Potentiometry up to 275°C: example of pH titrations of cobalt ferrite particles.

Hypothesis: High temperature potentiometry (or pH-metry) is needed for specific hydrothermal applications such as acid-base titration of metallic oxides particles to predict their reactivity toward dissolved species, or to describe the surface charge mechanisms. We present here a specific set-up, operational up to 300°C, H2-free and compatible with oxidizing conditions.

Experiments: An autoclave in Hastelloy with a zirconia pH probe has been used to titrate a suspension of cobalt ferrite (CoFe2O4), a typical corrosion product found in cooling circuit, up to 275°C. The technical choices of autoclave material and pH probe type were based on the objective of a high versatility of the set-up. Thus, reducible particles can be titrated thanks to the absence of H2 required in hydrogen electrode, and higher temperatures than those possible using Teflon-lined autoclaves can be reached.

Findings: The charge vs. pH curve has been determined at 120, 200 and 275°C and was found to be consistent with a surface complexation model (1-pK/constant capacitance model). To model the curves, the values of point of zero charge previously determined by mass titration were used and the capacitance was the only adjustable parameter. Its value was found to increase from 0.27 to 0.42 Fm(-2), from 25°C to 200°C, respectively, then decrease to 0.25 Fm(-2) at 275°C. These results show the possibility of such a set-up to determine chemical properties of mineral-solution interfaces or any else properties of aqueous fluid at elevated temperature.