Acid-base equilibria indicators

Surface reactions are most frequently observed for systems in which the reaction takes place very quickly. The faster the reaction, the thinner the layer of the solution round the dropping electrode in which the equilibrium is perturbed and the chemical reaction takes place. The thinner the reaction layer, the greater are the specific effects of the electrode surface on the reaction. Accordingly, one is more likely to obtain values of rate constants similar to values obtained by other methods for relatively slow than for fast reactions. Because acid-base equilibria are often rapidly established, it is not surprising that most waves affected by acid-base equilibria indicate a surface reaction, whereas other slower chemical reactions result in waves corresponding to volume reactions. 

Scheme VIII has the form of Scheme II, so the relaxation time is given by Eq. (4-15)—appjirently. However, there is a difference between these two schemes in that L in Scheme VIII is also a participant in an acid-base equilibrium. The proton transfer is much more rapid than is the complex formation, so the acid-base system is considered to be at equilibrium throughout the complex formation. The experiment can be carried out by setting the total ligand concentration comparable to the total metal ion concentration, so that the solution is not buffered. As the base form L of the ligand undergoes coordination, the acid-base equilibrium shifts, thus changing the pH. This pH shift is detected by incorporating an acid-base indicator in the solution.

An indication of the nature of the transition state in aromatic substitution is provided by the existence of some extrathermodynamic relationships among rate and acid-base equilibrium constants. Thus a simple linear relationship exists between the logarithms of the relative rates of halogenation of the methylbenzenes and the logarithms of the relative basicities of the hydrocarbons toward HF-BFS (or-complex equilibrium).288 270 A similar relationship with the basicities toward HC1 ( -complex equilibrium) is much less precise. The jr-complex is therefore a poorer model for the substitution transition state than is the [Pg.150]

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

With reference to a solvent, this term is usually restricted to Brpnsted acids. If the solvent is water, the pH value of the solution is a good measure of the proton-donating ability of the solvent, provided that the concentration of the solute is not too high. For concentrated solutions or for mixtures of solvents, the acidity of the solvent is best indicated by use of an acidity function. See Degree of Dissociation Henderson-Hasselbalch Equation Acid-Base Equilibrium Constants Bronsted Theory Lewis Acid Acidity Function Leveling Effect... 

The pL-independent IEs have therefore been interpreted580 as indicating that DOD shifts the acid-base equilibrium from (Fem—OO-) to the protonated intermediate (Fenl—OOH) and increases in that way the rate of synthesis of products formed via oxene chemistry (inverse SDIE) and decreases the rate of products formed through the peroxide chemistry... 

NMR spectroscopy, which was developed in the late 1950s as a most powerful tool for structural analysis of organic compounds, has also proven to be useful for acidity determinations. The measurement of the ionization ratio has been achieved by a variety of methods demonstrating the versatility of this technique. If we consider the general acid-base equilibrium Eq. (1.26) obtained when the indicator B is dissolved in the strong acid HA, then Up, and fcd, respectively, are the rates of protonation and deprotonation. 

Indirect Exchange Rates. In this case, the line shape is indirectly related to the acid-base equilibrium. Besides measuring intermolecular processes like the proton exchange rates, DNMR often has been used to measure intramolecular processes like conformational changes that occur on the same time scale. When the activation energy of such a process is very different in the acidic and basic forms for an indicator, DNMR can be used to measure the ionization ratio. 

The primary chemical interaction governing the operation of the pH optrode is the acid-base equilibrium of the indicator HA. 

The change in wave-heights shown in Fig. 2 can be considered as proof that an acid-base equilibrium accompanies the electrode process. This change can sometimes indicate the presence of an acid-base equilibrium when only inadequate experimental evidence can be obtained by other techniques3 (e.g. for protonation of a, (3-unsaturated ketones (51) or of azulenes (54). 

The plot of the pH-dependence (Fig. 18) indicates qualitatively a participation of an intermediate acid-base equilibrium. Evaluation of rate constants kr and kg is made difficult by the inaccessibility of the dissociation constant of reaction (24 b) which corresponds to protonation of a radical anion. ESR would be a suitable method for the determination of the dissociation constants of at least the more stable radical anions. Another possibility for obtaining at least an approximate value of the equilibrium constant is the measurement of the shifts of the half-wave potentials of the more negative wave at potential 3 with pH. Because the half-wave potential of this wave is known to be sensitive to the... 

FIGURE 7.4 Of the 16 chemistry topics examined (1-16) on the final exam, overall the POGIL students had more correct responses to the same topics than their L-I counterparts. Some topics did not appear on all the POGIL exams. Asterisks indicate topics that were asked every semester and compared to the L-I group. The topics included a solution problem (1), Lewis structures (2), chiral center identification (3), salt dissociation (4), neutralization (5), acid-base equilibrium (6), radioactive half-life (7), isomerism (8), ionic compounds (9), biological condensation/hydrolysis (10), intermolecular forces (11), functional group identification (12), salt formation (13), biomolecule identification (14), LeChatelier s principle (15), and physical/chemical property (16). 

Draw the products of each acid-base reaction. Indicate whether equilibrium favors the starting materials or the products. 

Electrolytes (Na, K, Ca, Mg) and blood sugar must be carefully monitored, and any deviation from the norm should be corrected immediately. The risk of hypophos-phataemia must be eliminated by early parenteral substitution. During refractory episodes, such as those involving the acid-base equilibrium and hyperhydration, haemodialysis is usually indicated. In hypoalbumin-aemia, substitution with salt-free albumin is necessary. With about 75% of patients, artificial respiration is called for, the aim being controlled hyperventilation. 

In this way it was possible to determine the pH-dependence of individual rate constants over a range of three pH-units. It was found that k[ is proportional to hydroxide ion concentration (Aii = A i[OH ]), that k i is practically pH-independent (ilj = and that the constant 2 depends on pH in the shape of a part of a dissociation curve, indicating that a rapidly established acid-base equilibrium precedes the rate-determining step. These differences in pH-dependence explain why at [0H ] < 0 3m, k[ (the value of which decreases Avith decreasing pH) can be neglected compared with (pH-independent) k i and why at [OH ] > 0 3m, is small compared with the rapidly increasing k. The rate of nucleophilic attack (with constant k[) decreases with increasing ethanol concentration. 

Since water is a much stronger base than alcohol, all hydrogen ions remain in the form of H2OH+ even after the addition of alcohol to an aqueous solution. Alcohol can, however, displace the acid-base equilibrium in aqueous solutions and this influence which alcohol exerts upon the color of an indicator depends not only upon the nature of the indicator but also upon the kind of acid-base system found in the solution. 

The acid-base equilibrium constants of the beta-blockers atenolol, oxprenolol, timolol, and labetalol were determined by automated potentiometric titrations. The pKg values were obtained in water-rich or water methanol medium (20% MeOH) to obviate the solubility problems associated with the compounds. The initial estimates of pKa values were obtained from Gran s method and then, were refined by the NYTIT and ZETA versions of the LETAGROP computer program. The resultant values were 9.4 (/ = 0.1 M KCI, 20% methanol) for atenolol, 9.6 (/ = 0.1 M KCI) for oxprenolol, 9.4 (/ = 0.1 M KCI, 20% methanol) for timolol and 7.4 and 9.4 (/ = 0.1 M KCI) for labetalol. The potentiometric method was found to be accurate and easily applicable. The operational criteria for applying the methodology are indicated. 

The necessary condition for the use of any acid-base equilibrium as an indicator one requires closely similar medium effects for the acid A and the conjugate base B upon changes of solvent, i.e. the correctness of the condition (1.3.18) should be maintained. 

Bott (1985) reported, however, an observation that strongly indicates the presence of the deprotonated form of the alkenediazonium salt 5.30. If this salt is kept with 0.05 equiv. OCDs K" for 20 h in the perdeuterated solvent mixture (CD3)2SO - CD3OD (5 2), almost complete exchange of the a-H for D is observed, but no nitrogen is evolved. It is likely that this exchange represents an acid-base equilibrium (5-7). The conjugate base 5.31 of this reaction is structurally related to the enol 5.25 with (at least) partial cumulative double bonds at the C(a)-atom ... 

This intramolecular ring to ring rearrangement, induced by amines, has been investigated in [BMIM][PFe] and [BMIM][Bp4]. With the exception of an acid-base equilibrium between the solvent and amine, the data seem to indicate that other interactions (such as substrate-ionic liquid, amine-ionic liquid and amine-amine) have scarce relevance, whereas the high dipolarity/polarizability of ionic liquids stabilizes the rate-determining transition state and so increases the reaction rate. 

The equilibrium parameters of Lux acid-base reactions in ionic media (solubility products of oxides and acid-base equilibrium constants) are essentially affected by the acidic properties of the molten alkaline halide mixtures, i.e., they are dependent on the constituent cation acidities. Therefore, one should consider the reverse problem - the estimation of the basicity indices of ionic melts on the basis of the calculated equilibrium constants. 

Intrinsically, an acid-base equilibrium exists in PILs that are prepared by proton transfer reactions from Bronsted acids to Bronsted bases. The thermal stabihty of PILs is dominated by the amount of neutral species (i.e., free acids and bases) because neutral species evaporate more easily than ionic species. Angell et al. [12] suggested that the difference between the pK values (ApJCJ of an add and a base is a good indicator of the equilibrium. Dai et al. [13] reported that protic ILs based on phosphazene or bicyclic guanidine superbases exhibit high thermal stability comparable to that of aprotic ILs. Ishiguro et al. [14] explored the... 

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