Excess positive charge

Certain perovskites with Pb on the A site are particularly important and show pronounced piezoelectric characteristics (PbTiO, PZT, PLZT). Different responses are found in BaTiO and PZT to the addition of donor dopants such as La ". In PZT, lead monoxide [1317-36-8] PbO, lost by volatilization during sintering, can be replaced in the crystal by La202, where the excess positive charge of the La " is balanced by lead vacancies, leading to... 

Acid milling dyes have medium neutral affinity and dyeing can commence on neutrally charged wool. If excess positive charges exist then rapid absorption and unlevel dyeing result. Usually acetic acid is sufficient to generate the requisite number of sites. 

The essential step in the dissolution reaction is the breaking of a siloxane bond Si—O—Si. This bond, although strong, is polar, and may be represented as (Si —0 ). The excess positive charge associated with the... 

Fig. 20.2 Helmholz double layer (H.D.L.) consisting of a plate of excess negative charges on the surface of the metal and a counterbalancing plate of excess positive charges (cations) in solution, the double layer as a whole being electrically neutral. The double layer can be regarded as equivalent to a capacitor in which the plates are separated by a distance ...

A simple model of the e.d.l. was first suggested by Helmholz in which the charges at the interface were regarded as the two plates constituting a parallel plate capacitor, e.g. a plate of metal with excess electrons (the inner Helmholz plane I.H.P.) and a plate of excess positively charged ions (the outer Helmholz plane O.H.P.) in the solution adjacent to the metal the... 

For 10-fold 13C labelled retinal, it has been shown that the differences between chemical shifts for polyene chain carbons of the chromophore in its native environment and detergent-solubilised system were small67 Analysis of the environment of the Schiff base has supported the model of stabilisation based on the protonation by a complex counterion. Three factors were responsible for the excessive positive charge in polyene (i) electronegative nitrogen, (ii) protonation and (iii) counterion strength. 

The r-process path is terminated by (neutron-induced or yd-delayed) fission near A max = 270, feeding matter back into the process at around Amax/2, followed by recycling as long as the neutron supply lasts, assuming sufficient seed nuclei to start the process off. The number of heavy nuclei is thus doubled at each cycle, which could take place in a period of a few seconds, yd-delayed fission also occurs after freeze-out, when the yd-decay leaves nuclei with A > 256 or so with an excessive positive charge (see Eq. 2.90). 

The carbon atom (within a carbenium ion) on which the excess positive charge may be considered to be largely concentrated. 

For p-type semiconductors, an accumulation layer forms when excess positive charge (holes) accumulate at the interface, which is compensated by negative ions of an electrolyte. Fig. 3.7(d). Similarly, a depletion layer forms when the region containing negative charge is depleted of holes, and thus positive counter ions... 

A representative potential distribution across the interface is shown in Fig. 3.9(c), taking the potential of the bulk solution as zero. The potential difference across the space charge region (psd occurs over a larger distance than that of the Helmholtz layer (pn). For an n-type semiconductor, (psc results from the excess positive charge of ionized donors in the bulk of the space charge region within the... 

For the correct interpretation of the luminescent bands, artificial apatite standards have been investigated, as nominally pure, as activated by different potential luminogen impurities. Natural carbonate-fluor-apatites not containing REE were heated with 1 - 5 wt. % of oxides of Eu, Pr, Sm and Dy at 900 °C in air and in vacuum. By changing the activation conditions the differentiation between isomorphous substitutions in different Ca-sites has been achieved. Under vacuum the compensation of the excessive positive charge by substitution of E by 0 is impossible and the luminescence centers in Ca(ll) sites may be less preferential. After heating at this temperature carbonate-fluor-apatite loses its carbonate content and becomes very similar to natural fluor-apatite. 

Fig. 6.4. A schematic representation of the charging of the solution side of an interface, (a) Shading indicates excess-charge density in the interphase region, (b) An exploded view shows that the positive charge in a lamina in the interphase exceeds the negative charge and there is a net, or excess, positive-charge density in the lamina.

Excess positive charge density, q( on solution side of interface... 

Almost all analyte ion inside the membrane in Figure 15-8b is bound in the complex LC+, which is in equilibrium with a small amount of free C+ in the membrane. The membrane also contains excess free L. C+ can diffuse across the interface. In an ideal electrode, R cannot leave the membrane, because it is not soluble in water, and the aqueous anion A-cannot enter the membrane, because it is not soluble in the organic phase. As soon as a few C+ ions diffuse from the membrane into the aqueous phase, there is excess positive charge in the aqueous phase. This imbalance creates an electric potential difference that opposes diffusion of more C+ into the aqueous phase. 

But the activity of C+ in the membrane (dAm) is very nearly constant for the following reason The high concentration of LC+ in the membrane is in equilibrium with free L and a small concentration of free C+ in the membrane. The hydrophobic anion R is poorly soluble in water and therefore cannot leave the membrane. Very little C+ can diffuse out of the membrane because each C+ that enters the aqueous phase leaves behind one R in the membrane. (This separation of charge is the source of the potential difference at the phase boundary.) As soon as a tiny fraction of C diffuses from the membrane into solution, further diffusion is prevented by excess positive charge in the solution near the membrane. 

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