Acidic Properties of Kaolinite in Water and Acetonitrile

Abstract

Potentiometric titration of weakly acidic sites on soil clays is limited in aqueous media by the acidic property of water which prevents sharp endpoints. Generally, it is not possible to obtain sharp endpoints in aqueous systems for acids with pKa > 8. In this study, kaolinite with K, H, or Al as the saturating cation was successfully titrated in water and acetonitrile using the glass‐calomel electrode system for potentiometric determination. Sharper endpoints and wider working potential ranges were obtained in acetonitrile than in the aqueous system. Titer of base needed to reach the final endpoint was greater in acetonitrile than in water; for example, K‐saturated clay required a negligible titer in water (0.4 meq/100 g clay) in contrast to acetonitrile, in which a significant titer (7.4 meq/100 g clay) was required. The larger titer in acetonitrile was attributed to (i) pH‐dependent sites for which a quantitative endpoint was not obtained in water due to the acid‐base properties of this solvent, and (ii) an increased surface acidity in acetonitrile. A comparable difference in total titers at the final endpoint was observed for the titration of H‐ of Al‐saturated kaolinite in water and acetonitrile.

Acidic Properties of Montmorillonite in Selected Solvents

“…The slightly larger titers observed when AN was used as the titration medium may have been due to the weaker acidic character of this solvent which would aid in the titration of weakly acidic pH-dependent sites (Kapoor, 1972;Loeppert et al, 1977). Also, the larger titers in AN may have been partially due to an increased acidic character of the pH-dependent sites in this solvent, as was suggested for kaolinite by Loeppert et aL (1977,1979).…”

“…This increased total titer may be attributed to pH-dependent sources of acidity as suggested by Kapoor (1972) and Loeppert et al (1977Loeppert et al ( , 1979 and possibly to Al-hydroxy contaminants.…”

“…Titration equipment and procedures for titration of clays were discussed by Loeppert et al (1977). In the present study a Radiometer combination electrode with a porousplug liquid junction was used for all potentiometric determinations.…”

“…Numerous attempts have been made to quantify sources of acidity in pure clays, organic matter, or soils by means of titration curves (Jackson, 1963;Volk and Jackson, 1964;Coleman and Thomas, 1967;Loeppert et al, 1977Loeppert et al, , 1979. Potentiometric and conductometric titrations with a variety of bases have been used to differentiate H + from A13 § at the clay surface (Low, 1955;Barshad, 1960;Coleman and Craig, 1961;Davis et al, 1962;Shainberg and Dawson, 1967;Rich, 1970).…”

Metal -surface reactions in mixed aqueous organic solvents

“…The effects of solvents on clay surface reactivity include increased surface acidity (Loeppert et al, 1977;Loeppert et al, 1979) in kaolinite, decreased surface potential of silicates, (Kosmulski and Matijevic, 1992), and decreased surface charge density coupled with increased solvent adsorption in rutiles (Kosmulski, 1990;Kosmulski, 1996). An increase in the surface charge density of goethite in acetone and methanol-water mixed systems with increasing cosolvent volume fraction as compared to aqueous systems was reported by Xue and Traina (1996a).…”