Time measurement-visual analysis of l-cysteine using the autocatalytic sodium sulfite/hydrogen peroxide reaction system and its application to length detection–flow analysis

Trace amounts of l-cysteine can function as a trigger, i.e., reaction initiator, in the autocatalytic sodium sulfite/hydrogen peroxide reaction system. Rapidly changing of pH after induction time is visually confirmed by color changing of bromothymol blue in this autocatalytic reaction. Based on this finding, μg L−1 levels of l-cysteine were measured over time using the autocatalytic reaction system. The determination range using the above method was 5.0 × 10−8–2.5 × 10−6 M, the detection limit (3σ) was 1.8 × 10−8 M (1.94 μg L−1), and the relative standard deviation was 2.41% at an l-cysteine concentration of 5 × 10−7 M (n = 5). This method was also applied to length detection–flow injection analysis. The determination range for the flow injection analysis was 2.0 × 10−7–1.0 × 10−5 M. The detection limit (3σ) was 1.4 × 10−7 M (17.0 μg L−1), and the relative standard deviation was 0.91% at an initial l-cysteine concentration of 10−6 M (n = 5).