A thermometric indicator method as a replacement for the filter paper spot test in the titration of clays using methylene blue

Large numbers of soil samples are routinely analyzed using methylene blue (MB) titration to determine clay content. Although, the MB filter paper spot test titration is simple, it is labor-intensive, subjective, and leaves no record for subsequent re-examination. Preliminary thermometric monitoring of the titration indicated a switch from endothermic to exothermic reaction during titration. We examined this nearly universal property of physicochemical processes as a potential replacement method to automate the MB clay titration, making use of literature data.We translated the isothermal enthalipmetric titration of montmorillonite using MB by Rytwo and Ruiz-Hitzky to an adiabatic thermometric titration (2003). The plot of temperature versus titrant volume gave two intersecting linear segments dependent on the titrated percentage of the clay's cation exchange capacity (CEC). The MB filter paper spot test endpoint reflects the first excess MB that persists in the solid suspension, which also happens to be a function of the clay's CEC. The spot test endpoint is correlated to the thermometric indicator point and, for montmorillonite, the two are related by an exact factor of two. Although this factor will naturally be dependent on the clay minerals in the samples, the analyses show that the thermometric indicator titration method is a viable replacement for the spot test. The proposed automation is commonplace as it is practiced with other types of titrations.

Research highlights
► Soils are analyzed manually by MB titration.
► The enthalpy is related to the CEC of clay.
► The MB spot test is related to CEC too.
► The spot test is twice thermometric point.
► Thermometric method is automatable.