Determining completion of excess salt removal from clays. Method comparisons

Three methods of determining the extent of excess salt removal during alcohol washing of clays were compared. Clays (<2 μm) separated from Wyoming bentonite, halloysite, and a Fargo soil (One, montmorillonitic, frigid Vertic Haplaquoll) were saturated with 0.5 M CaCl₂ or 1.0 M KCl. Excess salts were removed by eight consecutive washes with 95% ethanol. Vortex and ultrasonication mixing methods were also compared. After centrifugation, the supernatants were analyzed for excess salt by (i) the AgNO₃-Cl test, and by measuring (ii) electrical conductivity, and (iii) elemental concentrations. Elemental concentration and conductivity measurements indicated that substantial amounts of excess Ca and K (approximately 5 to 10% of CEC) remained in the clays when the AgNO₃-Cl test was first negative. Ultrasonication reduced the number of washings required to remove excess salt from clays when compared with vortex mixing. The results demonstrate that electrical conductivity is a rapid and reliable method of determining completion of excess salt removal.