Zero-charge

Thus, when titrating iodide with silver nitrate, coagulation occurs as soon as a slight excess of silver ion has been added (so that a point of zero charge has been surpassed). 

Figure V-8 illustrates that there can be a pH of zero potential interpreted as the point of zero charge at the shear plane this is called the isoelectric point (iep). Because of specific ion and Stem layer adsorption, the iep is not necessarily the point of zero surface charge (pzc) at the particle surface. An example of this occurs in a recent study of zircon (ZrSi04), where the pzc measured by titration of natural zircon is 5.9 0.1...

Material Potential-Determining Ion Point of Zero Charge... 

Table XI-1 (from Ref. 166) lists the potential-determining ion and its concentration giving zero charge on the mineral. There is a large family of minerals for which hydrogen (or hydroxide) ion is potential determining—oxides, silicates, phosphates, carbonates, and so on. For these, adsorption of surfactant ions is highly pH-dependent. An example is shown in Fig. XI-14. This type of behavior has important applications in flotation and is discussed further in Section XIII-4.

How should a molecule be divided into groups In some cases there may appecir to be a chemically obvious way to define the groups, especially when the molecule is a polymer that is constructed from distinct chemical residues. A particularly desirable feature is that each group should, if possible, be of zero charge. The reason for this can be understood if we recall how the different electrostatic interactions vary with distance from Section 4.9.1 ... 

Fig. 11. Effects of pH in the colloidal siUca-water system (1), where A represents the point of zero charge regions B, C, and D correspond to metastable gels, rapid aggregation, and particle growth, respectively. Positive and negative correspond to the charges on the surface of the siUca particle.

To a first approximation, the ions in both Helmholtz layers can be considered point charges. They induce an equal and opposite image charge inside the conductive electrode. When the electrode is negative to the point of zero charge, cations populate the inner Helmholtz layer. 

The above formulas combined with Eqs. (74) and (75) taken at zero charge density yield Eq. (54) for the differential capacitance. Eq. (82) can be used recursively to generate the derivatives of the differential capacity at zero charge density to an arbitrary order, though the calculations become rather tedious already for the second derivative. Thus, in principle at least, we can develop capacitance in the Taylor series around the zero charge density. The calculations show that the capacitance exhibits an extremum at the point of zero charge only in the case of symmetrical ions, as expected. In contrast with the NLGC theory, this extremum can be a maximum for some values of the parameters. In the case of symmetrical ions the capacitance is maximum if + — a + a, < 1. We can understand this result... 

FIG. 7 Parsons-Zobel plot of 1/Q as a function of the inverse Gouy-Chapman capacitance 1 /Cqc- The plot is calculated analytically from Eqs. (54) and (85) at zero charge density. The straight line represents the case = a = For the upper... 

This method is based on the principle that charged partides move towards electrodes in an electric field. A mixture of the required amino add and contaminant salts can be separated at a pH where the amino acid has a net zero charge (at the 1EP). 

The base of the aniline derivative has zero charge, and therefore H0 has to be used in 2-aminothiazole the heterocyclic nitrogen is protonated first and at much lower acidities than is the amino group (see Sec. 3.2). The protonation of the amino group of a cationic species therefore has to be treated using the H+ function (Rochester, 1970). 

An important point of the electrocapillary curve is its maximum. Such maximum value of y, obtained when q = 0, corresponds to the potential of zero charge (E ). The surface tension is a maximum because on the uncharged surface there is no repulsion between like charges. The charge on the electrode changes its sign after the... 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

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