Thesis (Ph.D.)--Chulalongkorn University, 2007

A gas diffusion sequential injection system with amperometric detection using a boron-doped diamond electrode was developed for the determination of sulfite/sulfur dioxide. A gas diffusion unit (GDU) was used to prevent interference from sample matrices for the electrochemical measurement. The sample was mixed with an acid solution to generate gaseous sulfur dioxide prior to its passage through the donor channel of the GDU. The sulfur 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 the GDU and turned to sulfite. Then the sulfite was carried to the electrochemical flow cell and detected directly by amperometry using the boron-doped diamond electrode at 0.95 V (versus Ag/AgCl). For sample volume of 50 [micro]L, this method was applicable in the concentration range of 0.2–20 mg L[superscript -1] for sulfite and a detection limit (S/N = 3) of 0.05 mg L[superscript -1] for sulfite was achieved. This method was successfully applied to the determination of sulfur dioxide in wines and the analytical results agreed well with those obtained by iodimetric titration. The relative standard deviations for the analysis of sulfite in wines were in the range of 1.0–4.1 %. The sampling frequency was 65 h[superscript -1].