NOTE SURFACE ACIDITY OF IMOGOLITE AND ALLOPHANE: A REPLY

We must raise an objection to many of the explicit statements and implicit assump-tions made by R. D. Laura in his criticism to our measurement of surface acidity of montmorillonite with the Hammett indicators (Henmi & Wada, 1974). Laura argued, the surfaces of montmorillonite are essentially basic, where he used the term surfaces as a synonym of the surface lattice groups of the mineral. We used the term surfaces to designate a clay surface system which includes not only the mineral attice groups but exchangeable cations and polar and/or nonpolar liquid molecules associated with them, and we never attempted to estimate the acid strength of the mineral lattice groups themselves. Laura criticized our measurement because none of the following three conditions for the use of the Hammett indicators is fulfilled: (i) montmorillonite surfaces must act as proton donors, (ii) the proton transfer on clay surfaces must strictly only be a function of relative acidities/basicities of montmorillonite surfaces and the indicator base and (iii) the phenomenon of acid colour development must be only due to protonation. The judgement whether the first condition is fulfilled or not is relate