Fig. 1. Effect of pretreatment parameters on the DPV response of 1 x 10e6 M folk acid. (a) Effect of upper potential limit; (b) effect of duration of potential cycle; (c) effect of frequency. Duration (a, c). 1 min; frequency (a, b), 10 Hz; upper potential limit (b, c) 2 V. Supporting electrolyte: 0.1 M perchloric acid.