Free gallium (III) determination with AGNES (Absence of Gradients and Nernstian Equilibrium Stripping)

Gallium is being extensively used in technological applications. Increasing emissions to the environment classify it as an emerging contaminant. Speciation information, such as the free gallium concentration, [Ga³⁺], is fundamental for understanding/predicting its bioavailability and potential toxicity to biota. In this work, [Ga³⁺] in aqueous solutions at pH 2 and 3 has been measured with AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). The deposition times to reach equilibrium, for a fixed accumulation factor or gain, were longer than those required with other metals such as Zn, Pb, Cd and In. This is attributed to the electrodic irreversibility of the couple Ga⁰/Ga³⁺ on the mercury electrode together with low concentrations of relatively poorly labile and/or poorly reversible (for the electrodic transfer) hydroxy complexes. When the AGNES-SCP variant was applied in the Hanging Mercury Drop Electrode with radius 141 μm, and the transition time was ≤10 s, the correction of the deposited mass with a depletion factor was essential. The speciation capacity of AGNES for Ga was evaluated with the phthalate ligand at pH 3 and the experimental results obtained were very similar to the theoretical results predicted with the stability constants in the NIST and Brown-Ekberg databases.