Point pristine of zero charge

The analysis of solids after dispersion in liquids offers a number of advantages because of the availability of a number of titration techniques, which augment the functional adsorption on dry surfaces. For acid-base potentiometrlc titration of Insoluble oxides in aqueous media the principle has already been explained in sec. I.5.6e and we shall return to it in sec. 3.7a. For amphoteric oxides the (pristine) point of zero charge can be measured it is determined by the difference between pK3,.,(jand pEjigg with some theoretical analysis these two constants can also be established individually. 

Here, [=SOH2] or [=SO ] represents the concentration of protonated or deprotonated surface sites, respectively, on the mineral surface, and the exponents are constants for each mineral. According to this model, the rate of dissolution of most oxides is slowest in solutions where pH = pHppzc, the pH of the pristine point of zero charge where the surface charge of the mineral of interest equals zero (Figures 3 and 4). Some authors include a separate rate term describing dissolution at near-neutral pH (= h2o[=SOH]). Above and below the pHppzc, oxides are predicted to show enhanced dissolution due to protonated and deprotonated surface sites, respectively. 

Figure 4 Log (dissolution rate) versus pH for quartz at 25 °C measured in various pH buffers in agitated batch reactors. The slope of the log rate — pH curve equals 0.3 above the pristine point of zero charge (Brady and Walther, 1992) (source Brady and Walther, 1990).

Tables A.l and A.2 (added in proof) present the pristine points of zero charge that...

Kosmulski, M.. Pristine points of zero charge of gallium and indium oxides, J. Colloid Intetf. Sci.. 238, 225, 2001. 

Kosmulski, M., Attempt to determine pristine points of zero charge of NbjO, TajO, and HfO2> Langmuir, 13, 6315, 1997. 

Table 1.1. Pristine points of zero charge at room temperature for a number of colloidal particle materials...

The PZC is usually measured in the presence of electrolytes. In many cases, one or both of the ions can adsorb on the surface. The PZC is then dependent on both the proper surface properties and also the content of the dispersion medium. If the medium does not contain any ions which specifically adsorb on the surface, the PZC is only dependent on the material in the colloidal particles. Such a PZC is known as a pristine point of zero charge (PPZC) (see Table 1.1). 

TABLE 1 Variability of Pristine Points of Zero Charge and the Consequence with Respect to Modeling Approaches... 

These measurements allow the determination of the IBP, and because no reference is needed in this technique, finally the pristine point of zero charge of a sorbent sample is obtained. Although information on charge at a certain distance from the particle surface is not really available, the pH, at which the particles do not move in an appUed field (or where the potential difference measured is zero), is an indication for the absence of charge on the probe particles. This particular pH is called the lEP. If the lEP coincides for different values of ionic strength and with a correctly determined CIP, this pH can be identified as the PPZC (at least of the sample). The relative charge obtained in a titration can then be renormalized to absolute charge. 

Model f is used with a 2-pK formalism this introduces supplementary parameters (at least one adjustable parameter) compared to model e, rramely the stability constants describing the acid-base behavior. For cases where the pristine point of zero charge is known, ApK can be introduced and only one additional parameter is required. ApK is defined as follows ... 

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