Measurement of pH and Eh in saline solutions for a better knowledge of the chemical environment in nuclear repositories

The mobility of heavy metals and radionuclides in aqueous systems strongly depends on the 2 geochemical master variables Eh and pH. Their potentiometric measurement in highly concentrated solutions which are typical for salt formations is complicated by liquid junction potentials that alter the cell potentials in an unpredictable way. The difference between apparent pH and the negative logarithm of H+ concentration (ΔpH) may amount to several pH units in highly saline solutions. This distorts geochemical modeling results significantly. For Eh measurements a rather small deviation is observed. For the calculation of the Fe2+/Fe3+ activities, further Pitzer ion interaction parameters valid in highly saline systems have to be developed