Ion-Containing Fluids

DH model yields a simple analytical expression for the Helmholtz energy, in the form 

In this equation, p, is the number density of ionic species i, e is the dielectric constant of the solvent, and the sum is over all ionic species. 

The DH theory has served as the basis for many semi-empirical models for electrolyte solutions, including the extended DH model, the models of Reilley and Wood and Scatchard et Perhaps the most widely 

A more fundamental approach is to attempt to model electrolyte solutions using statistical mechanical methods, of which there are two kinds of models (reviewed extensively elsewhere ° ) Born-Oppenheimer (BO) level models in which the solvent species as well as the ionic species appear explicitly in the model for the solution and McMillan-Mayer (MM) level models in which the solvent species degrees of freedom are integrated out yielding a continuum solvent approximation. Thus, for a BO level model, in addition to the interionic pair potentials one must specify the ion-solvent and solvent-solvent interactions for all of the ionic and solvent species. In this case, the interionic potentials do not contain the solvent dielectric constant in contrast to the MM-level models. Kusalik and Patey carefully discuss the distinction between these two approaches. 

The DH model (eqs 8.45 and 8.46), can also be derived from statistical mechanics as the solution of the mean spherical approximation (MSA) for an electro-neutral mixture of point ions in a continuum solvent i.e. it is an example of a MM-level model). It represents a limiting behaviour of electrolyte solutions and breaks down quickly for concentrations higher than 0.01 mol dm the extended DH model is accurate to a 0.1 mol dm . For more concentrated solutions, it is natural to consider replacing the point ions with finite-size ions this leads to the consideration of so-called primitive models (PMs), which are MM models consisting of an electro-neutral mixture of charged hard spheres in a continuum solvent. The simplest PM is the restricted primitive model (RPM) consisting of an equimolar mixture of equal-diameter charged hard spheres in a dielectic continuum i.e. p+=p = pj2, 7+ = 7 = a). 

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The application of silver-silver chloride electrode is limited to outside the body, e.g. skin electrodes for ECG or EEG recording. In this case they are made wet by a chloride ion containing fluid or paste. 

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