An on-site methodology has been developed for the direct determination of free Zn2+ with AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) in freshwaters. This implementation includes: i) the use of screen printed electrodes, which provide good limits of detection and easy transportation and deployment; ii) no need for sample purging; iii) a calibration in a synthetic river solution that reproduces well the speciation changes of the natural samples and allows the oxygen interferences to be minimised; iv) the addition of a background electrolyte up to 0.01 mol L-1 in both the calibration and freshwater samples; v) chemical stripping chronopotentiometry as the quantification stage of AGNES. This procedure minimizes the effects of working at low ionic strength and in the presence of dissolved oxygen. In the laboratory, the methodology was checked with different natural samples taken from rivers Garonne, Gave de Cauterets and Gave de Pau in the Pyrenees. Results appeared in good agreement with theoretical estimations computed from Visual Minteq. On-site measurements were performed for the first time with AGNES in the Gave de Cauterets in Soulom (France) and the results were corroborated with purged measurements performed in the laboratory.English language editing by Holly Pearson is acknowledged. The authors
acknowledge support for this research from the Communaut e
d’Agglom eration de Pau Pyr en ees (CDAPP), from the Spanish Ministerio de
Ciencia e Innovacio´n (project numbers CTM2012–39183 and CTM2013–
48967), from the European Community EFA15/08 (PyrMet) and from the
Comissionat d’Universitats i Recerca de la Generalitat de Catalunya (project
number 2014SGR1132)
