Heterometric Titration of Sodium Montmorillonite with Calcium Nitrate

In the heterometric titration of sodium montmorillonite with calcium nitrate, a titration curve comprising four stages is obtained. The first stage, attributable to the exchange reaction taking place at the sites of broken bonds, shows no change in transmittance upon addition of the salt. The second stage shows a decrease in transmittance attributable to the exchange reaction taking place in the interlayer space. The third stage demonstrates the completeness of the exchange reaction and the exclusive presence of calcium montmorillonite, since no change in transmittance takes place as the reagent is added. The fourth stage shows an increase in transmittance, but the reaction is obscure. The transition point, obtained by extrapolation of the straight portion of the second and third stages, is defined as the “endpoint” of the titration, and is equivalent to the exchange capacity of the clay.