Ionic media, constant

One major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a "constant ionic media". This concept is based on the general observation that the salt in seawater is close to constant in composition, i.e., the ratios of the major ions are the same from place to place in the ocean. Seawater in evaporative lagoons, pores of marine sediments, and near river mouths can be exceptions to this constancy. Consequently, the major ion composition of seawater can generally be determined from salinity. It has been possible, therefore, to develop equations in which the influences of seawater compositional changes on carbonate equilibria can be... 

All equilibrium constants in the present discussion are based on the concentration (not activity) scale. This is a perfectly acceptable thermodynamic scale, provided the ionic strength of the solvent medium is kept fked at a reference level (therefore, sufficiently higher than the concentration of the species assayed). This is known as the constant ionic medium thermodynamic state. Most modern results are determined at 25 °C in a 0.15 M KCl solution. If the ionic strength is changed, the ionization constant may be affected. For example, at 25 °C and 0.0 M ionic strength, the pXj of acetic acid is 4.76, but at ionic strength 0.15 M, the value is 4.55 [24]. 

The intrinsic equilibrium constants are postulated to be independent of the composition of the solid phase They remain conditional in the sense of the constant ionic medium reference state if interacting ions such as H+ are expressed as concentrations. 

The constant-capacitance model (Goldberg, 1992) assigns all adsorbed ions to inner-sphere surface complexes. Since this model also employs the constant ionic medium reference state for activity coefficients, the background electrolyte is not considered and, therefore, no diffuse-ion swarm appears in the model structure. Activity coefficients of surface species are assumed to sub-divide, as in the triplelayer model, but the charge-dependent part is a function of the overall valence of the surface complex (Zk in Table 9.8) and an inner potential at the colloid surface exp(Z F l,s// 7). Physical closure in the model is achieved with the surface charge-potential relation ... 

Potentiometric methods have eliminated the problems that beset earlier studies, due to the high electrolyte concentrations required for ideal electrode behavior. Following the so-called constant ionic medium principle [91], a large excess of an indifferent (or inert or swamping) electrolyte is added, so that the activity coefficients of the species can be considered constant when their concentration (very low compared to that of the indifferent electrolyte) are changed over a wide range. 

Electrodes sensitive to one of the ion-pair partners in the so-called constant ionic strength cell [95] proved to be valuable to measure the free ion concentration and to determine the stoichiometric equilibrium constant. The latter has a clear thermodynamic meaning if the ionic strength of the medium is indicated, since in this approach, the reference standard state is not the usual infinite dilution of all species dissolved in the solvent (y-> 1, as c -> 0), but is the infinite dilution of the reacting species in the constant ionic medium (7—> 1, as c 0 at 1 = constant) [7]. Even if the constant ionic strength attenuates the variation of liquid junction potentials, the lower the association constant, the lower the consistency of the obtained constant. 

The Ionic Medium Scale This convention can be applied to solutions that contain a swamping concentration of inert electrolyte in order to maintiiin a constant ionic medium. The activity coefficient, f = A /[A], beconries unity as the solution approaches the pure ionic medium, that is, when all concentrations other than the medium ions approach zero ... 

As equation 23 illustrates, a change in the activity scale convention merely changes kf. In an ideal constant ionic medium, equation 23 becomes... 

Both activity scales are thermodynamically equally well defined. In constant ionic medium, activity (= concentration) can frequently be determined by means of emf methods. 

Because we cannot define absolute values for the chemical potential, we restrict ourselves to give changes in chemical potential (resulting from a change in a chemical composition or a change in pressure or temperature). By choosing a suitable reference state, we are able to define the chemical potential (usually at r = 298.2 K and P = 1 atm). In equations 23-26, we define the reference state either as infinite dilution or as constant ionic medium. 

In a constant ionic medium, the concentration quotient becomes the equilibrium constant... 

Equilibrium constants are either or are defined in terms of the constant ionic medium activity scale. 

Equilibrium constants are defined for a constant ionic medium aotivity scale (Section 3.4). Equations 6-8 are derived as in Section 3.7. 

Using seawater as a constant ionic medium we can define pH by the relationship... 

If the measurements are made in the presence of a swamping inert electrolyte, for example, NaC104 (constant ionic medium), we would find, instead of equation 85 (compare Figure 3.2),... 

These authors used activities instead of concentrations, but at constant ionic strength (constant ionic medium scale), the rate law can be written in terms of concentrations. 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

Implicit in the recommendations of Pytkowicz and co-workers for pH measurements in seawater is the belief that the liquid junction potential between seawater of a given salinity and a saturated solution of potassium chloride is independent of the nature and concentration of solutes present at low concentrations in the seawater solvent. This would indeed be the case if seawater is a true constant ionic medium. Hawley and Pytkowicz (5) have estimated that this potential difiFerence amounts to 3.2 mV. As already indicated, this constancy of the liquid junction potential is essential for establishing an experimental scale of pmn. 

The constant ionic medium reference state determines the activity coefficients of the aqueous species. 

There is some uncertainty concerning this calculation. In [75LIB/TIA] Ni(C104)2 was used as a constant ionic medium (with a chloride concentration of 0.01 m), therefore, Ae can be calculated as Ae = e(NiCr, CIO4) - 8(Ni, CF) - s(Ni, CIO4). 

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