Single equilibration method for determination of cation and anion retentions by variable charge soils

A batch adsorption- and a titration-technique was combined and a simple method to determine the retentions of monovalent ions (M⁺ and A⁻) by variable charge soils was developed. A soil sample was successively washed with a concentrated solution of a salt (MA), water and acetone, and then air-dried. A portion of the soil sample was equilibrated with solutions of the salt (MA), the acid (HA) or the alkali (MOH), and pH and MA concentration in the equilibrium solution was determined. The M⁺ and A⁻ retentions were calculated from the amounts of M⁺ and A⁻ added and those found in the equilibrium solution. In this method, it was difficult to fix ionic concentration and pH of the equilibrium solution at a desired level. But the ion retentions could be graphically presented in two-dimensional diagrams by plotting the M⁺ and A⁻ retentions against pH—pC and pOH—pC, respectively, where C denotes the equilibrium MA concentration.