Lefèvre G, Berger G
Institut de Recherche de Chimie Paris, CNRS - Chimie ParisTech, 11 rue Pierre et Marie Curie, 75005 Paris, France.
IRAP, CNRS - Université Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France.
J Colloid Interface Sci. 2014 Sep 15;430:12-7. doi: 10.1016/j.jcis.2014.05.017. Epub 2014 May 22.
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.
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.
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.
对于特定的水热应用，如金属氧化物颗粒的酸碱滴定以预测其对溶解物种的反应性或描述表面电荷机制，需要高温电位滴定法（或pH测定法）。我们在此展示一种特定的装置，可在高达300°C的温度下运行，无氢气且与氧化条件兼容。
使用一个带有氧化锆pH探头的哈氏合金高压釜对钴铁氧体（CoFe2O4）悬浮液进行滴定，钴铁氧体是冷却回路中常见的典型腐蚀产物，温度高达275°C。高压釜材料和pH探头类型的技术选择基于该装置具有高通用性的目标。因此，由于氢电极中不需要氢气，可滴定可还原颗粒，并且能达到比使用内衬聚四氟乙烯高压釜更高的温度。
已在120°C、200°C和275°C下测定了电荷与pH曲线，发现其与表面络合模型（1 - pK/恒电容模型）一致。为了模拟这些曲线，使用了先前通过质量滴定确定的零电荷点值，并且电容是唯一的可调参数。发现其值从25°C时的0.27 Fm(-2)增加到200°C时的0.42 Fm(-2)，然后在275°C时降至0.25 Fm(-2)。这些结果表明这种装置有可能在高温下确定矿物 - 溶液界面的化学性质或水性流体的任何其他性质。
