binary IET separations in a Gradiflow BF200IET unit, was investigated. The isoelectric polyacrylamide-based hydrogels retained their functional and mechanical integrities when the methanol concentration in the hydro-organic solvent mixture was kept at or below 25% (v/v). The utility of the hydro-organic media was demonstrated in the purification of a hydrophobic ampholytic compound, technical grade 4-hydroxy-3-(morpholinomethyl) benzoic acid. Production rates as high as 7 mg/h were achieved using small, 15 cm2 active surface area isoelectric membranes.     

Acid-base equilibria in ethylene glycol--I: definition of pH and determination of pk-values of acid-base indicators

A pH-scale in ethylene glycol is defined, based on potentiometric measurements for a cell without liquid junction: glass electrode || 0.1M (HCl + KCl), ethylene glycol | AgCl:Ag; and a potentiometric-spectrophotometric method for the determination of pK-value of acid-base indicators in ethylene glycol on the basis of this pH scale is proposed, in which the pH scale is established and the pK-value determined in a single titration at constant ionic strength. The following pK-values of indicators are reported: Cresol Red-3.00, Thymol Blue-3.39, Bromophenol Blue-6.49, Bromocresol Green-7.38, Bromocresol Purple- 9.00 and Cresol Red (2nd change)-11.07. A linear relationship was found between the pK-values of four sulphonphthalein indicators in ethylene glycol and in water according to the following equation (pK(HI)(c))(EG) = 2.3 + 1.1 (pK(HI)(c))(W). An attempt is made to explain this dependence, based on the idea of solute-solvent interaction of species of the same charge type.     

Acid-base equilibria in ethylene glycol--II: autoprotolysis constants and acid-base properties of ethylene glycol and its mixtures

The acid-base behaviour of ethylene glycol and its mixtures with water (1, 5 and 10%), methanol, ethanol, isopropanol, nitromethane (each 10%) and ethyl methyl ketone-chloroform (5 + 5%) have been investigated by potentiometric titration, in a cell without liquid junction, equipped with a glass and a silver-silver chloride electrode. The autoprotolysis as well the protolysis constants of phthalic acid were determined for each mixture. The added solvents improve the properties of ethylene glycol, decreasing the viscosity without changing the acid-base behaviour of the ethylene glycol itself, which is favourable for the titration of weak bases. Water increases the basic, and nitromethane the acidic, properties of the mixture. Small quantities of water (ca. 1%) do not impair the titration conditions.     

Displacement processes on hydrophilic/hydrophobic surfaces in methanol-water mixtures

The adsorption from methanol/water and the enthalpy of displacement of methanol by water were studied on K-60 silica gel, on graphitized PRINTEX-80, and onn-alkylammonium vermiculites. The adsorption between the hydrophobized silicate layers was followed by x-ray measurements. The excess isotherms of alkylammonium vermiculites in methanol/water exhibit two maxima corresponding to the two steps of the individual isotherm. The equilibrium constant of the exchange of water by methanol is calculated. For calculating the adsorption capacities and molar adsorption potentials, a new equation is proposed which combines the adsorption excess quantities with free energy and enthalpy functions.     

Effect of anionic and nonionic water-in-oil microemulsions on acid-base equilibria of hydrophilic indicators

ApparentpK _a values (hereafter writtenpK) for the hydrophilic, negatively-charged indicators 2-hydroxy-5-bromo sodium benzenesulphonate (HBBS), 2-hydroxy-5-nitro sodium benzenesulphonate (HNBS), and 2-(5,7-dinitro-8-hydroxy) sodium naphthalenesulphonate (napthol yellow, NY) in the presence of water-in-heptane microemulsions of bis(2-ethylhexyl) sodium sulphosuccinate (AOT, anionic) and polyoxyethylene (4) dodecyl ether (Brij-30, nonionic) were determined spectrophotometrically. The variables studied were the type of buffer (borate, imidazole, nicotinamide, phosphate, and piperidine) and the ratios [water]/[surfactant] and [buffer]/[surfactant]. Partition experiments between water and heptane and UV-VIS spectra of the indicators in aqueous buffer in the presence of anionic and nonionic aqueous and reversed micelles showed that these indicators are confined within the second hydration shell of the microemulsion, i.e., are not adsorbed at the water-in-oil (W/O) interface. Buffer-independentpK values were obtained after the initial pHs of the solubilized buffer solutions were corrected for the ion exchange with counter-ion of the surfactant (AOT) and for the lower polarity of the micelle-solubilized water (Brij-30 and AOT). The micelle-inducedpK shifts were in the range 1.66–2.08 units for HBBS/AOT, 1.17–2.14 units for HNBS/AOT, 0.73–1.31 units for NY/AOT, 1.06–1.40 units for HNBS/Brij-30, and 0.12–0.33 units for NY/Brij-30. The observed pK shifts are explained as due to the effect of two factors, i.e., the lower polarity of the micelle-solubilized water (relative to bulk water, AOT, and Brij-30) and electrostatic perturbation of the equilibria by the negatively charged surfactant. The smaller micellar effect on thepK of NY was attributed to the extensive electron delocalization in its conjugate base.     

Microscopic acid-base equilibria of arginine

Three and two protonation macroconstants as well as twelve and four microconstants have been determined for arginine and citrulline, respectively. These data include microscopic basicity values of the arginine guanidino group, for which no reliable constants have been reported earlier. The determinations were carried out by a combination of potentiometric and deductive methods. The guanidino basicity proved to be extremely high, and can be characterized by microconstants between 14.7 and 15.0 log k units. The interactivity parameters could also be determined from the microscopic protonation constants. These values for the guanidino—amino and the guanidino—carboxylate interactions are 0.2 and 0.1 log k units, respectively. The pH-dependent distribution of all eight arginine microspecies is made visual by a microspeciation diagram. The microspeciation diagram is the most useful tool to predict bidentate binding isomerism in metal complex formation and site-specific bioligand—bioligand associations.     

Isothermal measurements of vapor-liquid equilibria for three n-alkane-chloroalkane mixtures

Results of isothermal vapor-liquid equilibrium (VLE) measurements for 1-chlorobutane with n-hexane and n-heptane at three temperatures and for 1,2-dichloroethane with n-heptane at two temperatures are reported.New constants of the Antoine vapor pressure equation for 1,2-dichloroethane are presented. The consistency of the new vapor-pressure data with published experimental data of heat of vaporization is checked.The VLE data are used for the determination of group interaction parameters of UNIFAC and of the quasichemical group surface interaction model (QUAGSIM).     

Effect of the solvent on the equilibria of acid-base indicators in aprotic and amphiprotic solvents

The dissociation constants of a series of indicators inN,N-dimethylformamide medium have been determined, and compared with those for the indicators in other aprotic solvents (dimethylsulphoxide and acetonitrile) and in some amphiprotic solvents (water, methanol, ethanol, propan-2-ol and tertbutanol). The effect of the solvent characteristics on the pK values have been evaluated, and the pK + p _ws ^t (H⁺) values shown to be linearly correlated with the Dimroth and ReichardtE _T ^N and acceptor number solvatochromic parameters.     

Spectrofluorometric characteristics and acid-base equilibria of serpentine

The corrected excitation and fluorescence emission spectra of serpentine hydrogen tartrate in aqueous solutions of varying acidity and in several solvents are reported, and the quantum yields and fluorescence sensitivity have been calculated. The ground state pK_a have been determined from the acidity dependence of the absorption spectra and the excited state pK_a have been determined using the Förster-Weller equation.     

Effect of acid-base equilibria on the donnan potential of layer-by-layer redox polyelectrolyte multilayers

In polymer films carrying an excess of fixed charge the electrostatic penalty to bring ions of same charge from the bathing electrolyte into the film sets a membrane potential (Donnan Potential) across the film-electrolyte interface. This potential is responsible for the ionic permselectivity observed in polyelectrolyte membranes. We have used electrochemical measurements to probe the dependence of the Donnan potential on the acid-base equilibrium in layer-by-layer self-assembled polyelectrolyte multilayers. The voltammperogram peak position of the Os(III)/Os(II) couple in self-assembled polyelectrolyte multilayers comprised of poly(allylamine) derivatized with Os(bpy)(2)PyCl+ and poly(vinylsulfonate) was recorded in solutions of increasing ionic strength for different assembly and testing solution pH. Protonation-deprotonation of the weak redox poly(allylamine) changes the fixed charge population in the as prepared (intrinsic) self-assembled redox polyelectrolyte multilayers. For films assembled in solutions of pH higher than the test solution pH, the Donnan plots (E(app) vs log C) exhibit a negative slope (anionic exchanger) while for films assembled at lower pH than that of the test solution positive slopes (cationic exchanger) are apparent. The ion exchange mechanism has been supported by complementary electrochemical quartz crystal microbalance. X-ray photoelectron spectroscopy and infrared reflection-absorption spectroscopy experiments demonstrated that the as prepared films have a memory effect on their protonation state during assembly, which leads to the observed dependence of the Donnan potential on the adsorption pH.     

Polarography of 4-nitrodiphenylamine (DPA) in methanol-water mixtures

The polarographic reduction of 4-nitro DPA in methanol-water mixtures of varying composition (30–80% v/v) in the pH range 1–12 was investigated. A single wave was observed in the reduction in these media at all pH values and solvent compositions. The number of electrons involved in the reduction was found to be six in all solutions indicating that 4-amino DPA is the final product. A mechanism consistent with these observations which involves the formation of 4-hydroxylaminodiphenylamine as an intermediate has been proposed.     

Graphical representation of polynuclear acid-base and complex equilibria

A calculation procedure for diagrams of log C(M) = f(pL) at various values of the ratio of analytical concentrations, C(L)/C(M), in polynuclear binary systems is described. The diagrams are useful in the resolution of analytical problems of practical interest involving acid-base equilibria (such as the preparation of buffer solutions) and complex equilibria. The presence of a solid phase is also considered.     

Effect of temperature on the acid-base properties of the alumina surface: microcalorimetry and acid-base titration experiments

Sorption reactions on natural or synthetic materials that can attenuate the migration of pollutants in the geosphere could be affected by temperature variations. Nevertheless, most of the theoretical models describing sorption reactions are at 25 degrees C. To check these models at different temperatures, experimental data such as the enthalpies of sorption are thus required. Highly sensitive microcalorimeters can now be used to determine the heat effects accompanying the sorption of radionuclides on oxide-water interfaces, but enthalpies of sorption cannot be extracted from microcalorimetric data without a clear knowledge of the thermodynamics of protonation and deprotonation of the oxide surface. However, the values reported in the literature show large discrepancies and one must conclude that, amazingly, this fundamental problem of proton binding is not yet resolved. We have thus undertaken to measure by titration microcalorimetry the heat effects accompanying proton exchange at the alumina-water interface at 25 degrees C. Based on (i) the surface sites speciation provided by a surface complexation model (built from acid-base titrations at 25 degrees C) and (ii) results of the microcalorimetric experiments, calculations have been made to extract the enthalpic variations associated respectively to first and second deprotonation of the alumina surface. Values obtained are deltaH1 = 80+/-10 kJ mol(-1) and deltaH2 = 5+/-3 kJ mol(-1). In a second step, these enthalpy values were used to calculate the alumina surface acidity constants at 50 degrees C via the van't Hoff equation. Then a theoretical titration curve at 50 degrees C was calculated and compared to the experimental alumina surface titration curve. Good agreement between the predicted acid-base titration curve and the experimental one was observed.     

pH and pK determinations by high-resolution solid-state 13C NMR: acid-base and tautomeric equilibria of lyophilized L-histidine

Acid-base properties of lyophilized powders of L-histidine have been systematically investigated using parent solutions at pH varying from 1.8 to 10. For the first time, high-resolution solid-state 13C NMR was shown to allow separate observation of all three acid-base pairs in the successive deprotonations of the carboxylic end, the imidazolium cation, and the terminal ammonio group of histidine. 1H CRAMPS NMR spectra directly visualize the absence of the N3-H(pi) tautomer in neutral and anionic species. Solid-state titration shifts are enlarged by approximately 1-4 ppm with respect to those measured in solution, permitting unambiguous observation of conjugate acid-base pairs. Calculated pK's from solid-state acid-to-base ratios r are found equal to those classically measured in solution at 0 degrees C with a similar ionic strength of 0.1 mol x dm(-3). This proves that natural-abundance 13C solid-state determinations of r can be used to measure pK's in parent solutions without recourse to full titration curves and subsequent curve-fitting procedures. Such an approach also leads to noninvasive characterizations of the acidity of lyophilized powders, i.e., to the prediction of in situ pH of products obtained after rehydration and solubilization of powders. These results show the possibility of measuring the pK of nonvolatile acidic substrates dissolved in any sublimable solvent through lyophilization of the investigated solutions; this leads the way to pH and pK determinations when electrochemical or spectrophotometric measurements are impossible or ambiguous, e.g., for concentrated solutions, polyacids, or mixtures of acidic solutes, and possibly to the establishment of pK scales in nonaqueous solvents and in melts.