Cation Exchange Capacity (CEC) is the sum of all the exchangeable cations on a mineral’s surface and is expressed as m.eq/100g of mineral. CEC of clays from samples such as oilsands and mature fine tailings (MFTs) is measured by the adsorption of methylene blue (MB) using the dot method.

MB is a cationic organic dye and is adsorbed by negatively charged clay mineral particles. Adsorption of MB is measured at a specific pH to ensure that only permanent charges on clay minerals are measured. The permanent charge in clay minerals occurs due to isomorphic substitution; Na+, K+, Ca2+ and Mg2+ are the cations that are replaced by MB in such reactions.  The adsorped quantity of dye on the clays is reported as the methylene blue index (MBI) in m.eq/100g minerals.  The comparison of the relative MBI values indicates the thresholds at which the presence of various clay minerals have an effect on observed behavior in oilsand samples, froth or tailings.