Monitoring methods Proton concentration

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

However, the danger of this approach is that the traps themselves may cause /3-proton elimination which lead to undetectably low concentration of active (or temporarily deactivated) chain ends. Malonate traps and H NMR analysis of the resulting terminal ester groups have been used successfully to monitor the growing chains in vinyl ether polymerizations [209], This technique measures the sum of dormant species and growing carbenium ions. Another method first used to monitor the active species in ring-opening polymerizations traps the active and dormant chain ends with phosphines [Eq. (135)]. 

Protonic diffusion in ice has been investigated by a spectroscopic method. This method is based on the isotope effect on molecular vibrations. The mass difference between hydrogen and deuteron results in a frequency difference by a factor of V2 for the stretch mode. The peak positions are well separated in the spectra and hence their heights are converted to the H(D) concentrations with good accuracy. The diffusion process is monitored by measuring the reflection spectra of an H2O/ D2O ice bilayer, for which the equation of diffusion is described in analytical form. The H/D mutual diffusion coefficient measured at 400 K shows a monotonic decrease by two orders of magnitude as the pressure increases from 8 to 63 GPa. 

Lee et al. (67) reported in 1993 that at high pH, complexes termed M-DNA form between the nucleobase pairs within calf thymus (CT), bacterial or synthetic DNA, and the divalent metal ions, Zn, Ni, or Co " ". The chemical nature of M-DNA has been investigated using methods that probe changes in DNA properties, but do not directly probe the coordination of the metal ions. Titration of a d(T-G)i5 d(C-A)i5 duplex with Zn " " at pH 8.5 monitored by NMR spectroscopy showed a decrease in the intensity of the imino peaks with the increasing concentration of Zn " " and the complete disappearance of these peaks when the Zn " " concentration became almost equal to that of the DNA base pairs (67). The transformation induced by Ni " " of B- to M-DNA for CT DNA in solutions with 1.1 mM base pair concentrations led to the release of 1.1 equiv of protons per base pair (68). Based on these results, Lee and co-workers... 

Most ligand pAa values can be determined via a potentiometric titration, where the pH is monitored after sequential additions of either acid or base. The requirement of millimolar ligand concentration is a major drawback of this method, especially for ligands with poor water solubility. Another common method for determination of protonation constants when a potentiometric titration is not possible is a spectrophotometric titration, where both pH and spectral changes are monitored upon the addition of acid or base. 

Indirect methods of measuring channel transport are based on measurements of indicators trapped within a vesicle bilayer membrane. " These measurement methods include Na-NMR UV/Vis or fluorescence spectroscopy of pH. metal ion concentration, or concentration of environment-sensitive indicators radiochemical techniques and pH stat techniques to monitor proton efflux. These methods give evidence that an active compound can alter membrane permeability, but it is difficult to establish a distinction between channel function, transport via carriers, or a nonspecific membrane disruption. ... 

A potentiometric titration belongs to chemical methods of analysis in which the endpoint of the titration is monitored with an indicator electrode that records the change of the potential as a function of the amount (usually the volume) of the added titrant of exactly known concentration. Potentiometric titrations are especially versatile because indicator electrodes suitable for the study of almost every chemical reaction used in titrimetry are now available. This technique is also frequently used in the study of operational conditions of visual titrimetric indicators proposed for general use in chemical analysis, as well as in the study of numerous reactions, such as protonation and complexation, which find their application not particularly in analytical measurements. The course of the potentiometric titration curve provides information not only about the titration endpoint position, but also the position and shape of the curve that may provide data about the processes accompanying the titration reaction. Another advantage is that the necessary apparatus is generally less expensive, reliable, and readily available in the laboratories. 

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