Dissociation, acid-base

Two useful characterization applications involving acid-base titrimetry are the determination of equivalent weight, and the determination of acid-base dissociation constants. 

The initial goal of the kinetic analysis is to express k as a function of [H ], pH-independent rate constants, and appropriate acid-base dissociation constants. Then numerical estimates of these constants are obtained. The theoretical pH-rate profile can now be calculated and compared with the experimental curve. A quantitative agreement indicates that the proposed rate equation is consistent with experiment. It is advisable to use other information (such as independently measured dissociation constants) to support the kinetic analysis. 

A frequently encountered pH-rate profile exhibits a bell-like shape or hump, with two inflection points. This graphical feature is essentially two sigmoid curves back-to-back. By analogy with the earlier analysis of the sigmoid pH-rate curve, where the shape was ascribed to an acid-base equilibrium of the substrate, we find that the bell-shaped curve can usually be accounted for in terms of two acid-base dissociations of the substrate. The substrate can be regarded, for this analysis, as a dibasic acid H2S, where the charge type is irrelevant we take the neutral molecule as an example. The acid dissociation constants are... 

Standard potentials Ee are evaluated with full regard to activity effects and with all ions present in simple form they are really limiting or ideal values and are rarely observed in a potentiometric measurement. In practice, the solutions may be quite concentrated and frequently contain other electrolytes under these conditions the activities of the pertinent species are much smaller than the concentrations, and consequently the use of the latter may lead to unreliable conclusions. Also, the actual active species present (see example below) may differ from those to which the ideal standard potentials apply. For these reasons formal potentials have been proposed to supplement standard potentials. The formal potential is the potential observed experimentally in a solution containing one mole each of the oxidised and reduced substances together with other specified substances at specified concentrations. It is found that formal potentials vary appreciably, for example, with the nature and concentration of the acid that is present. The formal potential incorporates in one value the effects resulting from variation of activity coefficients with ionic strength, acid-base dissociation, complexation, liquid-junction potentials, etc., and thus has a real practical value. Formal potentials do not have the theoretical significance of standard potentials, but they are observed values in actual potentiometric measurements. In dilute solutions they usually obey the Nernst equation fairly closely in the form ... 

Garcia-Soto, J. Fernandez, M. S., The effect of neutral and charged micelles on the acid-base dissociation of the local anesthetic tetracaine, Biochim. Biophys. Acta 731,275-281 (1983). 

Taking into consideration the acid-base dissociation equilibrium reaction of the ligand, one can obtain an expression for [L" ] and rewrite equation (61) as follows ... 

C. J. Drummond and F. Grieser, Absorption spectra and acid-base dissociation of the 4-alkyl derivatives of 7-hydtoxycoumarin in self-assembled surfactant solution Comments on their use as electrostatic surface potential probes, Photochem. Photobiol. 45, 19-34 (1987). 

The use of capillary electrophoresis (CE) during the synthetic drug development is described from the preclinical development phase to the final marketed stage. The chapter comprises the determination of physicochemical properties, such as acid—base dissociation constants (pKJ, octanol—water distribution coefficients (logP), and analysis of pharmaceutical counterions and functional excipients. 

Although the most active O2 evolution electrocatalysts exhibit Tafel slopes in the range 30-40 mV, in some cases slopes close to 60 mV have been reported. For these cases, a third mechanism has been proposed in which the formation of Oads is preceded by the acid-base dissociation of OHads- This mechanism is known for the name of its proposer, but it could be defined as follows. 

Izutzu, K. (1990). Acid-Base Dissociation Constants in Dipolar Aprotic Solvents, Chemical Data Series No. 35 Blackwell Scientific Publications, Oxford... 

From Izutsu, K. Acid-base Dissociation Constants in Dipolar Aprotic Solvents, Blackwell Science, Oxford 1990, and others. 

The latter reaction has been studied for a number of a-alcohol radicals and its rate was found to correlate with Taft s a values for the corresponding alkyl residues. It has been further shown that the a-alcohol radicals undergo acid-base dissociation at pH <13, yielding highly reactive reducing agents RHCO- the latter species transfer an electron to nitrobenzene at a diffusion-controlled rate (Asmus et al., 1966c). 

The experimental response may deviate from that expected under ideal conditions for the following reasons. First, there may be interactions between the adsorbed molecules that cause the surface activity to differ from the surface concentration [13,14]. Alternatively, double-layer effects, ion-pairing, acid-base dissociation, and dispersion of the formal potentials can cause similar deviations. A non-zero peak splitting may indicate intermolecular interactions between the redox centers or that switching the redox composition triggers a structural change within the supramolecular assembly, e.g. adsorbate reorientation or the formation of... 

The aqueous mobile phases used in RPLC allow the use of buffers in the mobile phase. This may lead to improved selectivity and efficiency. Secundary (ionic) equilibria other than acid-base dissociation may also be used (see section 3.3.2). 

To treat the phosphoric acid titrations step by step [16], one must identify all the reactions, including three acid-base dissociations, water dissociation, and two equations for the conservation of mass and charge for the six unknowns [H3PO4], [H2P04-], [HP04 2], [P04 3], [H+], [OH-] at any given pH, typically one of these six equations will be dominant. 

Figure 3. pH-Dependence of the effective Henry s law coefficient for gases which undergo rapid acid-base dissociation reactions in aqueous solution, as a function of solution pH, Buffer capacity of solution is assumed to greatly exceed incremental concentration from uptake of indicated gas. Also indicated at the right of the figure are Henry s law coefficients for non-dissociative gases. For references see (28). 

ACID-BASE DISSOCIATION EQUILIBRIA. STRENGTH OF ACIDS AND BASES The dissociation of an acid (or a base) is a reversible process to which the law of mass action can be applied. The dissociation of acetic acid, for example, yields hydrogen and acetate ions ... 

This equation, known as the Helmholtz-Smoluchowski equation, relates the potential at a planar bound surface region to an induced electro-osmosis fluid velocity 6. Recall that in the previous section surface charge was related to a potential in solution. In the following section surface charge will be related to the chemistry of the surface. A model for the development of surface charge in terms of acid-base dissociation of ionizable surface groups is introduced. 

In the absence of specific adsorption of electrolyte ions, surface charge is considered to originate from acid-base dissociation of ionizable groups. In terms of acid groups (AH) and basic groups (B), the respective pH-dependent equilibrium between surface sites and solution at the interface can be represented as... 

L. Finston and A. C. Rychtman A New View of Current Acid-Base Theories, Wiley, Chichester, 1982. [108] F. Strohbusch Neue Erkenntnisse der Sdure-Basen-Theorie, Chemie in unserer Zeit 16, 103 (1982). [109] R. A. Cox and K. Yates Acidity Eunctions - An Update, Can. J. Chem. 61, 2225 (1983). [110] (a) E. P. Serjeant and B. Dempsey Ionisation Constants of Organic Acids in Aqueous Solution, Pergamon Press, Oxford/U.K., 1979 (b) K. Izutsu Acid-Base Dissociation Constants in Dipolar Aprotic Solvents, Blackwell Scientific Publishers, Oxford/U.K., 1990. 

This relationship is often called the law of mass action when applied to semicondnctor doping statistics. The sitnation is again conceptnally identical to that for the equilibrinm constant relationship for aqneous acid/base dissociation. The relationship = [H+(aq)][OH (aq)] holds not only for the neutral liquid (i.e. the intrinsic , pH = 7 sample), but also for the proton and hydroxide concentrations in the presence of externally added sources of H+ (aq) or OH (aq) (i.e. the extrinsic or doped liquid). In the doped semiconductor, we are merely adding electrons or holes to control the carrier concentrations in the same way that the pH of water can be manipulated through the addition of acid or base. 

In order to determine to what extent these speculations have validity, it is necessary to be able to evaluate more quantitatively the relative contributions of these interactions to the free energies of protein and nucleic acid molecules in water and nonaqueous solvents. For this purpose, a substantial body of quantitative data is required concerning the properties of suitable model compounds in a variety of solvents, including their solubilities, acid-base dissociation constants, and thermodynamics of hydrogen bond formation. The dearth of pertinent data on hydrogen bonds in solvents of interest is particularly frustrating to even a semiquantitative evaluation of the scheme presented in Fig. 7. 

Though negligible, the electrochemical route is nevertheless critical with respect to the surface topography. This route is responsible for the pitting on terraces, as Fig. 23 shows [122]. This is because the electrochemical hydrolysis of the Si-H bonds is a two-step process initiated by an acid-base dissociation of the Si-H bond which avoids steric problems by liberating a terrace site to form a silanol group which is then removed (Fig. 27, bottom). Additives may reduce the formation of etch pits (see Figs. 24 and 25). 

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