Trace metal activities are important in relation to metal bioavailability in soils, but they are difficult to measure. We compared Cd2+ activities determined by four different methods in laboratory prepared solutions and soil saturation extracts. The methods include (i) the ion selective electrode, (ii) the Cd2+ addition method, (iii) the metal-ion chelate equilibria method, and (iv) GEOCHEM-PC, a computer chemical speciation program. The laboratory prepared solutions of known Cd concentrations ranged from 10-8 to 10-4 M Cd in a simple noncomplexing inorganic matrix to a complex matrix of inorganic and organic ligands that resembled a soil solution. There was good agreement between Cd2+ activities determined by the ion selective electrode and GEOCHEM-PC in test solutions for Cd concentrations > 10-6.5 M. There was also a good agreement between Cd2+ activities determined by the addition method and GEOCHEM-PC for test solutions of 10-8 and 10-7 M Cd and soil saturation extracts. Cadmium ion activities determined by the chelation method did not agree well with those determined by the addition method or GEOCHEM-PC.
(C) Williams & Wilkins 1995. All Rights Reserved.