speciation of Cadmium(II) Using Donnan Dialysis and Differential-Pulse Anodic Stripping Voltammetry in a Flow-Injection System

A flow-injection/Donnan dialysis/differential pulse anodic stripping voltammetry system was developed for the determination of free cadmium concentrations, [Cd2+], in solutions containing organically complexed Cd(II). A small dialysis cell with a strong cation-exchange membrane separating the sample and the receiver channels, was equilibrated in a flow-injection system. The ionic strength of sample and receiver solutions was 0.1 M, with NaNO3 as the bulk electrolyte. By determining a constant fraction of the Cd2+ associated with the membrane phase, [Cd2+] of the samples could be measured. Experimentally determined [Cd2+] corresponded well with those calculated, using tabulated stability constants, when citric acid, nitrilotriacetic acid, and oxalic acid were added as ligands. Thus, negatively charged and uncharged complexes were excluded from the membrane. Using the experimental design presented, [Cd2+]>5 × 10×8 M could be determined, but there is a great potential for increasing the sensitivity of the method. In solutions containing 1.0 μM Cd(II) and 200 mg fulvic acid/l, the inorganic fraction (Cd2+ CdNO3 + decreased from 57% to 10% when the pH increased from 4.04 to 5.51. In a soil solution from an orthic podzol, having a high concentration of dissolved organic carbon (22.4 mM), the inorganic fraction constituted 53% of the total Cd(II) concentration.