Solution chemistry charge balance

The principle of electroneutrality requires that the ionic species in an electrolyte solution remain charge balanced on a macroscopic scale. The requirement of electroneutrality arises from the large amount of energy required to separate oppositely charged particles by any significant distance against Coulombic forces (e.g., Denbigh, 1971). Because of this requirement, we cannot obtain a flask of sodium ions at the chemistry supply room, nor can we measure the activity coefficients of individual ions directly. 

Because we are generally able to define the chemistry of an aqueous solution containing n chemical elements by analytical procedures, n equations such as 8.48 and 8.49 exist, relating the bulk concentration of a given element mj to all species actually present in solution. Associated with mass balance equations of this type may be a charge balance equation expressing the overall neutrality of the solution ... 

Incorporating carbonate chemistry into the FREZCHEM model necessitates an explicit recognition of pH. In the pH range from 4 to 12, the following charge balance exists for the cations and anions in solution ... 

Well, in case you hadn t noticed, sodium ions don t seem to do much in chemistry. They are almost always spectator ions, because they don t participate in any of the chemical reactions. Their job is to provide a charge balance to the anions in solution. So, in calculating the pH of sodium acetate, we ignore sodium. The acetate ion, however, is the conjugate base of the weak acid, acetic acid. Therefore, the acetate ion is a base, and we can write this ionization equilibrium equation. 

Just like sodium ions, chloride ions are spectator ions in acid-base chemistry. Their job is to provide a charge balance to the cations in solution. So, in calculating the pH of lidocaine hydrochloride we ignore the chloride ion. Now we could draw out the structure or write the molecular formula of lidocaine and its conjugate acid, but it is tedious to do so. Let s do what most chemists do, and postulate a temporary abbreviation for these species. How about using L for lidocaine, and HL+ for its conjugate acid Now, we can write an equation for the acid ionization equilibrium reaction. 

The TLM (Davis and Leckie, 1978) is the most complex model described in Figure 4. It is an example of an SCM. These models describe sorption within a framework similar to that used to describe reactions between metals and ligands in solutions (Kentef fll., 1988 Davis and Kent, 1990 Stumm, 1992). Reactions involving surface sites and solution species are postulated based on experimental data and theoretical principles. Mass balance, charge balance, and mass action laws are used to predict sorption as a function of solution chemistry. Different SCMs incorporate different assumptions about the nature of the solid - solution interface. These include the number of distinct surface planes where cations and anions can attach (double layer versus triple layer) and the relations between surface charge, electrical capacitance, and activity coefficients of surface species. 

The basis for the discussion of adsorption on charged surfaces is the surface complexation model. The precept for this model is the use of the standard mass-action and mass-balance equations from solution chemistry to describe the formation of surface complexes. Use of these equations results in a Langmuir isotherm for the saturation of the surface with adsorbed species. There are of course other models that satisfy these precepts, but which are not generally referred to as surface complexation models, for example, the Stern model (J). 

In the previous calculations we used e (the extent of the reaction) as a calculation tool. However in aqueous chemistry calculations [4] a different calculation method, charge balance, is more convenient and widely used. We introduce it here and illustrate its use with examples of buffer solutions. 

In the broad field of physical chemistry, the Boltzmann distribution law is fundamental to the derivation of the Debye-Htickel theory of electrolyte solutions. In the more narrow arena of interfacial and colloid science, it is applied to the determination of the ionic atmosphere around charged interfaces. In that context, the charge cloud is more commonly referred to as the electrical double layer (EDL). The concept is illustrated schematically (Fig. 5.2) for the situation in which a particle possesses an evenly distributed charge that is just balanced by the total opposite charge, the counterions in the electrical double layer. 

The quantitative pH shift model [23] combined (1) a proton balance between the surface and bulk liquid with (2) the protonation-deprotonation chemistry of the oxide surface (single amphoteric site), and (3) a surface charge-surface potential relationship assumed for an electric double layer. Given the mass and surface area of oxide, the oxide s PZC, its protonation-deprotonation constants Kj and Kj (Figure 13.2), and the hydroxyl density, these three equations are solved simultaneously and give the surface charge, surface potential, and final solution pH. The mass titration experiment of Figure 13.4 can be quantitatively simulated, but perhaps the most powerful simulation is a comprehensive prediction of final pH versus initial pH, as a function of... 

So far in this book, the emphasis has been on the chemistry of acid-base balance. We now proceed to consider the relationship of the concentrations of the chemicals important in the context of acid-base balance to the composition of plasma. In the wider context of the physiology of body fluids such as extracellular and intracellular fluids, there are two constraints which are of importance. The first constraint is physico-chemical and applies to any aqueous fluid. It is called the principle of electroneutrality , which states that, in an aqueous solution, the number of positive charges equals the number of negative charges. For reference, this important principle is included in Table 5.3. 

---