Amperometric Determination of Sulfite by Gas Diffusion- Sequential Injection with Boron-Doped Diamond Electrode

Abstract

A gas diffusion sequential injection system with amperometric detection using aboron-doped diamond electrode was developed for the determination of sulfite. A gasdiffusion unit (GDU) was used to prevent interference from sample matrices for theelectrochemical measurement. The sample was mixed with an acid solution to generategaseous sulfur dioxide prior to its passage through the donor channel of the GDU. Thesulfur dioxide diffused through the PTFE hydrophobic membrane into a carrier solution of 0.1 M phosphate buffer (pH 8)/0.1% sodium dodecyl sulfate in the acceptor channel of theGDU and turned to sulfite. Then the sulfite was carried to the electrochemical flow cell anddetected directly by amperometry using the boron-doped diamond electrode at 0.95 V(versus Ag/AgCl). Sodium dodecyl sulfate was added to the carrier solution to preventelectrode fouling. This method was applicable in the concentration range of 0.2-20 mgSO3^2-/L and a detection limit (S/N = 3) of 0.05 mg SO3²-/L was achieved. This method wassuccessfully applied to the determination of sulfite in wines and the analytical resultsagreed well with those obtained by iodimetric titration. The relative standard deviations forthe analysis of sulfite in wines were in the range of 1.0-4.1 %. The sampling frequency was65 h^-1.

Keywords: Sequential injection; boron-doped diamond electrode; gas diffusion unit; sulfite.

Figure 1.

SIA manifold for the determination of sulfite: EC, electrochemical flow cell.

Figure 2.

Cyclic voltammograms of 1mM Na₂SO₃ in 0.1 M phosphate buffer (a) pH 4-7 and (b) pH 7-10 at the BDD electrode. The sweep rate was 50 mV/s. The area of the electrode was 0.07 cm².

Figure 3.

(a) Hydrodynamic voltammetric results for 10 mg SO₃²⁻/L (Signal) and 0.1 M phosphate buffer pH 8/0.1% SDS (Background) at 0.70-1.05 V (vs. Ag/AgCl). The SIA operating sequence is listed in Table 1. (b) Hydrodynamic voltammogram of signal-to-background ratios.

Figure 4.

SIA with amperometric detection results for 15 consecutive injections of 10 mg SO₃²⁻/L. Carrier was 0.1 M phosphate buffer pH 8 (-□-) and 0.1 M phosphate buffer pH 8/0.1% SDS (-●-). The SIA operating sequence is listed in Table 1.

Figure 5.

Relationship between the average peak current and the volume of the second plug of 2 M H₂SO₄. The concentration of sulfite was 10 mg/L.

Figure 6.

SIA with amperometric detection results at various flow rate of carrier for 10 mg SO₃²⁻/L. Carrier was 0.1 M phosphate buffer pH 8/0.1% SDS. The SIA operating sequence is listed in Table 1.

Figure 7.

Relationship between the average peak current and the sample volume of 10 mg/L sulfite.

Figure 8.

SIA with amperometric detection results for various concentrations of SO₃²⁻. The SIA operating sequence is listed in Table 1. The calibration curve is shown in the inset.