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Mechanism of Reaction of Neutral Fluoride Solution with Layer Silicates and Oxides of Soils¹

ABSTRACT

The rate of HCl titrable OH formed and reaction mechanism of layer silicates, oxides of Al and Fe, and soils in neutral 1N KF solution were investigated with reference to the possible stoichiometric relationships to mineral bonded fluoride, extracted aluminum and iron, and titrable acidity in the residue. With vermiculite, montmorillonite, kaolinite, halloysite, allophane, chlorite, gibbsite, hematite, goethite, Dodge soil, Cookeville soil and Alberni soil, the formation of HCl titrable OH was rapid initially but approached an apparent equilibrium after 24 hours. The rate of reaction was again rapid upon removal of the reaction products and renewal of the extractant. The amount of titrable OH per unit weight of sample increased with increasing the ratio of fluoride solution volume to sample weight. The hydroxyl formed ranged from 10 meq/100 g of Dodge soil B2 horizon to 281 meq/100 g of halloysite and was not accompanied by any detectable amount of mineral bonded fluoride in the water-ethanol washed residue. The amounts of aluminum or iron released in neutral 1N NH₄F, for example, 20 meq/100 g of hematite and 129 meq/100 g of Alberni soil, were nearly stoichiometric to the OH formed (19 to 131 meq/100 g, respectively) in neutral 1N KF extract. This strongly indicated that the fluoride complexing reaction primarily with Al and secondarily Fe from the minerals was the reaction mechanism. The increase in the potentiometric NaOH titer, to the inflection point at pH 7.5 to 8, of the solid phase after fluoride treatment and washing was in general proportional to the amount of OH formed in the KF solution. For example, for montmorillonite these values were 32 and 37 meq/100 g, respectively. This gives evidence that the mineral surfaces were disrupted (a conclusion confirmed electronoscopically) by reaction in fluoride solutions, yielding NaOH titrable groups in the mineral where Al and Fe had been removed. Ground quartz (<0.5µ) was rather inert toward the neutral fluoride solutions.

¹ Contribution from the Department of Soil Science, University of Wisconsin. Supported in part by the Research Committee of the Graduate School through a grant of funds from the Wisconsin Alumni Research Foundation and in part by science facilities grant G13793—Jackson from the National Science Foundation. Presented before Div. S-2 and S-5, Soil Sci. Soc. Amer. Meeting, Nov. 16, 1964.

² Graduate Research Assistant and Professor of Soil Science, respectively.

Received for publication November 6, 1964. Accepted for publication July 19, 1965.