Exchangeable aluminium on kaolinite

Three salt-free preparations of a ceramic kaolinite were made with different amounts and forms of (Al), where (Al) denotes aluminium ions balancing the isomorphous replacement charge of the clay. In two, (H) clay and (K/H) clay, prepared by washing with hydrochloric acid then water and potassium chloride/hydrochloric acid then water respectively, the (Al) was derived by decomposition of the clay itself. In the third, (Al/OH) clay, a known amount of aluminium together with sufficient alkali was added to give (Al) of the nominal form A1(OH)2.5. Electro-osmosis was used to determine the total charge carried by (Al). The moles of aluminium present as (Al) were estimated by series extraction of aluminium from the clay with potassium chloride/oxalate. Both the charge and the amount of (Al) were deduced from conductimetric titration curves of the clays with alkali. The isomorphous replacement charge of the clay was found to be 3.4 m-equiv./100 g. The average values deduced for the charge per (Al) ion were 3, 1.4 and 0.5 for the (H), (K/H) and (Al/OH) clays respectively. The potentiometric titration curves with alkali of the clays in 1N potassium chloride are discussed in terms of the form of (Al) present. The absorption of organic compounds by salt-free suspensions of kaolinite is also briefly discussed.