Determination of pH dependence

Kelly SD, Kemner KM, Fein JB, Fowle DA, Boyanov MI, Bunker BA, Yee N (2002) X-ray-absorption fine-structure determination of pH-dependent U-bacterial cell wall interactions. Geochim Cosmochim Acta 66 (in press)... 

Many biological and synthetic transport systems have a hydrophobic external and charged internal surface. These ubiquitous internal acids or bases frequently account for pH sensitivity. For this reason, it is advisable to record pH profiles as early as possible during the characterization of the novel molecule and continue in-depth studies at opti-ntized pH. > The best technique to determine pH profiles is the ANTS/DPX assay, since both probe and quencher are not very pH-sensitive (Section 3.1.2). However, most assays are applicable as long as systematic corrections are applied. Determination of pH dependences in planar bilayer conductance experiments is straightforward. 

The best known of the M/Mfiy electrodes that are used for the determination of pH are the Sb/SbjOj electrodes, but metals such as bismuth and arsenic filmed with their respective oxide act in a similar manner. Copper in alkaline solutions appears to behave as a pH-dependent Cu/CujO,... 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

Schaper, K.-J., Simultaneous determination of electronic and lipophilic properties [pKa, P(ion), P(neutral)] for acids and bases by nonlinear regression analysis of pH-dependent partittion measurements, J. Chem. Res. (S) 357 (1979). 

The occurrence of excited-state proton transfer during the lifetime of the excited state depends on the relative rates of de-excitation and proton transfer. The general equations will be presented first, but only for the most extensively studied case where the excited-state process is proton ejection (pK < pK) the proton donor is thus an acid, AH, and the proton acceptor is a water molecule. Methods for the determination of pK are then described and finally, the various cases of pH dependence of the absorption and fluorescence spectra are examined. 

Photocurrent onset, pH = 10.1, and quasi-Fermi level, 633 nFf - 0.85 V vs SCE, were determined from pH-dependent photocurrent measurements in the presence of methylviologen and glucose... 

Accurate measurement of pH is critically dependent on good analytical procedures, a fact that may not be appreciated by laboratory personnel [1,2]. The assumption is often made that if the electrode has been calibrated, there will be no variability in pH between laboratories. The pH measurement can erroneously be seen as merely dipping the electrode into the analyte and recording the value. In 1985, Davidson and Gardner [3] drew the following conclusion from their study Interlaboratory Comparisons of the Determination of pH in Poorly Buffered Fresh Waters ... 

Figure 15 Modified Pourbaix diagram for Ti02 illustrating the origins of pH-dependent band energetics and the pH-independent back-ET kinetics for covalently anchored dye species. The open circles are experimentally determined values of Ecb (combined electrochemical quartz microbalance and reflectance measurements). The driving force for the overall back reaction [coupled electron and proton transfer cf. Eqs. (10) and (11) for analogous reactions at Sn02] is pH dependent, but the driving force for the back ET in isolation [cf. Eq. (10)] is pH independent. (Data from Ref. 78.)...

For TiC>2, this difference is more than the unit of pK. That is because in the case of metal oxides, that contains some anionic or cationic contaminants, as was previously mentioned, beside adsorption reactions, that form the electric charge at the interface, some ion or isotope exchange processes take place. The contribution of these processes is visible in Fig. 9 that presents the adsorption as a function of pH dependence, as an increase of cation adsorption below pHpzc and/or anion above pHpzc. This method may be applied to the determination of complexion constants only of the very pure metal oxides. 

In colloidal solutions the measurement of the pH is always delicate, since it depends on the average distance between the oxide particles and the pH electrode. This explains why the latter is a function of the stirring rate of the solution. The best measure is the value obtained without stirring, the solid being far from the electrode and deposited at the bottom of the flask (a centrifugation is sometimes necessary). This measure can be considered as the pH of the bulk solution. This latter value, combined with the characteristics of the solution and the IEP of the oxide, allows calculation of the pH profile from the theory developed by Davis et al. [19]. Devices based on fiber-optic sensors [91] are being developed which might help resolve the difficult problem of the determination of pH of oxides in suspension. 

To determine the pH dependence of take the logarithm of both sides of Eq. 5.14. We... 

For the analysis of esters by RP-HPLC, it is very hard to avoid water because it will be present in the sample extraction solvent and even in the mobile phase. However, the kinetics of ester hydrolysis in solution can be studied by stopped flow injection experiments to determine compatibility with mobile phase. The same experiment must be repeated with different pH (from pH 1.2 to pH 6.8), since it is known that ester hydrolysis is pH-dependent. An example of pH-dependent ester hydrolysis is methylphenidate HCl. Methylphenidate HCl has an ester functional group, and its hydrolysis is pH-dependent. Refer to Table A15-1 for stability of methylphenidate HCl at different pHs [5]. The hydrolysis product of methylphenidate HCl is the free acid (refer to Figure A15-3) [5]. 

Ward, K.M. and Balaban, R.S. (2000) Determination of pH using water protons and chemical exchange dependent saturation transfer (CEST). Magnetic Resonance in Medicine, 44, 799—802. 

Hunkapiller et al. (31, 32) approached the problem in a radically different way. They determined the pH dependence of the parameters of the nuclear magnetic resonance spectrum for a bacterial protease which had been specifically enriched with at C-2 of its single histidine residue. The enzyme, a-lytic protease (33), is listed in Table I. The reasons for choosing it for this investigation are as follows. 

There are several studies that have been successful in determining the dissolution rate at conditions near seawater saturation. Acker et al. (1987) was able to employ very precise determinations of pH to measure the rate of dissolution of a single pteropod shell at different pressures from 15 atm to 300 atm. Because his measurements were at different pressures and is a function of pressure, he was able to determine whether the rate constant is indeed a function of K p. He found that Equation (9) fit his data better than (10), suggesting that the constant is not pressure dependent and the former is a more accurate universal rate law. An exponent oin= 1.9 was obtained for this surface-controlled dissolution reaction and a partial molal volume. Ay, of —39 cm mol (very close to the mean of the values determined in laboratory experiments for calcite) best fit the data. 

In this way it was possible to determine the pH-dependence of individual rate constants over a range of three pH-units. It was found that k[ is proportional to hydroxide ion concentration (Aii = A i[OH ]), that k i is practically pH-independent (ilj = and that the constant 2 depends on pH in the shape of a part of a dissociation curve, indicating that a rapidly established acid-base equilibrium precedes the rate-determining step. These differences in pH-dependence explain why at [0H ] < 0 3m, k[ (the value of which decreases Avith decreasing pH) can be neglected compared with (pH-independent) k i and why at [OH ] > 0 3m, is small compared with the rapidly increasing k. The rate of nucleophilic attack (with constant k[) decreases with increasing ethanol concentration. 

Table 1 Thickness of PVFA-co-PVAm layers adsorbed on to silicon wafers, determined by AFM, and IEP, determined by pH-dependent electrokinetic measurements in aqueous KC1 solution (10—3 mol L-1) using PVFA-co-PVAm/silica hybrid particles...

Fig. 5 (a) Dependence of Ex (30) values of several PVFA-co-PVAm/silica hybrid particles on the amino content in the PVFA-co-PVAm copolymer ET (30) values were determined from solvatochromic experiments using the adsorption of dye 2 from 1,2-dichlor-oethane (open circles), and toluene (filled circles) (b) Ej (30) values compared with the IEP values of several PVFA-co-PVAm/silica hybrid particles. ET (30) values were determined from solvatochromic experiments using the adsorption of dye 2 from 1,2-dichlor-oethane (open circles), and toluene (filled circles). IEP data were determined by means of pH-dependent electrokinetic experiments in aqueous 10-3 mol L-1 KCl solution... 

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