Experimental studies in high temperature aqueous chemistry at Oak Ridge National Laboratory Page: 4 of 12
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In this experimental program at ORNL, research is conducted to high temperatures and pressures using
several classical physical chemical methods of solution chemistry: potentiometry, electrical conductivity,
flow calorimetry, isopiestic measurements, flow densimetry, and volatility measurements. The range of
conditions accessible to these methods are listed in Table 1. The first two of these emphasize chemical
reaction equilibria and the next three, the excess thermodynamic properties of electrolytes. All involve
unique equipment developed at ORNL over a period of thirty years. In the remainder of this paper several
of these techniques will be described briefly along with some examples of their results and listings of
systems studied to this point. It will not be possible to be comprehensive on the published work but we
will give some general and a few selected specific references.
TABLE 1. Experimental Apparatus and the Range of Conditions Applicable
(P/bar) (t/*C) (Reference)
Electrochemical Cells satn. 25 - 300 (1,2)
(H2 concentration cells)
Electrical Conductivity 1 - 4000 25 -800 (3,4)
1 - 3000 25 -650 (5)
Flow Calorimeter 1 - 400 25 - 400 (6,7)
Isopiestic Apparatus sat. 110-250 (8,9)
Flow Densimeter 1-1000 25 - 500 (10)
Volatility Apparatus sat. 25 - 350 (11,12)
The use of hydrogen electrodes in a concentration cell configuration for the study of reactions involving
hydrogen ions at high temperatures was invented at Oak Ridge in 1969 (1). These cells represent
essentially a high temperature "pH meter" useable in many different ways limited mostly by the ingenuity
of the experimenter. These cells have since been used in a large number of studies of reactions such as
acid-base ionization, metal ion hydrolysis, metal complexation, precipitation, and most recently adsorption
reactions on solids at temperatures to 300*C. This work has been described recently in several recent
Figure 1 is a schematic drawing of the cell that shows the two Teflon compartments with a porous plug
joining them to complete the electrical circuit. The small inner compartment generally contains the
reference solution (R), usually a strong acid or base, and the large compartment contains the test solution
in which a titration can be performed. The cell representation is given below for the study of the
ionization of a weak acid, HA,
H2t/m ,CmNaj/mHAIMNV,mNO PtH2 (1)
Here the molality of NaCl, present as a supporting, non-complexing electrolyte, is high compared to the
other solutes, e.g., mwaa/ma - mNc/mm - 100 and consequently, the activity coefficients of the minor
components can be assumed to be equal in the two compartments. In this way the liquid junction potential
is minimized and is calculable by the Henderson equation.  The cell potential is given by
AE - RTF - In ([H ][H ]) + Ei (2)
Figure 2 shows the ionization constants for fourteen acids and bases from 25@ to 300*C measured with
this cell (13) to which a number of organic acids and bases could be added now. These data are one of the
most accurately known sets and have formed the basis for the "isocoulombic" approach for extrapolation
of temperature dependences (16-18). For this purpose reactions are written in the form of cations only, or
anions only, or with the same charges on both sides.
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Mesmer, R.E.; Palmer, D.A.; Simonson, J.M.; Holmes, H.F.; Ho, P.C.; Wesolowski, D.J. et al. Experimental studies in high temperature aqueous chemistry at Oak Ridge National Laboratory, article, January 1, 1996; Tennessee. (digital.library.unt.edu/ark:/67531/metadc685715/m1/4/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.