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Buffer Intensities and Equilibrium pH of Minerals and Soils: I. The Contribution of Minerals and Aqueous Carbonate to pH Buffering¹

ABSTRACT

Thermodynamic equilibrium considerations are used to calculate pH buffer intensities for a number of aqueous systems involving 16 common soil and rock minerals (viz. gibbsite, kaolinite, allophane, hematite, limonitic iron oxide, quartz, amorphous silica, calcite, siderite, hydrated magnetite + amorphous Fe(OH)₃, microcline, albite, anorthite, Mg-montmorillonite, two illites, and Mg-chlorite), gaseous CO₂ and 26 dissolved species. The calculation of buffer intensities is outlined for simple oxides and carbonates (analytical evaluation) and for silicate minerals during congruent and incongruent dissolution (graphical differentiation). Silicate minerals, carbonates and gibbsite provide strong buffering upon addition of strong acid under slightly alkaline to slightly acid conditions. Carbon dioxide at constant partial pressure is far more effective than any of the minerals considered in counteracting the effect of an increase in alkalinity.

¹ Contribution from the Department of Soil Science & Geology, Agricultural University, Wageningen, The Netherlands.

² Soil Scientists, Dep. of Soil Science & Geology, Agricultural Univ. P.O. Box 37, Wageningen, The Netherlands.

Received for publication July 25, 1973. Accepted for publication October 8, 1973.