High Temperature Potentiometry

Chemical Sciences Division, Oak Ridge National Laboratory, PO Box 2008, Building 4500S, Oak Ridge, TN 37851-6110, USA 

Department of Energy and Geo-Environmental Engineering The Energy Institute, The Pennsylvania State University, 207 Hosier Building 

The experimental data relevant to this chapter are to be found in the Appendices. Most of the studies cited in these Appendices mainly cover ranges of temperature below the 200 °C minimum limit generally adhered to in this book (see Table 3.1) and therefore whenever possible all of these lower-temperature results are also listed within each experimental summary. 

The use and application of electrochemical potentiomet-ric cells have been described recently by Lvov (2005) and Lvov and Palmer (2004), and in the latter book there are chapters dealing specifically with measurements of hydrolysis constants for homogeneous systems (Tremaine et al., 2004) and solubility/hydrolysis constants for homogeneous systems (Wesolowski et ah, 2004). 

A high-temperature potentiometric cell (system) consists of at least two electrodes at which the reversible electrochemical reactions take place. In such an equilibrium cell the electrode electric potential, E, can be related to the Gibbs energy, A, G of the corresponding electrochemical half-reaction as follows  

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At the Ostrava Mining University, the problems of determining basicity of the in situ oxide melts and of oxidizing properties of oxide melts were studied in the chemistry department by means of high-temperature potentiometry. A probe for measuring oxygen activity in oxide melts was thereby developed [174]. 

Potentiometry may also be used to determine activity coefficients of electrolytes the measured e.m.f. of an electrochemical cell is related to the activities of the ions. These measurements can yield very accurate values near room temperature for systems where reversible and reproducible electrodes have been developed. Potentiometry at high temperatures is much more difficult this is an area of active research. 

Resistivity measurements are done on meander shaped samples by the standard potentiometrie method in a stirred bath of liquid nitrogen relative to a dummy speeimen For this proeedure of residual re istometry the ultra-high measuring aeeuraey of 3.10 results below 300°C and of 3.10 for annealing treatments at higher temperatures. 

Another galvanic cell of highly practical and theoretical importance is the so-called standard cell (see Section 2.2.2), use of which has to be made as a calibration standard in non-faradaic potentiometry. For this purpose, the saturated Weston cell is the most accepted as its emf is reproducible, precisely known, only slightly temperature dependent in the region around 25° C (1.01832 V) and insensitive to unexpected current flows, if any. 

When thinking about the accuracy and precision of direct potentiometry, it must be kept in mind that 0.5 mV difference in cell voltage translates to almost 2% difference in sample concentration for univalent ions. For divalent ions, it is close to 4%, while for a trivalent ion, in which case S 20mV/decade, it is close to 6%. Therefore, for high accuracy, very well controlled measuring parameters such as cell temperature, junction potential, electrode selectivity, and so on, are needed. 

In routine blood analysis of electrolytes, where ion-selective electrodes are used nearly universally, very small concentration changes are sometimes determined with direct potentiometry. This requires potential stabilities and reproducibihties on the order of 10-100 pV, which is achieved in temperature controlled flow-through cells and with frequent, automated recalibrations between measurements/ In batch mode benchtop analyses with ISEs and in environmental monitoring applications, such a high precision is often not achieved. Precision and accuracy is mainly limited by variations in the liquid junction potential between the calibration and sample phases and by interferences from other sample ions, temperature fluctuations, and, if concentrations rather than activities are desired, variations in activity coefficients. 

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