Voltammetric determination of sulfur in biodiesel microemulsion using a silver solid amalgam electrode

Monitoring sulfur in biodiesel is of fundamental importance because even in low concentrations, it can harm the operation of the engine parts and increase the emission of toxic gases and particulate material. Hence, a simple, quick and sensitive adsorptive stripping voltammetry (AdSV) method based on a silver solid amalgam electrode (AgSAE) was developed to determine sulfur in biodiesel. The novel electrochemical method was evaluated through the linear sweep adsorptive stripping voltammetry (LSAdSV), square wave adsorptive stripping voltammetry (SWAdSV) and differential pulse adsorptive stripping voltammetry (DPAdSV) in a NH₃/NH₄⁺ buffer solution (pH 9.0), containing Na₂SO₃. The method was applied in biodiesel microemulsion samples under optimal conditions. To this end, a ternary phase diagram was constructed, employing three components: biodiesel/propan-1-ol/buffer solution. The microemulsion with the best response was found to be 25% NH₃/NH₄⁺ buffer (pH 9.0), 5.0% biodiesel and 70% propan-1-ol. The method reached a detection limit in the order of 10^-7 mol L^-1 of sulfur, and recovery values between 80% and 116%. The method was applied in the determination of sulfur in biodiesel, and the amount in the samples was found to be below the value stipulated by the regulatory agencies. The method can be a promising alternative for determining sulfur in the microemulsion of biodiesel, with the ability to provide a fast response regarding the quality of this biofuel.