Electrolyte pressure-volume-temperature

Review of the Experimental and Analytical Methods for the Determination of the Pressure-Volume-Temperature Properties of Electrolytes... 

The pressure-volume-temperature (PVT) properties of aqueous electrolyte and mixed electrolyte solutions are frequently needed to make practical engineering calculations. For example precise PVT properties of natural waters like seawater are required to determine the vertical stability, the circulation, and the mixing of waters in the oceans. Besides the practical interest, the PVT properties of aqueous electrolyte solutions can also yield information on the structure of solutions and the ionic interactions that occur in solution. The derived partial molal volumes of electrolytes yield information on ion-water and ion-ion interactions (1,2 ). The effect of pressure on chemical equilibria can also be derived from partial molal volume data (3). 

MILLERO Pressure-Volume-Temperature Properties of Electrolytes 593... 

When data at high temperatures and pressures began to be available, it was realized that the Born model was also capable of accounting for the large negative values of various partial molar properties of electrolytes at high temperatures (such as the partial molar volume, 10.2.4), and Helgeson and Kirkham (1976) used it in combination with other terms in their equation of state for aqueous species (Chapter 15). 

This reference book contains a compilation of thermodynamic data for about 2000 chemical compounds and aqueous ions (mostly inorganic). The thermodynamic properties tabulated are A-G , A,H , S , and C at 298.15 K, electrode potentials, enthalpies and entropies for phase transitions, A,G of inorganic aqueous ions from 25 to 350 C, partial molar heat capacities from 10 to 130 C, and the partial molar volumes of aqueous electrolytes at high temperatures and pressures. There are 1550 references given to the primary literature and to the literature evaluations of others. 

FIGURE 16.2 Representative base peak electropherograms from CZE runs of RPLC fractions, (a) Fraction 15 (5 peptide identifications) and (b) fraction 20 (19 peptide identifications). Column, bare fused silica capillary, 60 cm x 180 pm ODx30pm i.d. separation voltage, 15 kV observed CZE current, 1.91 p.A running electrolyte, 200 mm acetic acid + 10% isopropanol temperature, 22°C injection time, 10 s at 2 psi ( 4 nL total injection volume) supplementary pressure, 2 psi flow rate, 25nL/min spray voltage, 1.5 kV (reprinted with permission from Electrophoresis). 

What this comparison indicates is that as a first approximation, the terms A and B in equation (45) are not strongly dependent upon temperature and concentration. The pressure dependence of K for electrolyte solutions can be thus estimated from the properties of pure water. Since K = 1/B , the reciprocal of the 1 atm compressibility, it thus becomes possible to make reasonable estimates of vp from 1 atm specific volume data (v°) and compressibility data (B ). 

The electrolyte composition and pH should not be too detrimental. Aspects such as pressure, temperature, and injection technique may have an influence. Concerning the interface, the sheath flow should be optimized with respect to composition, pH, and flow rate. Furthermore, the positions of the fused-silica capillary as well as the API probe need to be carefully optimized. The mixing volume at the tip of the capillary must be kept at a minimum to avoid postcolumn band broadening caused by diffusion. 

The Criss-Cobble correspondence principle is useful for aqueous solutions to about 200 °C. At higher temperatures, the heat capacities Cp of ionic solutes such as NaCl26 at constant pressure rapidly become strongly negative and appear to be headed toward infinite negative values on approaching the critical temperature (which, incidentally, is somewhat higher for aqueous electrolyte solutions than for pure water). If, however, we examine the heat capacities Cy of aqueous electrolytes at constant volume,... 

In recent years we have undertaken a systematic investigation of the volumes and heat capacities of transfer of alkali halides and tetraalkylammonium bromides from water to mixed aqueous solvents (1-6). These properties are important because, when combined with enthalpies and free energies, they can be used to calculate the temperature and pressure dependences of various equilibrium properties of electrolytes in mixed solvents. Since the properties of electrolytes in mixed aqueous solvents are closely related to the corresponding properties of the nonelectrolyte in an electrolyte solution, infor-... 

However, p-jump techniques are not without fault (Takahashi and Alberty, 1969). Most chemical reactions are less sensitive to pressure than to temperature alterations. Thus, a highly sensitive detection method such as conductivity must be employed to measure relaxation times if p-jump is used. Conductometric methods are sensitive on an absolute basis, but it is also fundamental that the solutions under study have adequate buffering and proper ionic strengths. In relaxation techniques, small molar volume changes result, and consequently, even if a low level of an inert electrolyte is present, conductivity changes may be undetectable if pressure perturbations of 5-10 MPa are utilized (Takahashi and Alberty, 1969). 

The determination of molar volumes of molecular species in the liquid phase can be performed with the Rackett equation [58], requiring critical temperature, pressure and volume as well as a further fitting parameter. It is possible to calculate the molar volumes of electrolyte species using the two-parameter equation of Clarke (see Ref. [52]). 

From their calculations of the surface excess entropy and volume of the electric double layer at a mercury-aqueous electrolyte interface, Hill and Payne (HP) [147] postulated an increase in the number of water molecules in the Stern inner region as the surface charge a of about 30 piC/m2, which is consistent with the results of TC on a silver surface obtained some 30 years later. HP used an indirect method to determine the excess entropy and volume by measuring the dependence on temperature and pressure of the double layer capacitance at the mercury-solution interface. 

The physical meaning of the chemical potential relates to the amount by which the energy of a system would change when one additional particle i is introduced at fixed temperature and total pressure or volume (or entropy). The chemical potential is one part of the electrochemical potential. The chemical potential is equal to the electrochemical potential for neutral species or an electrolyte (neutral combination of charged ions), and for a pure phase (e.g., a metal, AgCl) the electrochemical potential is equal to the standard chemical potential. 

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