Oxide relative acidity value

Table 2.3. Relative acidity values (a) for some binary oxides of environmental importance...

This discussion should not be seen as explaining the acid/base character of oxides, i.e. their solubilities in water at various pH values. We are emphasising the close relationship between the acid/base behaviour of oxides and the nature of aqueous species. The dissolution of an oxide (other than a neutral oxide) in water, or in acids/alkalies, is an acid-base process, a chemical reaction rather than a mere separation of ions. The relative acid/base strengths of oxides are further discussed in Section 9.2. 

Enormous differences exist in the solubilities of the hydroxides, hydrous oxides, and acids of various elements. Moreover, the concentration of hydrogen or hydroxide ions in a solution can be varied by a factor of 10 or more and can be readily controlled by the use of buffers. As a consequence, many separations based on pH control are, in theory, available to the chemist. In practice, these separations can be grouped into three categories (1) those made in relatively concentrated solutions of strong acids, (2) those made in buffered solutions at intermediate pH values, and (3) those made in concentrated solutions of sodium or potassium hydroxide. Table 30-2 lists common separations that can be achieved by control of acidity. 

B x measure of the relative acidity/basicity of the surface sites, indicated by the PZC. For A1 and Fe oxides, PZC values are in the range of 7.5 to 9.5, denoting a high Bhx and strong proton bonding by surface sites. 

For Co electrodissolution in phosphoric acid, Sazou and Pagit-sasi32 i33 carried out a systematic study of the dynamic behavior in the voltage/extemal resistance parameter plane. The skeleton bifurcation diagram they found is typical for an NDR oscillator that is, bistability between stationary states occurs at high values of ohmic resistance, whereas oscillations are observed at relatively low values of the external resistance. However, from a chemical point of view, Co dissolution seems to be among the most complicated metal electrodissolution reactions because quite a number of different oxide species are involved. Explanations of the dynamics hardly go further than a general statement that the instabihties are due to the formation of a passive film in combination with an IR drop. 

It is shown in [4] that the presence of two types of centres possessing the discriminate reductive-oxidation and acid-base properties and participating in transformation processes of an alcohol molecule is an essential requirement to achieve high sensor response values when alcohol detection is mentioned. Alcohol detection is considered as a multi-step process involving both red-ox and acid-base interactions. Oxide phases within composites differ by oxygen-oxide surface bonding energy which can be the relative measure of oxide activity in oxidation reactions. The reactivity of... 

The activity of the preservative depends on the concentration in the aqueous phase and therefore on the partition coefficient (n-octanol/water). Because of its relatively favourable partition coefficient sorbic acid is a suitable preservative for o/w emulsions. Therefore it is often used in hydrophilic cream bases. Sorbic acid holds a carboxyl group that is deprotonated above pH 4—5. Sorbic acid only takes effect in the non-ionised form that means only in acidic solutions. However, at relatively low pH sorbic acid is degraded by oxidation. To prevent oxidation sorbic acid is often used in combination with potassium sorbate in order to obtain a pH value of 4-5. 

As a result, two composite systems derived from partially aliphatic polyimides— 5-(2,5-dioxotetra-hydrofurfuryl)-3-methyl-3-cyclohexene-l,2-dicarboxylic acid anhydride reacting with 4,4 -oxydianiline (DOCDA-ODA) and with iron oxide as Fe304—were prepared at different temperatures, 250 and 300°C (Nica et al. 2015). Correlations among the structural, morphological, thermal, and magnetic properties were established for possible application as humidity sensor devices. In addition, the influence of humidity on the electrical properties of poly(DOCDA-ODA)/iron oxide composite were discussed, which led to an analysis of the humidity sensitivity of pure polyimide and of a poly(DOCDA-ODA)/Fe304 with different filler contents of iron oxides at various values of relative humidity (RH). 

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