SURFACE ACIDITY OF MONTMORILLONITES

Abstract--Surface acidity of almost homoionic montmorillonites was measured by titrating selected Ham-mett indicators adsorbed on the clay with n-butylamine. As expected, the acidity is strongly affected by the exchangeable cations and the degree of hydration of the clay. Greater polarizing ability of the interlayer cations increases both the strength and number of acid sites per H 0 value. The acidity of 'activated ' or heated H-montmorillonite does not exceed that of untreated H-montmorillonite. The origin of the negative charge in the montmorillonite appears to affect he acidity of the clay. It is concluded that the very high acidities reported in the literature (Ho <-5-6) are apparent only and are due to physisorption of the indicator