Complexation of Hg(II) by humic acid studied by square wave stripping voltammetry at screen-printed gold electrodes

This paper reports the development of a sequential injection (SI) method to study the complexation of Hg(II) by humic acid (HA) using square wave anodic stripping voltammetry at a screen-printed gold electrode (SPGE). The SI system injected samples (in 0.020 mol L−1 NaNO3 and pH 6.0) to the flow cell during the deposition step and exchanged the medium to 0.050 mol L−1 HCl to perform the stripping step under stopped flow conditions. For sample volume=750 μL, flow rate=15 μL s−1 (deposition step), and square wave frequency=100 Hz, the detection limit was 20 nmol L−1 and the sampling throughput was 17 analyses per hour. Complexation of Hg(II) was studied using a 25.0 mg L−1 suspension of HA (72.5 μmol L−1 of ionizable sites), and total Hg(II) concentrations from 1 to 20 μmol L−1. The diffusion coefficient of the complex was (1.3±0.2)×10−7 cm2 s−1. A complexing capacity of 537 μmol g−1 was found. The log of the differential stability constant (log KDEF) decreased from 7.0 to 5.3 as the log of the degree of site occupation (log θ) increased from −1.6 to −0.5. The values of log KDEF were consistent with complexation of Hg(II) by carboxylic and phenolic groups.