Problematics and stability of on-line pH measurements in anaerobic environments: The jellied combined electrode.

Discrepancies of one pH unit and more have been observed after a few days, between continuous on-line in situ pH measurements and instant off-line pH measurements during anaerobic digestion of an agroindustrial wastewater. Concomitantly, the electrical resistance across the porous diaphragm of the on-line electrode increased, and a black clogging developed on its diaphragm. Measurements of the relative liquid junction potential in KCl or Na(2)S solutions excluded that high concentrations of ions such as, K(+), Na(+), Cl(-), HS(-), or S(2-) were the major cause of the drifts in pH values.