Fig. 4. (a) Effect of TDA concentration on the detection of As³⁺ at 0.625 and 1.25 mg L⁻¹, (b–e) effect of pH (3.0 to 6.0) of 5 mM Britton–Robinson buffer on the sensitivity of the linear plot between the A₆₄₅/A₅₂₅ ratio and As³⁺ ranging from 0.5 to 1.25 mg L⁻¹, and (f) the effect of reaction temperature on the detection of As³⁺ at 1.0 mg L⁻¹.