Simple Hydr oxides

The concentration of Pb(ii) in solutions of different initial pH in contact with PbO at 40°C as a function of time (0-24 h) was studied in [123]. Only numerical values are reported and units are not specified. 

Silica is more soluble than most other materials studied in this book. Not surprisingly, more studies have been devoted to the solubility of silica than to solubilities of less soluble materials. An overview of the older literature was presented in [124]. The solubility of silica was studied in [1787]. Solubilities of various silicas in the range from lO m (quartz) to 10 - M is reported in [126]. The solubility increased with specific surface area. Silicate concentration was measured at different pH values and NaCl concentrations in [127]. A solubility of silica of 10 M at pH 2 was found in [128]. 

The kinetics and temperature dependence of the solubility of silica were studied in [129] and the kinetics and ionic strength dependence in [130], in both cases at pH 2-10. The kinetics, pH dependence, and effect of alkali pretreatment were studied in [131]. Solubilities of 11 ppm for quartz and 116 ppm for amorphous silica are reported in [86]. The same study reports 10-80 ppm of silica in natural waters. 

The solubility of ZnO in water and in 0.001 M KCl was studied in [134], The solubility of original (commercial) and washed ZnO after 3-day equilibration was studied in [135]. Solubilities of Zn oxides and carbonates were studied in [136]. ZnO is soluble in dilute acids and bases, and remains sparingly soluble over a relatively narrow pH range. 

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TABLE 3.1 Zero Points of Simple Hydr(Oxides) (See Appendix for the most recent results)... 

Table 3.3 in this section is probably the first ever published compilation of PZC of mixed oxides and other complex materials. A few results have been presented in compilations of PZC discussed in Section D in addition to data on hydr(oxides). Table 3.3 presents data related to PZC of all inorganic materials (selected organic materials are discussed in Chapter 6) other than simple oxides whose PZC are collected in Table 3.1, composite materials having a layer structure (coatings. Section F), and salts of water soluble acids or bases (Section G). Most materials whose PZC are listed in Table 3.3 belong to one of the following groups ... 

Figure 3.2 shows the electrokinetic potential of a material as a function of the pH for different concentrations of inert electrolyte. Such plots are typical for simple and mixed (hydr)oxides and for many salts whose surface charging is pH dependent (Section 3.I.G). The increase in the ionic strength at constant pH depresses the absolute value of C potential, but the lEP is not affected. The electrokinetic curves in the presence of inert electrolytes are nearly symmetrical with respect to the pristine lEP (Figs. 3.84-3.100), when the electrolyte concentration is not too high. Very high concentrations of 1-1 electrolytes (>0.I mol dm ) induce effects characteristic for specific adsorption of cations, i.e. 

Simple, sparingly soluble (hydr)oxides. Within this class, compounds are sorted alphabetically by chemical symbol of the electropositive element (usually metal), then by degree of oxidation (lower degree of oxidation first), and then by degree of hydration (lower degree of hydration first). 

Interest in the variable surface charge/potential of metal (hydr)oxides started somewhat later 1-11). Also here people from a very different background have contributed to the development of this field of science. The emphasis and the approach followed has differed depending on ones background and scientific interest. The topic has been studied from a thermodynamic, colloid chemical, modelling, mineralogical, spectroscopic, surface chemical, theoretical chemical or practical point of view. For simple chemical reactions there will be relatively little conflict between the various points of view. The dissociation reaction of acetic acid can be written as ... 

The A1 edge sites were estimated to be a great deal more acidic than those in simple A1 (hydr)oxides such as corundum or gibbsite (Si site acidity appears to behave identically to Si sites on pure Si oxides, such as quartz). 

They also isolated the new metal by simple chemical methods. The zinc blende sample was dissolved in add and several metals, Pb, Cu, As and Sn, were removed by predpitation with hydr< en sulfide. The filtrate was oxidized and neutralized with ammonia in great excess. Zinc stayed dissolved whQe a predpitate of iron hydroxide, also containing indium, was obtained The hydroxide predpitate was dissolved in add, and iron (and some indium) was predpitated again by carefiil addition of sodium carbonate. From the filtrate indium hydroxide was predpitated by sodium carbonate in excess. They prepared indium by reduction of the oxide with charcoal in front of the narrow end of a blowpipe. The new metal was, according to the discoverers, a soft dudUe metal, with which one could mark a paper, and it was of lighter color than lead, similar to tin . 

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