Zero point charge measurement

Gil-Llambias, E.J. and Escudey-Castro, A.M., Use of zero point charge measurements in determining the apparent surface coverage of molybdena in MoO/y ALO, catalysts, 7. Chem. Soc. Chem. Commun., 478, 1982. 

Zero Point Charge (ZPO measurements. Potentiometric titration of samples (3.0 g) was carried out in an aqueous suspension (500 ml electrolytic KNO3 solution) according to the procedure reported by Parks (24),... 

The isoelectric points (lEP) of the individual materials and the zero point charge (ZPC) of the mixtures as defined by Parks [12], were determined by electrophoretic migration, measuring the zeta potentials as a function of the solution pH, as in previous studies [13] using a Zeta-Meter Inc. Instrument model 3.0+ Experiments were determined with 30 mg of approximately 2 pm diameter particles, suspended in 300 ml of 10 M KCl, adjusting the pH value with 0.2 M KOH and HCl solutions. Each curve obtained was recorded at least twice to ensure reproducibility. 

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...

The potential of zero charge measures, on a relative scale, the electron work function of a metal in an electrochemical configuration, i.e., immersed in a solution rather than in a vacuum. Converted to an absolute value (UHV scale) and compared with the classic electron work function of the given metal, the difference between the two quantities tells us what occurs from the local structural point of view as the metal comes in contact with the solution. 

As explained in Section 6.3.11, the inner potential difference—A( )—seems to encompass all the sources of potential differences across an electrified interface—Ax and A jf—and therefore it can be considered as a total (or absolute ) potential across the electrode/electrolyte interface. However, is the inner potential apractical potential First, the inner potential cannot be experimentally measured (Section 6.3.11). Second, its zero point or reference state is an electron at rest at infinite separation from all charges (Sections 6.3.6 and 6.3.8), a reference state impossible to reach experimentally. Third, it involves the electrostatic potential within the interior of the phase relative to the uncharged infinity, but it does not include any term describing the interactions of the electron when it is inside the conducting electrode. Thus, going back to the question posed before, the inner potential can be considered as a kind of absolute potential, but it is not useful in practical experiments. Separation of its components, A% and A f, helped in understanding the nature of the potential drop across the metal/solution interface, but it failed when we tried to measure it and use it to predict, for example, the direction of reactions. Does this mean then that the electrochemist is defeated and unable to obtain absolute potentials of electrodes ... 

The electrophoretic mobility of a protein solution may also be measured as a function of pH. By this technique it may also be observed that the colloid passes through a point of zero net charge at which its mobility is zero. The point at which charge reversal is observed electrophoretically is called the isoelectric point. 

Electrokinetic measurements at 25°C on silver iodide in 10 3 mol dm-3 aqueous potassium nitrate give d /d(pAg) = -35 mV at the zero point of charge. Assuming no specific adsorption of K+ or NO3 ions and no potential drop within the solid, estimate the capacity of the inner part of the electric double layer. Taking the thickness of the inner part of the double layer to be 0.4 nm, what value for the dielectric constant near to the interface does this imply Comment on the result. 

Hendershot, W. H. 1978. Measurement technique effects on the value of zero point of charge and its displacement from zero point of titration. Can. J. Soil Sci. 58 438-442,... 

The electroklnetlc counterpart of the point of zero charge is the isoelectric point (l.e.p.). that is the pAg, pH, etc. where by electroklnetlc methods no charge is measured. l.e.p. and p.z.c. are (very) different quantities because the former measures the situation where C = 0, that is where essentially the diffuse charge is zero, whereas the latter represents a zero surface charge. Only under pristine conditions are the two identical. We shall elaborate this distinction in sec. 3.8b. 

With smooth, nonporous surfaces the zero-point of charge and the isoelectric point usually do not differ much from each other. However, when porous particles, e.g., of activated carbons, are measured, the surface of the grains or particles may be acidic in character due to ageing while the internal surface is stiU basic. As mentioned before, aging in narrow pores is very slow due to diffusion restrictions. The electrokinetically measured lEP is determined by the -potential of the particle surface while the PZC is determined by the much larger interior surface of the particles [21]. 

Moriwaki, H., Yoshikawa, Y, and Morimoto, T., Oxide films on iron and nickel ultrafine particles studied with zero point of charge measurements, Langmuir, 6, 847, 1990. 

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