Abstract: Surface acidity of almost homoionic montmorillonites was measured by titrating selected Hammett 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 H0 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 the acidity of the clay.
It is concluded that the very high acidities reported in the literature (H0 < −5–6) are apparent only and are due to physisorption of the indicator.