Direct determination of the cation-exchange capacity of soils with automatic sample pretreatment in a flow system

An automatic method for the determination of the cation exchange capacity (CEC) in soils by use of a flown-injection system coupled to an open/closed loop sample pretreatment unit was developed. Cations were displaced from exchange sites on soil colloids by dissolved barium, excess reagent being subsequently removed and the solid retained in a filter cartridge located in the open/closed loop unit. The barium fixed in soil was then exchanged with magnesium(II) by adding magnesium sulphate, which aided to further remove barium as a sulphate precipitate. The clear liquid filtrate in the open/closed loop was finally driven to the loop of the injection valve and inserted into a flow system designed for the spectrophotometric determination of residual magnesium(II) with Eriochrome Black T. CEC values ranging from 6.6 to 20 mmol/100 g air-dried soil were obtained with an average relative standard deviation of ± 2.9%. About 10 samples per hour can be processed in this way.