Equilibrium simultaneous acid-base

In an early work by Mertz and Pettitt, an open system was devised, in which an extended variable, representing the extent of protonation, was used to couple the system to a chemical potential reservoir [67], This method was demonstrated in the simulation of the acid-base reaction of acetic acid with water [67], Recently, PHMD methods based on continuous protonation states have been developed, in which a set of continuous titration coordinates, A, bound between 0 and 1, is propagated simultaneously with the conformational degrees of freedom in explicit or continuum solvent MD simulations. In the acidostat method developed by Borjesson and Hiinenberger for explicit solvent simulations [13], A. is relaxed towards the equilibrium value via a first-order coupling scheme in analogy to Berendsen s thermostat [10]. However, the theoretical basis for the equilibrium condition used in the derivation seems unclear [3], A test using the pKa calculation for several small amines did not yield HH titration behavior [13],... 

The characteristic features of parameter estimation in a molecular model of adsorption are illustrated in Table 9.9, taking the simple example of the constant-capacitance model as applied to the acid-base reactions on a hydroxylated mineral surface. (It is instructive to work out the correspondence between equation (9.2) and the two reactions in Table 9.9.) Given the assumption of an average surface hydroxyl, there are just two chemical reactions involved (the background electrolyte is not considered). The constraint equations prescribe mass and charge balance (in terms of mole fractions, x) and two complex stability constants. Parameter estimation then requires the determination of the two equilibrium constants and the capacitance density simultaneously from experimental data on the species mole fractions as functions of pH. 

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

Thus, for every [H ], seven equations have to be solved simultaneously in order to compute the relative concentrations of each species present. Five mass laws (four stability expressions for the four different amine complexes and the acid-base equilibrium of NH4 -NH3) and two concentration conditions make up the seven equations. As concentration conditions one can formulate equations defining Cut- and NH3/. 

Many real-world applications of chemistry and biochemistry involve fairly complex sets of reactions occurring in sequence and/or in parallel. Each of these individual reactions is governed by its own equilibrium constant. How do we describe the overall progress of the entire coupled set of reactions We write all the involved equilibrium expressions and treat them as a set of simultaneous algebraic equations, because the concentrations of various chemical species appear in several expressions in the set. Examination of relative values of equilibrium constants shows that some reactions dominate the overall coupled set of reactions, and this chemical insight enables mathematical simplifications in the simultaneous equations. We study coupled equilibria in considerable detail in Chapter 15 on acid-base equilibrium. Here, we provide a brief introduction to this topic in the context of an important biochemical reaction. 

Accurate measurement of surface acidity constants for reactions 11 and 12 can be difficult (Davis and Kent, 1990 Dzombak and Morel, 1990), especially for natural systems (Stollenwerk, 1995). Acidity constants are often derived from acid-base titration data (Parks and de Bruyn, 1962), and constants for several metal oxides have been published. These published values have been used as initial estimates for modeling adsorption in natural systems with complex mineralogy. The acidity constants have then been optimized simultaneously with equilibrium constants for other solutes to give the best fit to experimental data (Goldberg and Glaubig, 1988b Kent et al, 1995 Kent et al., 2001). 

In many papers the term "superoxide" (HOO ) is used simultaneously with "superoxide anion radicals" (Oa )- However, this is under the physiological conditions incorrect The pKa value of this acid-base equilibrium is 4.8 [227] and, therefore, there is only a very small contribution of HOO at physiological pH (7.4). Therefore, the term "superoxide anion radical" should be exclusively used. The superoxide anion radical is both, a one-electron oxidant and a one-electron reductant. The reactions of O2 with many different biological substrates were studied in detail by the radiation chemists and a summary of the obtained second order rate constants is provided in [228]. However, not a single carbohydrate is mentioned in this comprehensive survey since no reaction could have been observed [229]. 

The interpretation of the viscosity-temperature behavior of these complex systems is difficult since many aspects of the melt conditions must be simultaneously considered. These include the chemical composition of the melt to establish the nature of the polymeric network including the amphoteric behavior of species like AI2O3 and Fe2O3> as well as the acid/base behavior of mixed valence constituents such as iron oxides, and the formation of immiscible liquid phases sometimes associated with the existence of several types of stable anions of significantly different size or charge in the system the nature of the container since some of it may dissolve and affect the composition of the melt the existence of a solid phase to establish the effect on the composition of the residual liquid phase (the solid phase may not be the one expected from related phase equilibrium studies) the relative amount of the liquid and solid phases to establish the composition of the liquid phase (this composition changes as the solid crystallizes out of... 

Despite the possibility of the simultaneous realization of these processes, the potentiometric curves of acid-base titration of Mo03 have no distinctive features as compared with the conventional ones [145]. However, the reverse titration curve differs appreciably from the direct titration results. Using the latter data the equilibrium constant of the formation of dimolybdate in the molten KCl-NaCl eutectic was calculated in Ref. [145] as... 

Describing the titration curve by the three different Eqs. (17.9) [or (17.8)], (17.12), and (17.10) [or (17.11)] is not fully satisfactory. Indeed they implicate that both antagonistic couples do not play a part simultaneously in the fixation of the equilibrium potential values (except at the equivalence point). This is a kind of negation of the very existence of the titration. Moreover, such a methodology also implicates that the titration curve is discontinuous, since three different equations are used. Finally, with this description, it is not possible to study the slope of the curve dEld( about the equivalent point. This is a drawback because it is this slope that governs the success of a titration, as we saw when we considered acid-base titrations. This is the basis for the sharpness index concept. 

Himeda, Y Komatsuzaki, N.O- Sugihara, H. Kasuga, K. Simultaneous Tuning of Activity and Water Solubihty of Complex Catalysts by Acid-Base Equilibrium of Ligands for Conversion of Carbon Dioxide. Organometallics. 2007,26,702-712. 

The reversible reaction between a solid solute and its ions in aqueous solution is never the sole process occurring. At the very least, the self-ionization of water also occurs, although we can generally ignore it. Other equilibrium processes that may occur include reactions between solute ions and other solution species. Two possibilities are acid-base reactions (Section 18-7) and complex-ion formation (Section 18-8). Calculations based on the Ksp expression may be in error if we fail to take into account other equilibrium processes that occur simultaneously with solution equilibrium. We have encountered the dissolution of Pbl2(s) several times already in this chapter. However, the dissolution process is, in fact, a lot more complex than we ve shown. As suggested below, there are many competing processes ... 

In this chapter, we see what acids and bases are and why they vary in strength. We shall use thermodynamics, particularly equilibrium constants, to discuss the strengths of acids and bases quantitatively and thereby develop our insight into their behavior. We then use our knowledge of equilibria involving acids and bases to examine systems in which more than one equilibrium is taking place simultaneously. 

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