Soil-Mineral Titrations

Soil or soil-mineral titrations are often used to establish surface acidity composition and acid-base behavior. Soil or soil-mineral surfaces are complex in nature owing to their large variation in functional group content and behavior. For example, the data in Figure 3.33 show that soil surface acidity is made up mostly by A1 and a smaller quantity of H+. The titration behavior of such soil would depend on amount of A1 present, affinity by which this A1 is adsorbed by the surface, degree of surface A1 hydroxylation, and finally the pKg values of the surface-associated H+. Commonly, two types of titrations are employed to evaluate soil or soil-mineral surfaces (1) conductimetric titration and (2) potentiometric titration. 

This implies that A1 in soils is found in various stages of hydroxylation, hence at various degrees of positive charge (e.g., 8+ to 18+). For this reason, various other anions, besides OH-, may be found associated with Al. Furthermore, because polymerization and production of crystalline Al(OH)3 takes time, titratable acidity, which represents a quick procedure, may not necessarily reflect soil-available acidity. A similar problem persists with KCl-extractable soil Al. Polymeric aluminum is strongly adsorbed by soil mineral surfaces and, for this reason, such Al may not be extractable with metal salts (e.g., KC1). 

The organic acid content of the organic matter can be measured by titration with base (estimated values of acid content for fulvic and humic acids are reported in Table 2.5). In soils, however, titrations neutralize mineral as well as oi ganic acidity, so that it is not possible to obtain a reliable estimate of the organic acid content. Nevertheless, an estimation of the contribution of soil organic matter to CEC has been made from the empirical relationship... 

Potentially mineralizable C and N are often measured by incubating a sample of field-moist soil at a known temperature in a sealed chamber containing an alkali trap. The C02-C accumulated in the trap is measured by acid titration and this represents the quantity of C mineralized. Alternatively, C02 in the headspace of the incubation chamber can be measured using a C02 analyser. The amount of N mineralized during incubation is calculated as the difference in extractable NH4+ - andNCV-N measured in the soil before and after incubation. Mineralizable N can also be measured in an open incubation system where the soil is leached periodically and NH4+- andNCV-N in leachates is measured (Stanford 1982). 

Jopony, M., and Young, S. D. (1987). A constant potential titration method for studying the kinetics of Cu2+ desorption from soil and clay minerals. J. Soil Sci. 38, 219-228. 

Bowden, J. W., A. M. Posner, and J. P. Quirk. 1977. Ionic adsorption on variable charge mineral surfaces Theoretical charge development and titration curves. Aust. J. Soil. Res. 15 121-136. 

The characteristic properties of some soils, studied by potentiometric titration and the surface complexation model, are shown in Table 3.12. The mineral composition of some soils are also provided (Table 3.13). 

Various chemical surface complexation models have been developed to describe potentiometric titration and metal adsorption data at the oxide—mineral solution interface. Surface complexation models provide molecular descriptions of metal adsorption using an equilibrium approach that defines surface species, chemical reactions, mass balances, and charge balances. Thermodynamic properties such as solid-phase activity coefficients and equilibrium constants are calculated mathematically. The major advancement of the chemical surface complexation models is consideration of charge on both the adsorbate metal ion and the adsorbent surface. In addition, these models can provide insight into the stoichiometry and reactivity of adsorbed species. Application of these models to reference oxide minerals has been extensive, but their use in describing ion adsorption by clay minerals, organic materials, and soils has been more limited. 

J. W. Bowden, A. M. Posner, and J. P. Quirk, Ionic adsorption on variable charge mineral surfaces Theoretical-charge development and titration curves, Aust. J. Soil Res. 15 121 (1977). N. J, Barrow, J. W. Bowden, A. M. Posner, and J. P. Quirk, An objective method for fitting models of ion adsorption on variable charge surfaces, Aust. J. Soil Res. 18 34 (1980). A. M. Posner and... 

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