Direct determination of free metal concentration by 1 implementing stripping chronopotentiometry as second stage

15 The electroanalytical technique Absence of Gradients and Nernstian Equilibrium 16 Stripping (AGNES) has been extended by applying stripping chronopotentiometry 17 (SCP) as the reoxidation stage in the determination of the free concentration of Zn 2+ , 18 Cd 2+ and Pb 2+ . This new approach, called AGNES-SCP, has been implemented with 19 screen-printed electrodes (SPE) and the standard Hanging Mercury Drop Electrode 20 (HMDE). Clear advantages of this variant have been shown: i) the easy resolution of the 21 peaks of different metals present in mixtures and ii) the sparing of blanks. A rigorous 22 computation of the faradaic charge along the SCP stage takes into account the 23 contribution of other oxidants, which can be efficiently measured at the end of the 24 deposition stage of AGNES. The free Cd concentration determined in an oxalate 25 solution at pH 6 with an HMDE as working electrode agreed well with values obtained 26 with a Cd Ion Selective Electrode. The free metal concentration measured using an SPE 27 for the system Cd and nitrilotriacetic acid (NTA) at pH=4.8 conformed also well with 28 Visual Minteq results. 29 30