Redox potential measurement in natural waters

Summary In spite of the importance of the redox potential (Eh) its experimental measurement in natural environment is still not entirely resolved. In the present work an optimization of the methodology to measure Eh is carried out. The behaviour and storing conditions of the standard solutions, the kind of reference electrode, as well as the kind of the working electrode are studied for calibration. From the results it is concluded that Eh measurements must be done in situ. As electrodes, Ag/AgCl electrode with sleeve-type junction and laminar Pt electrode, previously calibrated with Zöbell solution, should be used. A special cell that allows to carry out the measurement of Eh in ground water samples without variations in its composition and characteristics is proposed for in situ measurements. The proposed procedure is applied to Eh measurement of ground water of different chemical composition. Finally, the experimental values of Eh measured in these samples are applied to WATEQF program to know the distribution of the chemical species in these systems. Eh are mainly determined by the reduced forms Fe²⁺ and FeCO₃ and the oxidized forms amorphous Fe(OH)₃ and Fe₂O₃ (maghemite).