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 capac ity (CEC) in soils by use of a flow-injection system coupled to an ope n/closed loop sample pretreatment unit was developed. Cations were dis placed from exchange sites on soil colloids by dissolved barium, exces s reagent being subsequently removed and the solid retained in a filte r cartridge located in the open/closed loop unit. The barium fixed in soil was then exchanged with magnesium(II) by adding magnesium sulphat e, which aided to further remove barium as a sulphate precipitate. The clear liquid filtrate in the open/closed loop was finally driven to t he loop of the injection valve and inserted into a flow system designe d for the spectrophotometric determination of residual magnesium(II) w ith Eriochrome Black T. CEC values ranging from 6.6 to 20 mmol/100 g a ir-dried soil were obtained with an average relative standard deviatio n of +/-2.9%. About 10 samples per hour can be processed in this way.