IEPs

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...

A measure of a solute s ability to move through a conductive medium in response to an applied electric field ( iep) ... 

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O Grady, P. 2001. The Internet Encyclopedia of Philosophy, Thales of Miletus, http //www.utm.edu/ research/iep/t/thales.html January 12, 2004. 

Isoelectric point (IEP), 20 479 Isoelectronic point, 3 674 selected inorganic particles,... 

Mikhail, E. (1976). Observation and Least Squares. IEP Series. Harper Row, New York. 

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These data showed that the isolectric point (IEP) of Si02 was around pH 2, as expected. The binding of phosphate ions to the chrome surface were also estimated as a function of pH and ionic strength (Choi et al 2004). Further, both STM and AFM have been used to investigate the corrosion mechanisms of metals exposed to the aqueous phase. Since both STM and AFM can operate under water, this provides a variety of possibilities. 

IEP = ifosfamide, etoposide, cisplatin PEP = cisplatin, etoposide, paclitaxel EP = etoposide + cisplatin TP = topotecan+ paclitaxel E Carbo = etoposide + carboplatin. 

Specific adsorption of ions other than protons causes the pzc and the iep to shift along the pH scale (Stumm, 1992). Specifically adsorbed cations (anions) shift the titration curve and the point of zero proton condition at the surface (pznpc) to lower (higher) pH values, whereas the iep is moved to higher (lower) pH values. The shift of the iep of hematite to a lower pH by adsorbed EDTA and Cl" is shown in Figure 10.8. 

Investigators tend to report either the pzc or the iep, but ideally, when specifically adsorbing ions are present, both parameters should be measured. 

Iron oxides in soils and those used as pigments, are often associated with adsorbing species or with dopants such as phosphate, silicate and various metal ions their iep s/pzc s will differ from those of the pure compounds. Soils rich in Fe oxides, therefore, have pzc s of 4-7, i.e. lower than those of pure iron oxides (see Chap. 16). 

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